(A) Analysis of wall cracks caused by uneven settlement of foundation
All the loads of the house are finally transferred to the foundation through the foundation, and the stress of the foundation spreads with the depth under the load. The greater the depth, the greater the diffusion and the smaller the stress. At the same depth, it is always the largest in the middle and gradually decreases at both ends. It is precisely because of the diffusion of soil stress that even if the foundation stratum is very uniform, the stress distribution of the building foundation is still uneven, which leads to the uneven settlement of the building foundation, that is, there is more settlement in the middle of the building and less settlement at both ends, forming a slightly concave basin-shaped surface settlement distribution. When the geology is good and uniform, and the ratio of length to height of the building is small, the difference of uneven settlement of the building foundation is relatively small, which generally will not have much impact on the safe use of the building. However, when the house is built on muddy soil or soft plastic cohesive soil, the absolute settlement and relatively uneven settlement of the house may be relatively large because of the low strength and high compressibility of the soil. If the length and height of the building are relatively large, the overall stiffness is poor, and the foundation is not strengthened, then serious cracks may appear in the wall. Cracks appear symmetrically at both ends of the longitudinal wall, inclined to the direction of large settlement, about 45% along the door and window openings. It is a regular figure of eight, with small cracks in the upper part of the house and large cracks in the lower part. This kind of crack must be caused by uneven settlement of foundation caused by additional stress of foundation.
When the soil layer of building foundation is unevenly distributed and the soil quality is quite different, obvious uneven settlement often occurs at the junction of different soil layers or at different thicknesses of the same soil layer, which leads to cracks in the wall, and the cracks are large and small, and tilt in the direction of soft soil or thick soil layer.
In the case of large height difference or load difference of buildings, when there is no settlement joint, large uneven settlement cracks are easy to occur at the junction of high and low weights. At this time, the crack is located in the part with less layers and light load, and inclines upward to the part with more layers and heavy load.
When the compressibility of the soil at both ends of the house is large and the middle part is small, the settlement distribution curve will be convex. At this time, in addition to the cracks inclined outward at both ends of the longitudinal wall, vertical cracks often appear at the top of the longitudinal wall.
In a multi-storey house, when the bottom windowsill is too wide, the uneven settlement of the foundation is often caused by the concentrated transfer of wall load between windows, which leads to the reverse bending of the windowsill under the action of foundation reaction, resulting in vertical cracks in the middle of the windowsill.
In addition, if the foundation of the new building is located below the original building, the ratio of height difference H to clear distance L at the bottom of the new and old foundation should be less than 0.5~ 1. Otherwise, the foundation settlement caused by the new house load will cause cracks in the original house and wall. Similarly, in the construction of adjacent high-rise and low-rise buildings, the construction should be organized according to the principle of high before heavy, low before light; Otherwise, if the low-rise building is built first and then the high-rise building, it will also cause cracks in the low-rise wall.
From the above analysis, we can see that the distribution of cracks is closely related to the height-width ratio of the wall. Houses with large ratio of length to height have poor rigidity and poor deformation resistance, and are prone to cracks. Because the length-height ratio of the longitudinal wall is greater than that of the transverse wall, most cracks occur on the longitudinal wall. The distribution of cracks is closely related to the distribution curve of foundation settlement. When the settlement distribution curve is concave, cracks mostly occur in the lower part of the house, and the width of cracks is large or small. When the settlement distribution curve is convex, cracks often appear in the upper part of the house, and the width of cracks is large or small. The distribution of cracks is closely related to the mechanical properties of the wall. Due to stress concentration, cracks often occur at doors and windows, plane turning points and height changes. Because the wall was destroyed by shear, its principal tensile stress was 45. So the crack is also inclined at 45 degrees.
In order to prevent wall cracking caused by uneven settlement of foundation, soft soil foundation and uneven foundation should be treated first, but when drawing up foundation reinforcement treatment scheme, foundation treatment and superstructure treatment should be combined to make them work together. We can't simply start with foundation treatment, otherwise it will not only cost a lot; And the effect is also poor. In the treatment of superstructure, there are: changing the shape of the building; Simplify the building plane; Reasonable settlement joint; Strengthen the overall stiffness of the house (such as increasing transverse walls, increasing ring beams, adopting mat foundation and box foundation, etc.). ); Adopt light structure, flexible structure and so on.
(2) Analysis of wall cracks caused by temperature stress.
General materials have the property of thermal expansion and cold contraction, and the deformation of building structure due to the change of surrounding temperature is called temperature deformation. If the structure is free from any constraints and can deform freely when the temperature changes, then no additional stress will be generated in the structure. If the structure is constrained and cannot deform freely, additional stress or temperature stress will be generated in the structure. Structural expansion value caused by temperature stress.
Wall cracks caused by temperature stress generally have the following situations:
1. splay crack
As shown in Figure 4-6, when the outside temperature rises, the external wall itself will elongate along the length direction, but the elongation value of the roof part (especially the reinforced concrete roof directly exposed to the atmosphere) is much larger. From the cutting of the joint between the roof and the wall, it can be seen that the extension of the roof will produce additional horizontal thrust on the wall, so that the wall will be pushed by the roof to produce shear stress, and shear stress and tensile stress will also cause principal tensile stress. When the principal tensile stress is too large, it will produce splayed cracks on the wall. Because the distribution of shear stress is generally zero in the middle and the maximum at both ends, splayed cracks mostly occur at both ends of the wall, generally occupying two or three bays and occurring on the top wall.
2. Horizontal cracks and angular cracks
In flat-topped houses, sometimes there are longitudinal horizontal cracks and corner cracks along the top of the external wall at the bottom of the roof panel or near the top ring beam, which are caused by the outward or inward pulling force caused by the elongation or shortening of the roof. Corner crack is actually a form of horizontal crack, which is connected by horizontal crack of external transverse wall and horizontal crack of vertical wall. In this case, there are generally no open cracks below. Sometimes, horizontal cracks in the outer longitudinal wall will also appear at the top window sill.
3, parapet root and vertical cracks
Due to the extension or shortening of the roof, the parapet root is pushed or pulled outward or inward, and the parapet outside the parapet root masonry is inclined outward, forming horizontal cracks. Sometimes, due to the shrinkage of reinforced concrete roof, the parapet may be in an eccentric compression state, which leads to the vertical cracking of the upper part of the parapet.
In addition, local vertical cracks are easy to appear in the walls on both sides of the stairwell or at staggered floors, which is caused by the large tension caused by floor shrinkage.
There are many and complicated reasons that affect the expansion and contraction cracks of houses. These are just some common situations. In order to reduce the influence of temperature stress, reasonable expansion joints can be adopted; Avoid dislocation of floor slab and expansion joint; Strengthen roof insulation; Use linoleum and talcum powder or iron sheet to isolate the roof panel from the wall, and leave a certain gap at the root of the parapet, so that it can expand freely and have expansion space; Planting roofs in the water storage roof area; The daughter has a constructional column on the wall; Technical measures such as strengthening the weak links of the structure and improving its tensile strength.