The analysis of quality problems in construction engineering is the premise of correctly formulating the treatment scheme of quality accidents and the basis of clarifying the responsibility of quality accidents. Therefore, the analysis of quality problems requires comprehensive, accurate and objective; The nature, harm, cause and responsibility of the accident cannot be omitted. There must be scientific argumentation and judgment; Very reasonable: only when the theory is well founded can we achieve the goal of unified understanding.
First, the wall crack analysis
Wall cracks are common quality problems in mixed structures, which are caused by uneven settlement of foundation, temperature stress, earthquake force, expansion force, frost heaving force, load and construction quality. The characteristics of wall cracks caused by uneven settlement of foundation and temperature stress are analyzed as follows:
(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. There are regular patterns along the openings of doors and windows, 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 of the bottom surface of the new and old foundation should be less than 0.5~ 1. Otherwise, the foundation will settle due to the load of the new building, resulting in cracks in the original building 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 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 just 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.
Second, the collapse analysis of cantilever structure
There are many examples of cantilever structure collapse, one is the whole overturning collapse; Second, it collapses along the roots of cantilever beams and plates. The main reasons are:
1. The stability moment is less than the overturning moment.
The stability of cantilever structure is maintained by weight or external tension, and the overturning safety factor is required to be not less than 1.5. If the stability moment is less than the overturning moment, it will inevitably lose stability, overturn and collapse. Such as awning and cantilever beam, when the weight (brick height) on the beam can not meet the stability requirements, the support and formwork will be removed, leading to collapse accidents.
2. Improper formwork support scheme
The stress at the root of cantilever structure is the largest. After pouring concrete, the strength is not enough, the formwork support will settle, and the concrete at the root will crack immediately. After the formwork is removed, it will break and collapse from the root. If the cantilever structure is variable cross-section, the formwork will be made into equal cross-section shape during construction, which will reduce the cross-section of the root and cause collapse accident after formwork removal.
3. Dislocation and deformation of steel bars
The negative bending moment at the root of cantilever structure is the largest, and the main reinforcement should be arranged on the upper part of beam slab. If the reinforcement is placed at the lower part during construction, or the downward deformation is too large when it is trampled, or the anchorage length is not enough, the root will collapse after formwork removal.
4. Building overload
The bending moment at the fixed end of cantilever structure is proportional to the applied load. If the construction load exceeds the design load, cracks will appear at the root of the formwork when it sinks. Especially when pouring concrete from the root outward, with the increase of load; Template deformation, but also easy to produce cracks in the root, leading to fracture after ripping.
5. Early form removal
Many collapse accidents of cantilever structures are caused by premature formwork removal and insufficient concrete strength. Therefore, the specification stipulates that the concrete formwork removal strength of cantilever beam slab with span less than 2m should be greater than or equal to 70%; For a cantilever beam slab with a span of more than 2m, the concrete formwork removal strength is 100%.
Three, reinforced concrete column hoisting fracture accident analysis
(1) accident overview
The C-pillar of an engineering project is a column with equal section and the length is l2m；; The cross section is 40 omm * 6 omm, with symmetrical reinforcement, with 4 industries per side 16 and 2 industries per kloc-0/2 for structural reinforcement; The strength grade of concrete is C20, and it has reached 100% strength during hoisting. The column is prefabricated horizontally and hoisted at one point; The lifting point is 2m away from the top of the column; When just lifted off the ground, cracks appeared between the lifting point about 4.8m away from the column foot and the column foot. The cracks ran through both sides along the bottom surface, and the maximum width was 1.3mm, which led to the fracture of the column.
(2) Cause analysis of the accident
The main reasons for this accident are: when the column is prefabricated and hoisted horizontally, the stress on the lifting point is inconsistent with that when it is used; The choice of lifting point is unreasonable, the lifting torque is too large, and its bending strength and crack resistance can not meet the requirements. The analysis and calculation are as follows:
The selection of 1. lifting point does not conform to the principle of minimum lifting torque MDm.
The lifting moment of the column is closely related to the position of the lifting point, so it is damaged. The principle of selecting lifting point is that the lifting bending distance must be minimum. Therefore, when lifting a column with equal cross section, the absolute value of |Mmx|=| MD|, that is, the maximum positive bending distance in the span should be equal to the negative bending distance of the lifting point. Accordingly, the lifting point position is 0.293L (L from the top of the column (L is the length of the column). When l is 12m, the distance between the lifting point and the top of the column should be 0.293x 12 = 3.5m, and the original lifting point is 2m away from the top of the column, which does not conform to the principle of minimum lifting point moment. When lifting, the absolute value of the maximum bending moment of mid-span must be greater than the absolute value of the negative bending moment at the lifting point, so the crack occurs at the section of the maximum positive bending moment of mid-span.
2. The bending strength of the column during hoisting is not enough.
Now, according to the minimum lifting bending moment, if precast horizontally, the bending strength of the column can not meet the requirements. The inspection results are as follows:
(1) The calculated load g is that the gravity density of reinforced concrete is 25,000 N/m and the dead weight is 0.4× 0.6× 25,000 = 6,000 N/m; The dynamic load coefficient is 1.3~ 1.5. If 1.5 is taken, the calculated load is q =1.5× 6000 = 9000 n/m. 。
(2) Calculation diagram
According to the principle of minimum bending during hoisting, the hoisting point is 3.5m away from the top of the column, and the column foot does not leave the ground during hoisting, and the column just hangs off the ground, similar to the cantilever simply supported beam.
3. The crack resistance of the column is not enough when it is hoisted.
According to the code for construction acceptance, the crack width in the tensile area of reinforced concrete members during hoisting is not more than 0.2~0.3mm, and the crack width is related to the tensile stress of steel bars. The greater the tensile stress of steel bars, the greater the crack width. Therefore, the tensile stress of steel bars is often used to control the crack width in column hoisting. As long as the tensile stress of steel bar meets the requirements of the following formula, it shows that the crack width is within the allowable range and can meet the requirements of crack resistance. It shows that the crack resistance can not meet the requirements.
(3) Lessons learned from the above accidents should be as follows:
(1) Because the lifting force of the column is different from the service force, the lifting calculation must be carried out.
(2) When the lifting force is different from the use force, the selection of lifting points should conform to the principle of minimum lifting torque to avoid damage due to excessive lifting torque. For example, in this case, according to the principle of minimum lifting point moment, when the lifting point is 4.8m away from the top of the column, the absolute value of the positive bending moment across the span is equal to the absolute value of the negative bending moment at the lifting point, both of which are 55. 125XlO. However, when the original lifting point is 2m away from the top of the column, the maximum mid-span bending moment is 103.68x 1o.n.mm, which shows that the mid-span bending moment of the original lifting point is 1.88 times larger than that determined according to the principle of minimum lifting point bending moment. There is a big crack in the column 4.8m away from the column foot, which leads to fracture, which also proves that the lifting moment of this section is the largest.
(3) When the lifting force is consistent with the use force, the selection of lifting points should meet the requirements of the use force as much as possible, such as the two lifting points of a simply supported beam should be close to both ends of the beam; The two lifting points of the cantilever beam should be on the two fulcrums of the beam.
(4) When the bending strength and crack resistance can't be satisfied after hoisting calculation, turn over and hoist first. For example, if a body crane is used in this example, bending strength and crack resistance can be satisfied. If the passing crane is still not satisfied, the lifting point can be increased from one point to two points to reduce the lifting point torque or take temporary measures.
In addition, in order to facilitate positioning and centering, and ensure the safety of hoisting, the center line of the hook should be aligned with the center of gravity of the component when binding; When horizontal members are hoisted and bound at two points, two lifting ropes should be used respectively; For equivalent section members, it is also required that the two lifting points are symmetrical left and right, and the two lifting ropes have the same length; The horizontal included angle of the lifting rope should be greater than or equal to 60. Should not be less than 45. ; It is forbidden to drive with oblique crane and hanging objects.
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