Strengthening piers and lintels
Partitions and lintels are among the most loaded areas of walls and are therefore often reinforced.
Traditionally, steel and reinforced concrete frames are used to strengthen the walls, although in some cases it is advisable to plaster on a mesh or lay over with bricks.
For small vertical and inclined cracks, the walls are reinforced with reinforcing mesh made of wire with a diameter of 3-5 mm with a cell of 100x100 mm (Table 4.4, item 1). The meshes are welded to form a closed loop. For a better fit of the mesh to the wall, use pins (nails) 100-150 mm long, driven into the joints of the masonry. Shotcrete or a layer of plaster 15-20 mm thick is applied to the reinforced wall.
In case of large vertical cracks, the wall is reinforced with a steel cage (Table 4.4, item 2), which is mounted on the pre-plastered and leveled surface of the wall. The cage is a structure of longitudinal corners 50x50 (45x45) mm and strips of steel strip 50x5 mm welded to them with a pitch of 300-500 mm. In this case, the pitch of the slats should not exceed smallest size pier. To create pre-stress in the casing and improve its work together with the brickwork, the strips are sometimes heated to a temperature of 150-200°C before welding.
However, this method of prestressing the cage is quite labor-intensive and difficult to implement, so it is rarely used. More technologically advanced is prestressing, which is achieved using a mortar prepared with prestressing (expanding) cement and injected into the gap between the corners and the brickwork.
Partitions with a complex configuration and surface damage are reinforced using a reinforced concrete cage (Table 4.4, paragraph 3). The cage is made of concrete class B15-B20 and reinforced with a spatial frame consisting of longitudinal and transverse rods. The thickness of the reinforced concrete cage and the cross-sectional area of the longitudinal reinforcement are determined by calculation.
Table 44
Methods for strengthening (replacing) the pier
| No. | Strengthening methods. Gain sketch | Reinforcement elements | |
| Item no. | Material, dimensions | ||
| Plastering on mesh | Nails l=100-150 Wire mesh, class. Вр1 Ø=3…5 mm; cell 100x100 Cement-sand mortar M100; δ=15-20 | ||
Steel clip  | Corner 50x50x5 Planks 50x5 in increments of 300-500 | ||
Reinforced concrete cage  | Longitudinal reinforcement Class. AII, AIII Ø=6..12 Transverse reinforcement class. AI Ø=6…8 Concrete class. B15-B20 δ=40-60 | ||
Replacing the wall  | Racks Boards δ=30-40 Boards δ=50-60 Wooden wedges New wall |
In a project for strengthening long-length piers (when their length is two or more times greater than the thickness), it is necessary to provide for the installation of additional connections passing through the pier masonry.
In case of significant damage to masonry, it may be advisable replacing the wall with a new one. The partition is shifted (replaced) after preliminary unloading. For this purpose, wooden racks are installed in the window openings adjacent to the pier, which are covered with boards to ensure rigidity and stability. The load from the lintels is transferred to the racks through wooden wedges driven into the rack against each other (Table 4.4, item 4). After installing the wall, the gap between the new and old masonry is caulked with a rigid mortar.
It is important to note that the materials for laying a new wall and repairing the wall must have similar physical and mechanical characteristics. This eliminates uneven deformations of the wall and possible overstress of the pier.
Damage to lintels over door and window openings is usually observed in old buildings with great physical wear and tear, and is characterized by the appearance of vertical cracks and the loss of individual masonry stones.
Jumpers strengthen steel corners (channels) or reinforced concrete beams installed in advance built nests(Table 4.5). Reinforcement angles are combined when welding with horizontal plates, and channels - with plates or bolts. The load from the lintel, perceived by the steel elements, is transferred to the walls by means of a strip steel suspension or through steel beams of an angle or channel profile, placed in holes punched in the wall.
An effective method of increasing the strength of masonry at small eccentricities () is the device clips : steel, reinforced concrete And mortar.
The most common elements reinforced by the clip are pillars and piers. Columns, as a rule, have a rectangular cross-section with an aspect ratio of no more than 1.5, which facilitates the efficient operation of the clips, which limit transverse deformations in the section. The walls have an elongated shape in plan, usually with an aspect ratio of more than two. At the same time, for the effective use of the clips, additional connections are installed in the form of coupling bolts or anchors. The permissible distances between ties (anchors, clamps) are no more than 1000 mm and no more than two wall thicknesses in length and height - no more than 750 mm. The connections are securely fixed in the reinforced masonry.
Steel clip- this is a system of longitudinal elements of an angle profile (Fig. 14.5), installed on the solution in the corners or protrusions of the structure and transverse elements (planks) welded to them in the form of strip or reinforcing steel, as well as support pads (when reinforcing the entire column or pier, when part of the forces from the overlying structures is transferred to the longitudinal elements). The pitch of the planks is taken to be no less than the smaller cross-sectional size and no more than 500 mm.
R 
is. 14.5. Reinforcement of stone structures with a steel cage: 1 – reinforced structure, 2 – corner, 3 – strip, 4 – cross brace, 5 – strip, 6 – anchors, 7 – bolt, 8 – support corner, 9 – steel plate
To increase the efficiency of reinforcement, it is recommended to tighten the transverse bars. To do this, from the side of two opposite edges, strips are welded to the longitudinal elements only at one end. Then the strips are heated to 100...120°C and the second free end is welded to the vertical corners while heated. When the planks cool, the reinforced structure contracts.
15. Construction of window and door openings in existing masonry.
Works on replacing block lintels start with the installation of temporary fastenings. Furrows (fines) are punched on both sides of the jumper alternately. The height and width of the grooves must correspond to the height and width of the lintel being replaced and have a gap of about 40...60 mm for a tight fit of the newly connected elements with the existing masonry. Punching begins from the weakest points of the old jumper.
Before installing steel replacement beams made of profile steel (angles, channels), the latter are wrapped in mesh. When installing beams, ensure that the gaps between the brickwork and the structure being installed are carefully filled with a mortar of at least M100 grade. After filling with mortar, the steel beams are bolted together. The spacing of the tie bolts is taken to be no more than 500 mm for spans of no more than 2400 mm and no more than 800 mm for spans of more than 2400 mm. The distance from the ends of the profile to the coupling bolt is taken to be at least 100 mm.
A similar method is used when constructing new openings in existing walls (Fig. 14.14).
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is. 14.14. Construction of a new opening in existing walls: 1 – channel, 2 – tie bolts, 3 – mortar, 4 – steel mesh, 5 – opening to be arranged
The number of the channel profile of steel lintels for a specific opening width for different wall thicknesses is indicated in the table. 14.2. After installing the lintel elements and hardening the mortar, openings are made under the lintels.
If there are defects and damage in the jumpers, they are used to increase their strength. steel linings, representing an elastic support for the elements (Fig. 14.15, 14.16). The linings are made of profile steel of an angle or channel profile. The profiles are connected to each other by strips made of strip steel.
Rice. 14.15. Reinforcement of flat lintels with overlays: 1 – longitudinal corners, 2 – transverse strips, 3 – end corners, 4 – opening
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is. 14.16. Reinforcement of the arched lintel: 1 – reinforcement pads for the arched lintel, 2 – strip, 3 – vertical corner, 4 – support corner
Reinforcement with corners is carried out on both sides of the damaged lintel using cement mortar of a grade not lower than M100. To do this, clear the horizontal seam to a depth of 70 mm in the supporting parts of the lintels. Gaps between the corners and the jumper are not allowed. At the ends of the lintel, holes are punched for installing sections of corners or strips for the entire thickness of the wall from one, then from the other end. Corners (strips) are welded to the ends of the longitudinal corners. Along the length, the corners are connected with planks with a pitch of no more than the thickness of the wall and no more than 500 mm. Connecting strips can be replaced with meshes welded to the bottom edge of the corners. The dimensions of the corners are determined by calculation.
If the height of the corner flange is insufficient and the opening is large, it is recommended to install hangers in the form of inclined strips made of strip steel, 4 mm or more thick, or round steel with a diameter of 10...16 mm with end anchors in the upper part of the wall above the piers. At the bottom, the pendants are welded to the longitudinal corners of the frame (Fig. 14.17).
R 
is. 14.17. Reinforcement of flat lintels using hangers: 1 – reinforcement pads, 2 – strip steel hangers, 3 – holes for hangers, 4 – support pad, 5 – bolt, 6 – existing opening
Jumpers can be reinforced by reducing the opening width due to the installation of additional rows of masonry on the side of the opening with the obligatory bandaging of old and new masonry.
During the operation of stone structures from various reasons signs of their destruction may appear - open cracks appear in the elements (see Fig. 5.27). Such structures can continue to be used after they have been strengthened by enclosing the masonry.
The need for reinforcement may also arise when operating conditions change, for example, when loads increase as a result of reconstruction of buildings, construction of superstructures, etc.
The cages, which must fit tightly to the brickwork, are made of steel, reinforced concrete, and reinforced. The masonry enclosed in a cage operates under conditions of limited lateral expansion (the cage prevents the expansion of the masonry), which increases its load-bearing capacity by 2-2.5 times. Including pillars and piers with cracks in the frame can completely restore their load-bearing capacity. The most efficient operation of the yoke to which the load is transferred (the yoke rests on the upper and lower structures); in this case, it not only restrains the transverse expansion of the masonry, but also absorbs part of the load, unloading the reinforced element.
Steel frames are made by placing rolled steel angles on mortar at the corners of pillars and piers. The corners are combined with strips made of strip steel, which are welded in increments of no more than 500 mm and no more than the smaller side of the cross-section of the reinforced element. To protect the steel frame, it is covered with a layer of cement mortar 25-30 mm thick over a metal mesh, which ensures reliable adhesion of the solution, or the cage is painted (Fig. 5.34, a).
The reinforced plaster casing is made of vertical rods and clamps and is plastered with M75, M100 mortar with a thickness of 30-40 mm (Fig. 5.34, b). Similarly, you can make a reinforced concrete cage, taking the thickness of the cage 40-120 mm.
Rice. 5.34. Reinforcement of the wall with clips: a) steel clip;
b) reinforced plaster casing; 1 - pier; 2 - corners;
3 - strips 35x5-60x12 mm; 4 - plaster; 5 - vertical rods 0 8-12 mm; 6 - clamps 0 4-10 mm
Examples of column calculations
Example 5.1. Using the data from Example 3.7, calculate the steel column for the store building. The column is made of rolled I-beams with parallel flange edges. Load N = 566.48 kN (in fact, the loads from the weight of steel beams and steel columns are less than the loads taken from example 3.7, in which the loads are determined from the weight reinforced concrete beams And brick column, but for comparison of calculation results in examples 5.1, 5.2, 5.3, 5.4 the loads are assumed to be the same). We accept the reliability coefficient for responsibility as y„ = 0.95; load taking into account the reliability coefficient for responsibility 566.48 0.95 = 538.16 kN. The column is actually two floors high, but the design length is taken to be equal to the height of one floor, since its fastening in the ceiling 1e/- 3.6 m is taken into account. The design diagram of the column and its cross-section are shown in Fig. 5.35.
1. Determine the group of structures according to the table. 50* SNiP P-23-81*; columns belong to structure group 3. We accept steel C245 in accordance with GOST 27772-88 (when accepting steel, you should take into account whether a given rolled product is made from this steel or not, since often a certain type of rolled product is made from limited types of steels (see Appendix 1, table. 2).
2. Determine the design resistance of steel according to the table. 2.2, taking into account that the I-beam refers to shaped steel, and having previously specified its thickness / to 20 mm, /^ = 240 MPa = 24 kN/cm2.
3. When calculating stability, we accept the operating condition coefficient ус = 1 (Table 2.3). We set the flexibility of the column X-100, which corresponds to the buckling coefficient Ф ~ 0.542 (Table 5.3). Determine the required area:
4. Determine the required minimum radius of gyration (for a given flexibility X = 100): / = 4/A. = 360/100 = 3.6 cm.
5. Based on the required area and radius of gyration, we select an I-beam according to the assortment of I-beams with parallel flange edges. The closest fit is the I-beam 23Ш1, which has the following characteristics: A = 46.08 cm2; /x= 9.62 cm; 4= 3.67 cm.
6. Check the selected section:
We determine the greatest actual flexibility (the greatest flexibility will be relative to the y-y axis, since the radius of gyration from
With the same design lengths, the cross-sections of the columns are different. The smallest cross-section is a steel column, the largest cross-section is a column made of unreinforced brickwork. The cross-section of a wooden column is smaller than the cross-section of columns made of reinforced concrete and brickwork.
Tasks for independent work
Problem 5.1.
Select the cross-section of the main steel column made of rolled I-beams: the load acting on the column is N - 300 kN; reliability coefficient for responsibility % = 0.95; steel C 235; operating condition coefficient ус= 1; design column length 1^=6 m.
Problem 5.2.
Determine the load-bearing capacity of a steel secondary column made from a rolled I-beam 20K2. The load acting on the column, 20 kN, is applied at the center of gravity of the section; steel C245; operating condition coefficient ус = 1; design length 1e/= 5.0 m.
Problem 5.3.
Check the strength of the centrally compressed brick pillar. Load acting on the pillar, N - 340 kN; N,= 250 kN. Reliability coefficient for responsibility = 0.95. Pole section 510x640 mm; sand-lime brick M75; cement-lime mortar M50. Design diagram - hinged fastening of the pillar to supports; column height H = 4.2 m.
Problem 5.4.
Select the cross-section of a centrally compressed brick pillar. Estimated length /0 = 2.8 m. Load N - 120 kN, N - 100 kN. Reliability coefficient for responsibility y„ = 0.95. Clay brick of plastic pressing M75; cement-lime mortar M75.
Problem 5.5.
Check the strength of a centrally compressed brick column made with mesh reinforcement. The column is subject to a load of N- 380 kN. The reliability coefficient for responsibility is 0.95. Column section 640x640 mm. Clay brick of plastic pressing Ml25; cement-lime mortar M50. The column is reinforced with meshes made of reinforcement class VR-1, 04 mm. The pitch of the reinforcement bars in the meshes (cell size) is 60 mm; mesh pitch 5= 154 mm.
Problem 5.6.
Select the cross-section of a wooden rack made of timber; the stand is hinged at the ends, the length of the stand is / = 2.0 m. The load is applied at the center of gravity of the section, N - 15 kN. Reliability coefficient
responsibility for responsibility = 0.9. Material: birch; grade 2. Temperature and humidity operating conditions B2 (operation outdoors in a normal zone, for such operating conditions coefficient TV = 0.85). When determining the design resistance of birch, the design resistance determined for pine (spruce) wood should be multiplied by the coefficient tp (Table 2.5), which takes into account another type of wood, and the coefficient tb, which takes into account operating conditions. The maximum flexibility of the stand is Xmax = 120.
Problem 5.7.
Check the load-bearing capacity of a wooden stand made of logs. Material: spruce, grade 3; operating conditions A3 (coefficient tb = 0.9). The load acting on the rack is applied at the center of gravity of the section, N - 150 kN. Reliability coefficient for responsibility y„ = 0.95. The rod is hinged at both ends, length /== 3.0 m. Log diameter D= 180 mm. Extreme flexibility of the Xmax-120 rack.
Problem 5.8.
Select the class of reinforcement and the diameters of the transverse rods for the reinforced concrete column, determine their pitch if the longitudinal rods of the column frame are taken to have a diameter of 25 mm, A-III.
Problem 5.9.
Calculate a reinforced concrete column. Load acting on the column, N= 640 kN; N(= 325 kN. Reliability factor for responsibility UP = 0.95. The load is applied with a random eccentricity. Column section is 350x350 mm, symmetrical reinforcement. Column height H = 4.9 m, the ends of the column are hinged. Reinforcement - longitudinal class A- II; transverse Вр-1. Heavy concrete class B20; ул2 - 0.9.
Problem 5.10.
Determine the reinforcement of a reinforced concrete column with random eccentricity and construct its cross-section. Load: N- 1800 kN; N,= 1200 kN. Reliability coefficient for responsibility y„ - 0.95. Estimated column length /0 = //skin!NY = 7.0 m.
Column section 400x400 mm. Heavy concrete class B30; yb2 - 0.9. Longitudinal and transverse reinforcement class A-III.
Problem 5.11.
Check the load-bearing capacity of a reinforced concrete column subjected to a load of N = 250 kN. Load applied
with random eccentricity; long-term part of the load A, = 125 kN; reliability coefficient for responsibility y„ = 0.95. Design column length /0 = 3.0 m. Symmetrical reinforcement Ax = L5 = (2,022 mm). Class A-Sh fittings. Heavy concrete, concrete strength class B20; y = 0.9. The cross section of the column is 300x400 mm (Fig. 5.39).
Problem 5.12.
Select reinforcement for a reinforced concrete column with random eccentricity. Estimated column length /0 = 6.0 m. Column cross-section 400 x 500 mm. Reinforcement is symmetrical, A5 -LE. Load: II= 700 kN, long-term part of the load 525 kN. Coefficient
reliability factor for responsibility y„ ~ 1.0. Heavy concrete class B25, concrete operating condition coefficient yb2 = 0.9. Longitudinal reinforcement of class A-II, transverse reinforcement should be taken, based on the required diameter, class A-I or Bp-1.
§ 6.5. Technology for strengthening brick walls, pillars, piers
When reconstructing residential buildings with masonry walls, there is a need to restore the load-bearing capacity or strengthen the masonry elements due to increased loads from the floors being built on. During long-term operation of buildings, signs of destruction of piers, pillars and masonry walls are observed as a result of uneven settlement of foundations, atmospheric influences, roof leaks, etc.
The process of restoring the bearing capacity of masonry should begin by eliminating the main causes of cracking. If this process is facilitated by uneven settlement of the building, then this phenomenon should be eliminated using known and previously described methods.
Before acceptance technical solutions When strengthening structures, it is important to assess the actual strength of load-bearing elements. This assessment is carried out using the method of destructive loads, the actual strength of the brick, mortar, and for reinforced masonry - the yield strength of steel. In this case, it is necessary to fully take into account the factors that reduce the load-bearing capacity of structures. These include cracks, local damage, deviations of the masonry from the vertical, disruption of connections, support of slabs, etc.
As for strengthening brickwork, the accumulated experience of reconstruction work allows us to identify a number of traditional technologies based on the use of: metal and reinforced concrete frames, frames; on the injection of polymer-cement and other suspensions into the body of the masonry; on the installation of monolithic belts along the top of buildings (in cases of superstructure), prestressed ties and other solutions.
In Fig. 6.40 shows typical design and technological solutions. The presented systems are aimed at comprehensive compression of walls using adjustable tension systems. They are made of open and closed types, with external and internal locations, and are provided with anti-corrosion protection.
Rice. 6.40. Structural and technological options for strengthening brick walls A- diagram of strengthening the brick walls of the building with metal strands; b,V,G- nodes for placing metal strands; d- layout diagram of a monolithic reinforced concrete belt; e- the same, with cords with centering elements: 1 - metal cord; 2 - tension coupling: 3 - monolithic reinforced concrete belt; 4 - floor slab; 5 - anchor; 6 - centering frame; 7 - support plate with hinge
To create the required degree of tension, turnbuckles are used, access to which must always be open. They allow additional tension to be produced as the strands lengthen as a result of temperature and other deformations. Compression of brick wall elements is carried out in places of greatest rigidity (corners, junctions of external and internal walls) through distribution plates.
To uniformly compress the masonry walls, a special design of the centering frame is used, which is hinged on the support-distribution plates. This solution ensures long-term operation with fairly high efficiency.
The locations of the tie rods and centering frames are closed various kinds belts and do not violate general form facade surfaces.
For elements of walls, piers, pillars that have damaged brickwork, but have not lost stability, local replacement of the masonry is carried out. In this case, the brand of brick is taken to be 1-2 units higher than the existing one.
The work technology provides for: installation of temporary unloading systems that absorb the load; dismantling fragments of damaged brickwork; masonry device. It must be taken into account that the removal of temporary unloading systems should be carried out after the masonry has gained strength of at least 0.7 R KL . As a rule, such restoration work is carried out while maintaining the structural design of the building and the actual loads.
Techniques for restoring unplastered brickwork are very effective when it is necessary to maintain the original appearance of facades. In this case, the bricks are very carefully selected in terms of color and size, as well as the material of the seams. After restoration of the masonry, sandblasting is carried out, which makes it possible to obtain updated surfaces where new areas of the masonry do not stand out from the main body.
Due to the fact that stone structures perceive mainly compressive forces, the most effective way to strengthen them is to install steel, reinforced concrete and reinforced cement cages. In this case, the brickwork in the cage operates under conditions of all-round compression, when transverse deformations are significantly reduced and, as a result, resistance to longitudinal force increases.
The design force in the metal belt is determined by the dependence N= 0,2R KJl × l× b, Where R KJl - design chipping resistance of masonry, tf/m2; l- length of the section of the reinforced wall, m; b- wall thickness, m.
To ensure normal operation of brick walls and prevent further opening of cracks, the initial stage is to restore the bearing capacity of foundations using reinforcement methods that eliminate the occurrence of uneven settlements.
In Fig. 6.41 shows the most common options for strengthening stone pillars and piers with steel, reinforced concrete and reinforced cement frames.
Rice. 6.41. Reinforcement of pillars with steel frames (a), reinforced frames (b), meshes and reinforced concrete frames ( V,G) 1 - reinforced structure; 2 - reinforcement elements; 3 - protective layer; 4 - panel formwork with clamps; 5 - injector; 6 - material hose
The steel frame consists of longitudinal corners for the entire height of the reinforced structure and transverse strips (clamps) made of flat or round steel. The pitch of the clamps is taken to be no less than the smaller cross-sectional size, but not more than 500 mm. To enable the cage to work, gaps must be injected between the steel elements and the masonry. The solidity of the structure is achieved by plastering with high-strength cement-sand mortars with the addition of plasticizers that promote greater adhesion to masonry and metal structures.
For more effective protection A metal or polymer mesh is installed on the steel frame, along which a solution 25-30 mm thick is applied. For small amounts of work, the solution is applied manually using a plastering tool. Large volumes of work are performed mechanized with the supply of material by mortar pumps. To obtain a high-strength protective layer, shotcrete and pneumatic concrete installations are used. Due to the high density of the protective layer and high adhesion with masonry elements, the joint work of the structure is achieved and its load-bearing capacity is increased.
The construction of a reinforced concrete jacket is carried out by installing reinforcing mesh around the perimeter of the reinforced structure and fastening it through clamps to the brickwork. Fastening is carried out by using anchors or dowels. The reinforced concrete frame is made from a fine-grained concrete mixture of at least class B10 with longitudinal reinforcement of classes A240-A400 and transverse reinforcement - A240. The pitch of the transverse reinforcement is taken to be no more than 15 cm. The thickness of the cage is determined by calculation and is 4-12 cm. Depending on the thickness of the cage, the technology of work production changes significantly. For frames up to 4 cm thick, concrete application methods are shotcrete and pneumatic concrete. The final finishing of the surfaces is achieved by installing a plaster covering layer.
For frames up to 12 cm thick, inventory formwork is installed around the perimeter of the reinforced structure. Injection tubes are installed in its shields, through which a fine-grained concrete mixture is injected under a pressure of 0.2-0.6 MPa into the cavity. To increase adhesive properties and fill the entire space, concrete mixtures are plasticized by introducing superplasticizers in a volume of 1.0-1.2% of the cement mass. Reducing the viscosity of the mixture and increasing its permeability is achieved by additional exposure to high-frequency vibration through contact of the vibrator with the jacket formwork. Quite good effect
gives a pulsed mixture supply mode, when short-term effects of increased pressure provide a higher velocity gradient and high permeability.
In Fig. 6.41, G a technological scheme for carrying out work by injecting a reinforced concrete cage is given. The formwork is installed to the full height of the structure, ensuring a protective layer of reinforcement filling. Concrete injection is carried out in tiers (3-4 tiers). The process of finishing the concrete supply is recorded by control holes on the opposite side from the injection site. For accelerated hardening of concrete, systems of thermoactive formwork, heating wires and other methods of increasing the temperature of hardening concrete are used. Dismantling of the formwork is carried out in tiers when the concrete reaches its stripping strength. Hardening mode at t= 60 °C ensures stripping strength during 8-12 hours of heating.
Reinforced concrete cages can be made in the form of elements of permanent formwork (Fig. 6.42). In this case, the outer surfaces can have a shallow or deep relief or a smooth surface. After installing permanent formwork and fastening its elements, the space between the reinforced and enclosing structure is sealed. The use of permanent formwork has a significant technological effect, since there is no need to dismantle the formwork, and most importantly, the finishing cycle of work is eliminated.
Rice. 6.42. Strengthening pillars using architectural concrete cladding formwork 1 - reinforced structure; 2 - reinforced frame; 3 - cladding elements; 4 - monolithic concrete
Thin-walled elements (1.5-2 cm) made of dispersed reinforced concrete should be considered the most effective permanent formwork. To engage the formwork in the work, it is equipped with protruding anchors, which significantly increase adhesion to the concrete being laid.
The design of mortar clips differs from reinforced concrete ones in the thickness of the applied layer and composition. As a rule, to protect the reinforcing mesh and ensure its adhesion to the brickwork, plaster cement-sand mortars with the addition of plasticizers are used, which increase the physical and mechanical characteristics. The technology of construction processes is practically no different from performing plastering work.
To ensure the joint operation of the frame elements along its length, which exceeds 2 or more times the thickness, it is necessary to install additional transverse links across the masonry section. Strengthening brickwork can be done by injection. It is carried out by injecting cement or polymer cement mortar through pre-drilled holes. As a result, the monolithic nature of the masonry is achieved and its physical and mechanical characteristics are increased.
Quite stringent requirements are imposed on injection solutions. They must have low water separation, low viscosity, high adhesion and sufficient strength characteristics. The solution is injected under pressure up to 0.6 MPa, which provides a fairly wide penetration zone. Injection parameters: the location of the injectors, their depth, pressure, composition of the solution in each specific case are selected individually, taking into account the cracking of the masonry, the condition of the seams and other indicators.
The strength of masonry reinforced by injection is assessed by SNiP II-22-81*"Stone and reinforced stone structures." Depending on the nature of the defects and the type of injected solution, correction factors are established: tk = 1.1 - in the presence of cracks from force effects and when using cement and polymer-cement mortars; tk= 1.0 - in the presence of single cracks from uneven settlements or in the event of a breakdown in the connection between jointly working walls; tk = 1.3 - in the presence of cracks from force effects during the injection of polymer solutions. The strength of the solutions should be in the range of 15-25 MPa.
Strengthening brick lintels is a fairly common phenomenon, which is associated with a decrease in the load-bearing capacity of spacer masonry due to weathering of seams, adhesion failure and other reasons.
In Fig. Figure 6.43 shows design options for strengthening jumpers using various types of metal plates. They are installed by punching grooves and holes in the brickwork and are subsequently cemented with cement-sand mortar over a mesh.
Rice. 6.43. Examples of strengthening lintels of brick walls A,b- by placing linings made of angle steel; V,G- additional metal jumpers made of channel: 1 - brickwork; 2 - cracks; 3 - corner linings; 4 - strip overlays; 5 - anchor bolts; 6 - channel overlays
To redistribute forces on reinforced concrete lintels due to increased loads on the floors, metal unloading belts are used, made of two channels and united by bolted connections.
Strengthening and increasing the stability of brick walls. Strengthening technology is based on creating an additional reinforced concrete jacket on one or both sides of the wall (Fig. 6.44). The technology of the work includes the processes of preparing and cleaning the surface of the walls, drilling holes for anchors, installing anchors, attaching reinforcing bars or mesh to the anchors, and monolithization. As a rule, for fairly large volumes of work, a mechanized method of applying cement-sand mortar is used: pneumatic concrete or shotcrete, and less often manually. Then, to level the surfaces, a grout layer is applied and subsequent operations related to finishing the wall surfaces are performed.
Rice. 6.44. Strengthening brick walls with reinforcement A- separate reinforcement bars; b- reinforcement cages; V- reinforcing mesh; G- reinforced concrete pilasters: 1 - reinforced wall; 2 - anchors; 3 - fittings; 4 - plaster or shotcrete layer; 5 - metal cords; 6 - reinforcing mesh; 7 - reinforced frame; 8 - concrete; 9 - formwork
An effective method of strengthening brick walls is the installation of reinforced concrete one- and two-sided racks in grooves and pilasters.
The technology for installing double-sided reinforced concrete racks involves the formation of grooves to a depth of 5-6 cm, drilling through holes along the height of the wall, fastening the reinforcement frame using ties and subsequent monolithization of the resulting cavity. For grouting, cement-sand mortars with plasticizing additives are used. A high effect is achieved when using mortars and fine-grained concrete with preliminary grinding of cement, sand and superplasticizer. In addition to great adhesion, such mixtures have the property of accelerated hardening and high physical and mechanical characteristics.
When constructing one-sided reinforced concrete pilasters, the installation of vertical grooves is required, in the cavities of which anchor devices are installed. The reinforcement cage is attached to the latter. After its placement, the formwork is installed. It is made from separate plywood panels, united with clamps and attached to the wall with anchors. Fine-grained concrete mixture is pumped using pumps in layers through holes in the formwork. A similar technology is used for double-sided installation of pilasters with the difference that the process of fastening the formwork panels is carried out using bolts that cover the thickness of the wall.
Tkachev Sergey
The inspection of stone and reinforced masonry structures is carried out taking into account the requirements of SNiP 11-22-81 “Stone and reinforced masonry structures”, as well as “Recommendations for strengthening masonry structures of buildings and structures”.
Before the examination stone structures it is necessary to identify their structure by highlighting the load-bearing elements. It is especially important to take into account the actual dimensions of load-bearing elements, the design diagram, evaluate the magnitude of deformations and destruction, identify the conditions for supporting beams, slabs and other bendable elements on the masonry structure, the condition of the reinforcement (in reinforced masonry structures) and embedded parts. The size and nature of defects and the presence of typical damage (chips and cracks) directly depend on the above conditions.
For strength determination In masonry, mechanical tools and devices are used, as well as ultrasonic devices. Using hammers and chisels through a series of blows, you can approximately assess the qualitative condition of the stone and concrete structures. More accurate data is obtained using special hammers, i.e., mechanical devices based on the assessment of traces or results of impacts on the surface of the structure being tested. The simplest, although less accurate, tool of this type is the Fizdel hammer. A ball of a certain size is pressed into the striking end of the hammer. By means of an elbow strike, creating approximately the same force different people, a trace-hole remains on the surface under study. According to the size of its diameter c. Using a calibration table, the strength of the material is assessed .
More precision instrument is the Kashkarov hammer, when using which the force of the ball hitting the material under study is taken into account by the size of the mark on a special rod located behind the ball.
But the most modern and accurate mechanical action devices are spring ones: the device of the Academy of Public Utilities of the RSFSR, the Central Research Institute of Building Structures. The operating principle of these devices is based on taking into account a certain impact force caused by the release of a charged spring. A device of this type is a housing in which a spiral spring is placed, connected to a hammer rod. After pressing the trigger, the spring is released and the firing pin strikes. In the TsNIISK device, the impact force can be set to 12.5 or 50 kg/cm 2 for stone materials of varying strength.
To determine the bends and deformations of vertical surfaces, their shape and the nature of deviations from verticality and plane, use a level with a special attachment that allows sighting, starting from 0.5 m instead of the minimum 3.5 m when there is no nozzle.
The relief of vertical surfaces is revealed by the method of sighting the instrument from one of its positions on the rod, applied horizontally to pre-designated points of the surface being examined. The results of measuring the deformations of horizontal or vertical surfaces are plotted on diagrams on which, for clarity, lines of equal deviations from the horizontal or vertical are revealed, like horizontals planes. The cross-section is given equal to 2-5 mm depending on the degree of deviation or violation of the position or local defects of the element being examined and its overall dimensions.
However, first of all, it is necessary to find out the nature of the negative changes in the masonry and determine whether the process of crack formation has stabilized, or whether their number and opening width are increasing over time. For this purpose, they are installed in the masonry itself beacons. The beacon is a strip of plaster, glass or metal that covers both sides of the crack. Lighthouses made of plaster and glass will burst if the deformation that caused the cracks continues.
| Instruments for diagnosing the strength of a material: a - Fizdel hammer; b something Kashkarova; c - TsNIISK pistol: 1 - calibrated ball; 2 - angular scale; 3 -
calibration table; 4-replaceable rod for fixing the impact mark |
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Measuring vertical surface deformations using a level with an optical attachment: a-plan; b- wall surface; c - section; 1 - level; 2 - rail; 3 - places where the slats are applied; 4 - lines of equal deviations from the plane |
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Beacons for monitoring the condition of cracks: /-crack; 2-plaster and alabaster mortar; 3- wall material; 4- plaster beacon; 5 - glass lighthouse; 6 - metal plate; 7 - marks every 2-3 mm; 8 - nail |
By measuring the magnitude of the divergence of the halves of the beacon, the nature of the change in the crack or its stabilization is determined. A metal beacon is attached to one side of the crack, and it can move along its other edge, along its other side, where the initial and subsequent positions of the end of the beacon are recorded. The simplest beacon is paper beacon, which is a strip of paper glued to a crack; with further expansion of the crack, the paper beacon breaks.
Cracks in load-bearing masonry structures correspond to the stages of crack formation (or stages of masonry operation under compression). With efforts in masonry F
, not exceeding the effort Fcrc
, in which cracks appear in the masonry, the structure has a load-bearing capacity sufficient to withstand the existing load, cracks do not form. Under loads F 
Fcrc
the process of crack formation begins. Since masonry does not resist tension well, there are cracks on stretched surfaces (areas)
appear much earlier than the possible destruction of the structure.
The main reasons for the formation of cracks are:
1) poor quality of masonry (poor mortar joints, non-compliance with dressings, backfilling in violation of technology, etc.);
2) insufficient strength of the brick and mortar (cracks and curvilinearity of the brick, non-compliance with the drying technology during its manufacture; high mobility of the mortar, etc.);
3) the combined use in masonry of stone materials of different strength and deformability (for example, clay bricks together with silicate bricks or cinder blocks);
4) use of stone materials for purposes other than their intended purpose (for example, sand-lime brick in conditions of high humidity);
5) low quality of work performed in winter time(use of bricks that have not been cleared of ice; use of frozen mortar, absence of anti-frost additives in the solution);
6) failure to make temperature-shrinkable seams or an unacceptably large distance between them;
7) aggressive environmental influences (acidic, alkaline salt exposure; alternating freezing and thawing, moistening and drying);
8) uneven settlement of the foundation in the building.
It is no coincidence that the foundation settlements are indicated last condition for the occurrence of cracks in masonry. It should be borne in mind that during the period of mass construction in masonry, mortars without antifreeze additives were used, thin, non-plastic, i.e. very cheap. All this contributed to abundant education shrinkage
cracks that must be separated from the clean surface during examination sedimentary
cracks that have a specific, easily identifiable character.
Let's consider the process of crack formation in masonry during compression
First stage- the appearance of the first hair cracks in individual stones. An effort Fcrc
at which cracks appear at this stage depends mainly on the type of mortar used in the masonry:
- in masonry with cement mortar F crc = (0.8 - 0.6) F u ; ;
- in masonry with complex mortar F crc = (0.7 - 0.5) F u ;
- in masonry with lime mortar F crc = (0.6 - 0.4) F u ,
Where F u—
destructive force.
Second stage— germination and unification of individual cracks. This stage begins and proceeds more intensely along the southern facade of the building, which experiences the greatest temperature fluctuations in the atmospheric environment. In addition, the growth of cracks is observed when external drains are improperly organized or their system is disrupted in places where the masonry periodically gets wet.
Third stage– further formation of large surfaces of destruction and exhaustion of the strength of the masonry.
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The photograph shows a structure with an attic resting on an internal transverse wall. On the free part of the roof, a slope was created for an organized external drainage system, but the corner of the building was significantly wetted. The arrow points to a developing crack that appeared after one year of operation of the reconstructed structure. |
Defects in brickwork and their causes: a-wear from 20 to 40%; b-wear 41-60%; c- overloaded walls with wear up to 40%; d - the same, with greater wear; d - exposure of brickwork due to wear of plaster |
When analyzing the pattern of cracks, it should be remembered that the appearance of individual cracks in the dressing stones indicates overstress in the masonry. Crack development in the second stage indicates significant overstressing of the masonry and the need to unload or strengthen it.
When large destruction surfaces form, it is advisable to replace the masonry with a new one or strengthen it with a structure that can fully withstand the operational load.
During the operation of the structure, cracks may open due to an unreasonably long length of the temperature block or due to the absence of a temperature-shrinkage seam at all. During the period of reconstruction with the construction of bay windows, hanging elevators, installing additional and attic floors Cracks may appear in the masonry due to the insufficient area of support of the lintels on the wall and the low strength of the masonry, from overloading the wall and the low strength of the masonry. There are other possible causes of cracking. For example, chaotically located cracks often occur in structures that are in close proximity to the place where piles are driven, or in old buildings, the wear of the brickwork reaches 40% or more.
Strength bricks and stones must be determined in accordance with the requirements of GOST 8462-85, solution— GOST 5802-86 or SN 290-74. The density and moisture content of masonry is determined in accordance with GOST 6427-75, 12730.2-78 by establishing the difference in the weight of the samples before and after drying. The frost resistance of stone materials and mortars, as well as their water absorption, is established according to GOST 7025-78.
Samples for testing are selected from lightly loaded structural elements, provided that the materials used in these areas are identical. Samples of bricks or stones must be intact without cracks. Irregularly shaped stones are cut into cubes with edge sizes ranging from 40 to 200 mm or drill out the cylinders (cores) diameter from 40 to 150 mm. To test solutions, cubes with edges from 20 to 40 are made mm, composed of two mortar plates glued together with gypsum mortar. Samples are tested for compression using standard laboratory equipment. Areas of brick (stone) from which samples were taken for testing must be completely restored to ensure the original structure.
Technology for restoring and strengthening brickwork
As noted above, the brick buildings of mass-produced residential buildings had high reliability and a significant margin of safety. But long service life, violations technical specifications contents could cause significant damage to load-bearing brick walls. Depending on the visible damage and condition of the structures, the loads acting on them, and other factors that impede normal operation, during reconstruction measures are taken to restoration load-bearing capacity of brickwork. In addition, when increasing the number of floors of a structure or otherwise increasing the construction volume of a structure, the need arises for strengthening brick structures.
Recoverybearing capacity of masonry comes down to sealing and localizing cracks. Naturally, this problem must be solved after identifying and eliminating reasons that caused cracking:
1) eliminate or stabilize uneven foundation settlements by strengthening foundations or foundations;
2) change the conditions for transferring the load to the cracked pier in order to redistribute the load over a larger area;
3) redistribute the loads to other (or even additional) structures in case of insufficient strength of the masonry itself.
It should be noted that sealing cracks should also accompany measures to strengthening brick structures, which are necessary when loads increase and it is impossible to redistribute them to other elements of the structure.
Technologically, sealing cracks in brick walls can be done using one of the following methods or a combination of them.
Injection of cracks - injection of solutions of liquid cement or polymer-cement mortar, bitumen, resin into cracks in damaged masonry. This method of restoring the load-bearing capacity of masonry is used depending on the type of structure, the nature of its further use, the available injection capabilities, and most importantly, if the crack is local and has a small opening. It can be done using various materials. Depending on their type they distinguish silicization, bituminization, smolization And cementation. Injection allows not only to monolith the masonry, but also to restore and, in some cases, increase its load-bearing capacity, which occurs without increasing the transverse dimensions of the structure.
The most widely used are cement and polymer-cement mortars. To ensure injection efficiency, Portland cement grade of at least 400 with a grinding fineness of at least 2400 is used. cm 2 /g, with a cement paste density of 22 - 25%, as well as Portland slag cement grade 400 with low viscosity in liquefied solutions. Sand for the solution is used fine with a fineness modulus of 1.0 - 1.5 or finely ground with a grinding fineness of 2000-2200 cm 2 /g. To increase the plasticity of the composition, plasticizing additives in the form of sodium nitrite (5% by weight of cement), polyvinyl acetate PVA emulsion with a polymer cement ratio P/C = 0.6 or a naphthalene-formaldehyde additive in the amount of 0.1% by weight of cement are added to the solution. .
Quite stringent requirements are imposed on injection solutions: low water separation, required viscosity, required compressive and adhesive strength, low shrinkage, high frost resistance.
At small cracks in a clutch (up to 1, 5 mm) use polymer solutions based on epoxy resin (epoxy ED-20
(or ED-16) - 100 wt.h.; modifier MGF-9 — 30 wt.h.; hardener PEPA – 15 parts by weight; finely ground sand - 50 wt.h), as well as cement-sand mortars with the addition of finely ground sand (cement - 1 parts by weight; superplasticizer naphthalene-formaldehyde – 0.1 parts by weight; sand - 0.25 parts by weight; water-cement ratio – 0.6).
At more significant opening of cracks use cement-polymer solutions of composition 1:0.15:0.3 (cement; PVA polymer; sand) or 1:0.05:0.3 (cement: plasticizer sodium nitrite: sand), W/C = 0.6 , sand fineness modulus M k =1. The solution is injected under pressure up to 0.6 MPa. The density of crack filling is determined 28 days after injection.
The solution is injected through injectors with a diameter of 20-25 mm. They are installed in special drilled holes after 0.8-1.5 meters along the length of the crack. The diameter of the holes should ensure installation of the injector tube on the cement mortar. Hole depth – no more 100 mm, the injector tube is fixed in the hole with caulked tow.
Injecting cracks up to 10 mm wide with cement-sand mortar:
1- masonry; 2- crack; 3- holes for injectors every 800-1500 mm; 4- steel injector tube; 5- tow, caulked with glue; 6- solution supply
Installation of reinforcing steel brackets
used in methods for restoring the bearing capacity of masonry when cracks open more 10 mm. To do this, a recess is made in the masonry using a milling cutter to the size of the bracket. The bracket is secured with bolts along the edges, the crack itself is usually injected with cement-sand mortar and caulked with a rigid mortar.
Installation of brackets made of reinforcing steel: 1-reinforced wall; 2-crack in the wall, injected with cement-sand mortar after installing brackets; 3-brackets made of reinforcing steel; 4-groove in the masonry, selected with a milling cutter; 5-recesses at the ends of the groove, made with a drill; 6-filling grooves and recesses with cement-sand mortar
At significant damage masonry a network of cracks staples perform double-sided, in this case the masonry experiences double-sided compression. Development of numerous end-to-end cracks can be stopped by using a staple instead strip steel plates , which are installed in increments of 1.5-2 wall thicknesses.
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Double-sided brackets made of reinforcing steel with bolts: 1- masonry; 2- through crack; 3- plates made of strip steel; 4- coupling bolts; 5 holes in the wall |
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The damage can be so significant that in some cases partial dismantling and re-building of the destroyed brickwork is required. Typically this is done with the device inserts of brick locks equipped with an anchor .
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Wide, more 10 mm, crack ( 1 ) intercepted by a one- or two-sided overlay ( 2) , no longer made of strip steel, but of rolled metal, which is attached to the wall with anchor bolts. In this case, the overlay is called anchor. Along the entire length of the development of the crack, the damaged brick is removed to a thickness of two bricks and replaced with reinforced masonry on cement-sand mortar, called brick castle (3-4
).
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Partial or complete filling of openings with masonry: 1- reinforced partition; 2- window openings; 3- reinforced brickwork of grade M75-100 on mortar M50-75; 4- seam, wedged with a metal plate and caulked with cement-sand mortar |
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Scheme for unloading brick walls: 1 - jumpers, 2 - boards 50-60 mm; 3- racks with a diameter of more than 20 cm; 4 - wooden wedges; 5- temporary fastening of racks |
Increased bearing capacity and stability of piers can be ensured increasing cross-sectional area
, device of various clips
or metal frame.
Increasing cross-sectional area The wall is reached by increasing its width. In this case, new sections of masonry are laid out on both sides of the wall, which is securely tied to the old one, and, if necessary, reinforced. Damaged load-bearing walls are unloaded, the cross-sectional area of the walls increases, and the area decreases accordingly window openings, That's why window units subject to replacement.
When leaning on a reinforced wall truss structure or the wall deviates from the vertical by more than 1/3 of the thickness of the brick, the wall is first unloaded by placing temporary wooden or metal pillars on gypsum mortars.
Main ways reinforcement of brickwork,
are well-tested methods of device clips, build-ups
or shirts,
divided into reinforced concrete
And mortar
. When amplified reinforced concrete frames, jackets And build-ups class B10 concrete and class A1 reinforcement are used, the transverse reinforcement spacing is taken to be no more than 15 cm. The thickness of the holder is determined by calculation and varies from 4
before 12 cm.
Mortar clips, shirts And building up, also called plastering, differ from reinforced concrete because they use grade 75-100 cement mortar, which protects the reinforcement reinforcement.
Construction of a reinforced concrete frame effective in case of surface destruction of the material of piers and pillars to a small depth or when deep cracks occur, when widening of the piers is possible. In the first case, the destroyed sections of the pier are cleared to a depth of at least the thickness of the reinforced concrete casing, and the section of the pier does not change as a result of its construction. In the second case, the cross-section of the pier is increased due to the installation of a reinforced concrete cage.
The technological process of installing a reinforced concrete frame of piers consists of removing window fillings, clearing damaged areas or cutting down the pier to the required depth, removing window quarters, installing reinforcement, installing formwork, concreting, maintaining concrete, removing formwork and dismantling scaffolding. The working reinforcement of a reinforced concrete cage can be pre-stressed by heating up to 100-150° C (for example, heating by electric current).
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Construction of reinforced concrete frames: a-without increasing the cross-section of the pier; b-with magnification sections pier |
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Construction of a pre-stressed plaster frame: 1-reinforced wall; 2-metal plates with holes for cords; 3-strand-connections; 4 holes in the wall for cords; 5-reinforcing bars welded to the plates and tightened in pairs; 6- plaster made of cement-sand mortar; 7-reinforcement mesh tied to bars |
Instead of reinforcing cages when reinforcing, it is possible to use wire mesh with a diameter of 4-6 mm with cell 150x150 mm. In both cases of reinforcement, both meshes and frames are attached to the reinforced surface with pins (anchors).
For large areas, additional tie clamps are installed in steps of no more than 1m at medium length75 cm.
The formwork of the reinforced concrete frame is built up from the bottom up during the concreting process. To construct reinforced concrete frames, the shotcrete method is used, in which formwork is not required. In this case, it is applied under pressure to the reinforced surface of the wall. concrete mixture using a cement gun. The advantage of this method of constructing a reinforced concrete frame is the mechanization of the concreting process. The reinforced concrete cage increases the load-bearing capacity of the element enclosed in it by 2 times
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Reinforced concrete frame clamps: 1- reinforced wall surface; 2- fittings with a diameter of 10 mm; 3- tie clamps with a diameter of 10 mm; 4 - holes in the masonry; 5 - concrete frame; 6- reinforcement cages |
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| Construction of a plaster or reinforced concrete jacket: 1-reinforced wall; 2-armholes; 3-plaster jacket 30-40 mm or reinforced concrete jacket 60-100 mm thick; 4-reinforcement with a diameter of 10 mm; 5-reinforcement with a diameter of 12 mm; 6-metal pins | Construction of a reinforced concrete core: 1-reinforced wall; 2-openings; 3-post (core) made of reinforced concrete;
4-niche cut into the pier; 5-reinforcement frame; 6-concrete |
Mortar shirts and extensions
differ from clips only in one design feature - they are made one-sided. The shirt can be made not over the entire width of the wall - in the form core.
Sometimes steel clips for reinforcing brickwork on constantly used buildings are left without protective coating mortar or concrete, arranging metal carcass
gain.
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| Reinforcement of piers with a metal frame: a- narrow pier; b- wide pier; 1-brick element; 2-steel corners; 3-bar; 4-cross link |
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Construction of overhead belts from corners: 1-reinforced partition; 2-corners of overhead belts; 3-cross bars; 4-pin bolts; 5-plaster with cement-sand mortar on a metal mesh |
The construction of a metal frame of piers is less labor-intensive and material-intensive than the construction of a reinforced concrete frame, and is widely used.
Preparation for the installation of metal frames of piers consists of unloading the piers, removing the filling of window openings and cutting down the quarters. With this method, corner steel racks are installed at the corners of the piers to their full height and tightly adjusted to the piers, which are connected after 30-50 cm in height with strip steel, butt-welded to the corner flanges. Then the wall is covered with wire metal mesh and plastered.
The metal frame can be placed on the wall or embedded flush into it. In the second case, before installing the frame, the corners of the walls are cut off and horizontal grooves are punched in the places where the metal connecting strips are installed.
After installing the frame, the gaps between metal elements and the wall is carefully caulked with mortar. If the lintels resting on the pier are also destroyed, it becomes more effective to strengthen the pier by adding racks from the corners. In this case, the racks are made slightly longer than the distance between the lintel and the floor. At the top they are attached to the exposed reinforcement of the jumpers, and at the bottom to an overhead belt made of channel, mounted on the body of the object being reconstructed. The racks are straightened in pairs with clamps, thus creating pre-stress. Straightening, breaks, cuts in the flanges of the corners are welded.
Gain corners buildings are also advisable to produce using channel overlays length 1.5-3 m. Overlays can be placed on both the outer and inner surfaces of the wall. They are connected to the brickwork using tie bolts installed in pre-drilled holes. The coupling bolts are located along the height of the reinforced part of the masonry through 0.8-1.5 m.
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Connecting the racks from the corners: 1-reinforced partition; 2-openings; 3-racks of unequal corners, curved to the side; 4-break lines; 5-fold part; 6-exposed reinforcement; 7-welding; 8-solution |
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If local deformations occur and to prevent further crack opening, it is carried out by strengthening interface zones longitudinal and transverse walls of the building unloading beams . Unloading beams are installed in previously punched grooves on one or both sides of the wall at the level of the top of the foundation or lintels of the first floor.
Double-sided beams through 2-2.5 m connected by bolts with diameter l6-20 mm, passed through previously drilled holes in the beams and wall. One-sided beams are installed on anchor bolts, the smooth ends of which are secured in the wall by installing them with cement mortar into previously drilled sockets. Beam connections with bolts are secured with nuts. Step anchor bolts 2-2.5 m.
The gaps between the beam flanges and the brickwork are carefully caulked cement mortar composition 1:3. For the manufacture of unloading beams, a channel or I-beam No. 20-27 is used. In places where walls break, cracks are installed on each floor using clamps made from scraps of rolled stock with a length of at least 2 m. Before installing the bracket-screed, a groove is cut out in the wall so that the screed is installed flush with the surface brick wall. Holes for bolts are drilled in the wall and in the screed according to the markings. 20- 22 mm, with the help of which the bracket-screed is attached to the wall. The distance from the crack to the bolt installation site must be at least 70 cm. Before installation, the tie-brace is wrapped with wire mesh or wire. 1-2 mm. After installing the structure, the crack and the fine are carefully sealed with a brand solution M100.
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Installation of metal plates (frame) when reinforcing a building: 1-deformed building; 2-cracks in the walls of the building; 3-linings made of channels or metal plates; 5-pin bolts; 6-fine for installing plates, sealed with mortar; 7-holes in the walls for bolts, after installing the bolts they are caulked with mortar |
Typically, development cracks related to uneven settlement of foundations, requires additional measures not only to increase the load-bearing capacity of the masonry, but the rigidity of the entire structure as a whole. Gross violations of masonry technology, unacceptable operating conditions of the structure, as in the case of uneven settlement of foundations, cause not only the development of cracks in window and doorways, but also violations of the verticality of enclosing structures.
In places tearing off external walls from internal ones to restore the rigidity of the building, connections are made from metal frames or reinforced concrete dowels. In this case they say that the building reinforced.
However, most often, after eliminating the causes of uneven settlement of the foundation, the building needs tightening the body generally. Perhaps the only way to do this is to creation of tension belts .
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Construction of external stressed belts: 1-deformed building; 2-steel rods; 3-roll profile from angle No. 150; 4 turnbuckles; 5-weld seam; 6- cracks in the walls of the building; 7-hole in the wall filled with cement-sand mortar
It should be emphasized here that the most common mistake in strengthening the body brick buildings with a hard design diagram is the creation vertical stiffening discs(laying in or reducing the area of window openings, installing vertical metal frames, etc.), while the most important thing here is horizontal stiffening disk. The tension belt, also called "bandage", is made from reinforcing bars with a diameter 20-40 mm connected by turnbuckles.
In rare cases, rolled steel is used instead of reinforcement. The result is a reinforcing element that absorbs both tensile and compressive forces, called spacer connection. Spacer ties are installed at the roof level and at the level of interfloor ceilings; they can be located both on the outside and on the inside of the structure.
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| Construction of internal stress zones: 1-deformation building; 2-steel rods with nuts; 3-metal plates; 4 turnbuckles; 5-holes in the walls, which are sealed with mortar after packing the strands; 6-cracks in the walls of the building | |
Strengthening interfloor ceilings residential buildings of series 1-447 is determined by the presence of short cracks and fragmentation of brick stone in the places where the floor slabs support. The main cause of destruction is usually an insufficient support area for the floor slab or the absence of a distribution pad.
Most effective technique amplification is a mounting technology steel rods And spacer ties under the floor slab, since, as already noted, the creation of a horizontal rigidity disk in buildings of this type is of paramount importance. However, this is a very expensive and time-consuming method; it is only possible with complete reconstruction with resettlement of residents. Therefore, they try to fulfill local strengthening damaged structures.
Local reinforcement, depending on the type of floor slabs, during partial or phased reconstruction is carried out by:
—increasing the support area of the beam using metal or reinforced concrete racks, the force from which is transmitted outside the destruction zone;
-increasing the support area of the slab by means of a belt fixed in the zone of destruction of the masonry;
- devices under the end of the floor slabs of reinforced concrete pads.
Calculation of brick elements reinforced with reinforcement and clips
Longitudinal reinforcement , designed to absorb tensile forces in eccentrically compressed elements (at large eccentricities), in bending and tensile elements, in reinforcing brickwork during reconstruction, is quite rare, so it is not considered in this section. However, with the growth seismic danger of some areas of central Russia due to underground workings and other anthropogenic factors, as well as when laying railways and highways near residential areas, longitudinal reinforcement is used when lining thin (up to 51 cm) brick walls of reconstructed buildings.
Mesh reinforcement masonry sections significantly increases the load-bearing capacity of reinforced elements of stone structures (pillars, piers and individual sections of walls). The effectiveness of mesh reinforcement during reinforcement is determined by the fact that reinforcing meshes laid in the horizontal seams of masonry sections prevent its transverse expansion during longitudinal deformations caused by acting loads, and thereby increase the load-bearing capacity of the masonry body as a whole.
Mesh reinforcement is used to strengthen masonry made of bricks of all types, as well as ceramic stones with slot-like vertical voids with a row height of no more than 150 mm. Reinforcement of concrete and masonry with mesh reinforcement natural stones with a row height of more than 150 mm little effective.
For masonry with mesh reinforcement, mortars of grade 50 and higher are used. Mesh reinforcement is used only for flexibility or, as well as for eccentricities located within the core of the section (for rectangular sections e 0<0,33 y). При больших значениях гибкостей и эксцентрицитетов сетчатое армирование не повышает прочности кладки.
For example, it is required to determine the cross-section of longitudinal reinforcement for a brick pillar 51 x 64 cm, height 4.5 m. The pillar is made of ordinary clay bricks of plastic pressing brand 100
on brand solution 50
. In the middle section of the column, the reduced calculated longitudinal force acts N p=25 t, applied with eccentricity e o =
25 cm towards the side of the section having size 64 cm.
We reinforce the column with longitudinal reinforcement located in the tension zone outside the masonry. We reinforce the compressed zone of the cross section of the column structurally, since when the reinforcement is located externally, frequent installation of clamps will be required to prevent buckling of the compressed reinforcement, which will require additional waste of steel. The installation of structural reinforcement in the compressed area is mandatory, as it is necessary for attaching the clamps.
Cross-sectional area of the pillar F=51 x 64 = 3260 cm 2. R=l5 kgf/cm 2(at F > 0.3 m 2). Design resistance of longitudinal reinforcement made of steel class A-1R a =l900 kgf/cm2.
We take stretched reinforcement from four rods with a diameter of 10 mm F a =3.14 cm 2.
Determine the height of the compressed section zone X at h 0 =65 cm, e=58 mass media b=51 cm:
1.25-15-51 x (58-65+)-1900 -3.14-58 = 0,
and from the resulting quadratic equation we determine x= 35 cm<
0.55h o =36 cm.
Since the condition is satisfied, the load-bearing capacity of the section is determined by at =1000:
pr = = =7
hence = 0.94.
Section bearing capacity
0.94(1.25 x 15 x 51 x 35-1900 x 3.14) =25.6 t >N p =25 t.
Thus, with the adopted reinforcement cross-section, the bearing capacity of the column is sufficient.
Complex structures are made of masonry reinforced with reinforced concrete working together with the masonry. It is recommended to place reinforced concrete with outside masonry , which allows you to check the quality of the laid concrete, the grade of which should be taken to be 100-150.
Complex structures are used in the same cases as masonry with longitudinal reinforcement. In addition, it is advisable to use them, just like mesh reinforcement, to strengthen heavily loaded elements under axial or eccentric compression with small eccentricities. The use of complex structures in this case makes it possible to sharply reduce the cross-sectional dimensions of walls and pillars.
Elements reinforced with clips are used to strengthen pillars and piers that have a square or rectangular cross section with an aspect ratio of no more than 2.5. The need for such reinforcement arises, for example, when adding existing buildings. Sometimes it is necessary to strengthen masonry that has cracks or other defects (insufficient strength of the materials used, poor quality of masonry, physical wear, etc.)
Clips, as well as mesh reinforcement, reduce transverse deformations of masonry and thereby increase its load-bearing capacity. In addition, the clip itself also absorbs part of the load.
In the previous sections, three types of clips were considered: steel, reinforced concrete and reinforced plaster .
Calculation of elements made of brickwork, reinforced with clips, under central and eccentric compression at small eccentricities (not beyond the core of the section) is carried out according to the formulas:
with steel frame
N n [(m to R + ) F+R a F a ];
with reinforced concrete frame
N n [(m to R + ) F+m b R pr F b +R a F a ];
with reinforced plaster casing
N (m R + ) F.
The values of the coefficients are accepted:
at central compression=1 and =1;
with eccentric compression (by analogy with eccentrically compressed elements with mesh reinforcement)
1 — , where
N p - reduced longitudinal force; F- masonry cross-sectional area;
F a- cross-sectional area of the longitudinal corners of the steel cage installed on the mortar, or the longitudinal reinforcement of the reinforced concrete cage;
f b - cross-sectional area of the concrete cage enclosed between the clamps and the masonry (without taking into account the protective layer);
R a - design resistance of the transverse or longitudinal reinforcement of the cage;
- buckling coefficient, when determining the value A accepted as for unreinforced masonry;
t to - masonry operating conditions coefficient; for masonry without damage t to=1; for masonry with cracks t to =0,7;
t b - concrete operating conditions coefficient; when transferring the load to the holder from both sides (bottom and top) t b
=1; when transferring the load to the holder from one side (bottom or top) t b=0.7; without direct load transfer to the holder t b =0,35.
- percentage of reinforcement determined by the formula
x 100,
Where f x- cross-section of the clamp or crossbar;
h And b- dimensions of the sides of the reinforced element;
s- distance between the axes of the transverse bars with steel cages ( hs b, but no more than 50 cm.) or between clamps for reinforced concrete and reinforced plaster clamps (s15 cm).
For example, in the middle section of the pier measuring 51x90 cm, located on the ground floor of the building, after completion of construction of the superstructure, the calculated longitudinal force will apply N n =60 t applied with eccentricity e O = 5 cm, directed towards the inner edge of the wall. The partition is made of sand-lime brick, grade 125, with mortar, grade 25. The height of the wall (from the floor level to the bottom of the precast reinforced concrete floor) is 5 m. It is necessary to check the load-bearing capacity of the wall.
Pier section F= 51 x 90 = 4590 cm 2 > 0.3 m 2.
Design resistance of masonry R = l4 kgf/cm 2. Distance from the center of gravity of the section to its edge in the direction of eccentricity
y = = 25.5 cm; = =0.2<0,33,
the eccentricity is within the core of the section. We design the wall for eccentric compression with low eccentricity. The elastic characteristics of sand-lime brick masonry on mortar grade 25 is = 750.
Reduced flexibility of the wall np == 11.3.
Buckling coefficient = 0.85.
Coefficient taking into account the influence of eccentricity = 0.83.
Let us determine the load-bearing capacity of the pier:
0.85 x 14 x 4590 x 0.83 = 45,200kgf
Since the load-bearing capacity of the wall turned out to be insufficient, we reinforce it with a frame made of steel isosceles angles measuring 60x60 mm, d=6 mm. The corners are installed on the mortar in the corners of the wall and connected to each other by strips of strip steel with a section of 5x35 mm, welded to the corners at a distance s=50 cm along the height of the wall.
Next, we determine the load-bearing capacity enhanced pier. Coefficient of masonry operating conditions t k =1. Design resistance of steel strips R a =1500 kgf/cm2. Sectional area of the plank f x= 0.5x3.5= 1.75 cm 2. Design resistance of the corners of the cage (the load is not transferred to the corners) R a =430 kgf/cm 2. Sectional area of corners F a=6.91x4=27.6 cm 2. Next, we determine the coefficients and , =0,83, =1-=0,61 and the corresponding percentage of reinforcement: =x100=0.21%
Hence the load-bearing capacity of the reinforced wall will be:
0.83.0.85[(14 +0.61хх)4590+430 x27.6]=63800kgf > N p =60000 kgf
The load-bearing capacity of the reinforced wall is sufficient. 
