The accident and its causes are analyzed as follows:
After the roof partially collapsed, the design was reviewed and no problems were found. During the construction review, the following problems were found:
1) The deep beam is designed as C20 concrete, and no test block is left during construction. Afterwards, it was found that its strength grade was only about C7.5 At the fracture of the beam, it was obvious that the sand was not washed clean, and the aggregate was mixed with impurities such as clods, lime particles and pigeon eggs leaves.
2) The cement used in concrete is 400 ordinary portland cement produced locally, but it only reaches 350 after inspection, so the strength of concrete will be affected to some extent when it is prepared as 400 cement during construction.
3) It is found that the deep beam is broken on one side, and there is almost no reinforcement in the tensile width 1/3. This arrangement of main reinforcement makes the beam in bending, shearing and torsion state under the roof load, thus making the supporting part of the beam produce torque.
4) Wall inspection found no quality problems.
Based on the above construction problems, it can be considered that the fracture of deep beam is mainly due to the large shear stress caused by torque and shear force, and the low concrete strength of beam leads to the shear failure of beam. Among them, concrete aggregate contains excessive clods and other harmful impurities, which is the main reason for the low strength of concrete.
2, concrete freezing or curing temperature is too low accident.
A project is a three-story brick-concrete structure, with cast-in-place reinforced concrete floor, longitudinal wall bearing and lime-soil foundation (below). After construction, the second layer of concrete was poured in June 5438+that year 10. The main structure was completed in June 5438+the following year 10. During the renovation project in April, it was found that there were oblique cracks in the girders of each floor.
Its phenomenon:
1) cracks are mostly oblique, with an inclination of 50 ~ 60, and most of them occur within the spacing of 300mm steel hoops. There is a vertical crack near the middle of the beam.
2) Oblique cracks are dense at both ends and rare in the middle (it is worth noting that there are oblique cracks at the longitudinal reinforcement truncation); Its position along the beam height direction is mostly below the neutralization axis, and some of it passes through the beam height.
3) The crack width is about 0.5 ~ 1.2 mm near the beam end and about 0.1~ 0.5 mm near the span; The crack depth is generally less than 1/3, and individual ends penetrate; The number of cracks in each beam varies from 4 to 22, generally from 10 to 15.
The accident and its causes are analyzed as follows:
1) Construction reason: No special maintenance measures were taken when watering the beam and slab on the second floor, and scaffolding and bricks were built on the slab 2 hours after watering. 165438+1When the three-layer cast-in-place slab was poured in early October, the indoor temperature was 0 ~ 1℃, and no heat preservation measures were taken. According to the test data, the strength of concrete after 2 1d is only 42.5% of the theoretical strength of 28d, and only 52% after one month. Therefore, the early freezing of concrete is an important reason for this quality accident. In addition, the low cement content of concrete (only 2 10kg/m3, slightly less than the minimum value of 225kg/m3) is also one of the factors.
2) Design reasons: First, the stirrup spacing is too large. Article 7.2.7 of the Code for Design of Concrete Structures stipulates that "when the beam height is 500mm and v < 0.07fcbh0, the maximum spacing of stirrups in the beam is 200mm." However, the stirrup spacing in this project is 300mm, which is also the reason why inclined cracks often appear between stirrups. The second is that the longitudinal reinforcement in the middle of the beam span is cut off. Article 6. 1.5 of Code for Design of Concrete Structures stipulates that "the longitudinal tensile reinforcement shall not be cut off in the tensile zone". However, some longitudinal tensile steel bars in the beam of this project are truncated in the middle of the span, and oblique cracks appear in the truncated parts, which shows that the tensile steel bars have a certain effect on the shear bearing capacity of the beam section, and also shows that the provisions of the code are the most appropriate.
3) Comparing the reasons of construction and design, it is obvious that the early freezing of concrete during construction is the main cause of the quality accident of this project.
Accident reinforcement scheme:
Because there are a lot of oblique cracks on the beam, it is easy to break the brittle section and lead to the fracture of the beam, so it must be strengthened. The strengthening scheme is to wrap a U-section beam outside the original beam, which is designed to bear all the bending moment and shear force of the original beam, and set reinforced concrete columns and foundations along the wall at the end of the U-section beam as the support of the strengthened beam.
3, concrete early shrinkage accident cases
An office building is a cast-in-place reinforced concrete frame structure. When the concrete reaches the predetermined strength and the floor formwork is removed, it is found that there are many irregular microcracks on the floor, as shown in Figure 2. 16. The crack width is 0.05 ~ 0. 15 mm, which sometimes runs from top to bottom, but its overall feature is that there are more cracks on the plate than under the plate.
Accident cause analysis and treatment measures:
1) It is found that the meteorological conditions during the construction period are: 9: 00 am, temperature 13℃, wind speed of 7m/s and relative humidity of 40%; At noon, the temperature is 15℃, the wind speed is 13m/s (maximum instantaneous wind speed 18m/s) and the relative humidity is 29%. At 5 pm, the temperature 1 1℃, the wind speed 1 1m/s, and the relative humidity was 39%. Concrete pouring is carried out under this very dry condition. Due to unusually dry and strong wind, cracks appeared in concrete shortly after solidification. According to relevant data, when the wind speed is 16m/s, the evaporation rate of concrete is four times that of no wind. When the relative humidity is 10%, the evaporation rate of concrete is more than 9 times that of 90%. According to these parameters, the evaporation rate of this project under the above meteorological conditions can reach 8 ~ 10 times of usual.
2) Therefore, it can be considered that dry air and strong wind are the main reasons for more water-loss shrinkage cracks in floors that are in contact with the atmosphere, while there are fewer water-loss shrinkage cracks in floors that have been protected by formwork. After testing, the cast floor is reserved block, and the bearing capacity of the floor is tested, which can meet the design requirements.
3) This shows that the shrinkage of concrete in water at the initial crack has no effect on the bearing capacity of the floor. However, for the durability of buildings, resin injection method should also be used for reinforcement.
4, concrete pits Angle honeycomb exposed.
And empty accident cases.
The reinforcement of the platform and column of the theater is shown in Figure 2. 19(a) and (b). 14 reinforced concrete column, 13 has serious honeycomb phenomenon. The details are as follows: the total side area of the column is 142m2, and the honeycomb area is 7.4 1 m2, accounting for 5.2%; Among them, K4 is the most serious, and the exposed reinforcement area in the honeycomb alone is 0.56m2.. The exposed rebar position is above the ground 1m, which is the overlapping part of rebar (Figure 2. 19c).
Accident cause analysis:
1) Concrete pouring height is too high. For columns over 7m in height, there are no holes for pouring concrete in the formwork. When pouring concrete, there are no facilities such as stringing and storage, which violates the provisions of "the free pouring height of concrete shall not exceed 2m" and "the pouring height of columns shall not exceed 3.0m" in the construction acceptance specification, resulting in segregation of concrete during pouring.
2) The thickness of pouring concrete is too thick, and the requirements for tamping are lax. During construction, a vibrating rod was not used, but a 6-meter-long wooden pole was used for tamping, and the thickness of each pouring was wrongly specified to bear the load of concrete (about 40 cm thick), and it could be rammed for 30 times after pouring. This regulation violates the boundary of "the pouring thickness of columns shall not exceed 20cm" in the construction acceptance specification.
3) The design clear distance of lap joint of column reinforcement is too small, only 3 1 ~ 37.5 mm, which is less than the requirement that the clear distance of column longitudinal reinforcement should be ≥50mm in the design specification. In fact, the clear distance of some exposed tendons is 0 or10 mm.
Accident handling plan:
Remove all loose concrete around the honeycomb; Plug wet sacks on the chiseled surface, and the concrete will be soaked to a thickness of at least 40 ~ 50 mm after 24 hours; According to the honeycomb size, support a template with a bell mouth, as shown in Figure 2. 19 (e); Pouring C30 (old concrete C20) pisolite concrete containing early strength agent; Maintenance 14 day and night; Chisel off the concrete at the bell mouth after formwork removal. In addition to the above reinforcement measures, ultrasonic flaw detection should be carried out to find out whether there are hidden dangers.
5. Accident cases of improper handling of concrete construction joints
In the lobby of a conference room, the roof panel is precast floor, and the main beam, ring beam and rain cover are cast-in-place C20 reinforced concrete members (Figure 2.27). During construction, beam concrete is poured first, and then ring beam and rain cover concrete are poured. For some reason, there are improper construction joints at the joint of beam end and ring beam (Figure 2.27a), and the concrete at the construction joints is not properly handled. Because there is no lateral constraint on the concrete there, it cannot be vibrated, which actually forms scattered piles.
Accident cause analysis:
1) Construction joints shall be left at the position with the maximum shear force at the beam end;
2) The concrete strength grade at the construction joint obviously does not meet the design requirements, even lower than C 10, which seriously affects the shear bearing capacity and bond strength of the beam end;
3) The old and new concrete cannot be connected.
4) Carefully chisel the concrete at the beam end into the shape shown in Figure 2.27B, and partially prefabricate the floor to strengthen the shear resistance of the beam end.
6, concrete corrosion accident cases
A hotel area in Beijing is a six-story two-span continuous beam with cast-in-place reinforced concrete inner frame structure and prestressed hollow floor. The first and second floors around the house are 490mm thick bearing brick walls, and the second floor is 370mm thick bearing brick walls. The ground floor of the whole building is 5.0m high, which is used as a restaurant, and the floors above the ground floor are 3.60m high, which is used as a guest room. The middle column at the bottom is circular in cross section, with a diameter of 550mm, equipped with 9 secondary steel bars with a diameter of 22 and 6 @ 200 stirrups, as shown in Figure 2.35. Single-column reinforced concrete stepped foundation with a bottom area of 3.50m× 3.50m; The surrounding load-bearing wall is a brick strip foundation with a bottom width of 1.60m, which is calculated according to the foundation bearing capacity fk= 180Kn/m2 (the bearing layer is cohesive soil), and the design value of the foundation bearing capacity after the correction of the foundation width and depth is considered.
The first floor of the reinforced concrete project of the house is constructed in winter. In order to prevent concrete from freezing, chlorine salt with cement content of 3% is added when pouring concrete.
Two years after the completion of the project, one day, a concrete fragment with a diameter of about 40mm suddenly fell near the top of the A-pillar of the ground floor restaurant. In order to prevent houses from collapsing, restaurants and hotels have to suspend business and check the cause of the accident.
Accident cause analysis:
1) In the design of building structure, the load imposed on the column by the two-span continuous beam is estimated as 50% of the total live load of each span (the other 50% is borne by the load-bearing wall), which is less than 25% of the load transmitted to the column by the theoretically accurate two-span continuous beam.
2) Although both the column foundation and the load-bearing wall foundation are designed according to fk= 180Kn/m2, the calculated settlement of the strip foundation under the load-bearing walls on both sides is estimated to be about 45mm, which is obviously larger than the calculated settlement of the foundation under reinforced concrete columns (estimated to be about 34mm). Although the settlement difference between them is11mm-0.002l = 0.002× 7000 =14mm, this is allowed; However, the load-bearing wall supporting the continuous beam is "soft" (the settlement is relatively large). However, the columns supporting the continuous beam are "hard" (the settlement is relatively small), which leads to the adjustment of floor load to the column direction, so that the actual load borne by the middle column is greater than the design value, while the actual load borne by the load-bearing walls on both sides is less than the design value.
3)( 1) and (2), the actual load of the column will be much greater than the design value.
4) Although the column is designed as a reinforced concrete compression member with a circular section of ￠550, it is equipped with nine secondary longitudinal bars with a diameter of 22, and the reinforcement ratio is 1.44%, which is sufficient from the point of view of section bearing capacity, but the stirrup configuration is unreasonable, which shows that the stirrup section is too thin and the spacing is too large, and there is no additional stirrup.
5) In winter construction of bottom concrete project, chloride antifreeze is used in concrete pouring, which has a salt pollution effect on concrete and a catalytic effect on the corrosion of steel bars in concrete. In fact, from the analysis of the actual situation of the steel bars at the damaged position of the bottom column, the longitudinal bars and stirrups have been corroded. The diameter of stirrups is less than 6, and it is less than 5.2 after corrosion, and the loss rate of cross section is about 25%. Such a thin stirrup is difficult to bear the lateral tension caused by the lateral buckling of nine secondary steel bars with a diameter of 22 on the column end section. In this way, the stirrup is bound to break at its weakest point, and the concrete protective layer will peel off and the concrete fragments will fall off after the break.
7, improper reinforcement configuration accident cases
There is a cast-in-place reinforced concrete awning in the first floor window of a department store, the length of which is 1200mm, as shown in Figure 2.36(a). When the formwork is removed after the concrete reaches the design strength, the quality accident of the canopy root fracture suddenly occurs, as shown in Figure 2.36(b), which is in the form of a door curtain.
Accident analysis:
The reinforced bar is dislocated (only 20mm away from the formwork, as shown in Figure 2.36c). Originally, the stressed steel bars were arranged according to the design, and the steel bar workers left after binding. Before pouring concrete, some "good-hearted people" saw that the awning steel bar floated on the lintel stirrup and the stressed steel bar was placed at the top of the awning (traditionally, the stressed steel bar was always placed at the bottom of the component), so they temporarily changed the stressed steel bar into the lintel stirrup and stuck it on the template. When pouring concrete, the site personnel did not check the position of the reinforced bar, so the above accident happened.
8. Accident cases caused by steel bar position configuration during construction.
The original design section and reinforcement of the frame column of a project are shown in Figure A above. When the column foot is inserted, the two rows of 5 25 are changed to 3 25 by mistake (Figure B). This mistake was discovered only after the concrete pouring of the column foundation was completed.
Accident case handling method:
1) Add 2/25 steel dowel on the short side of the column.
2) In order to ensure the anchorage of new steel dowel, the two short sides are welded with 3-25 transverse reinforcement and 3-25 short sides respectively, and the second step is 500mm higher. When heightening the steps, the original foundation surface should be chiseled, cleaned, sealed and poured to improve the concrete level, and a layer of ￠6@200 steel mesh should be laid on the surface of the new steps.
3) Stirrup was provided at the bottom of the column within 500mm of the original design, and now it is increased to 1000mm.
9, beam root fracture accident
1) The maintenance workshop (ground floor) and dormitory of a county highway section of this project are two-story brick-concrete structures with a construction area of 556m2. See Figure 3-62 for the local plan and section of the roof.
2) See Figure 3-63 for the dimensions and reinforcement of cantilever beam of roof layer, and C 18 for concrete. The roots of 7 cantilever beams were found to be broken during the form removal process.
Accident cause analysis:
1) There is no test data for the actual strength of concrete, and it is found that the density of concrete is very poor and there are many cracks. At that time, the water-cement ratio was not determined by trial mixing.
2) The main stress reinforcement of the cantilever beam is seriously displaced downwards.
3) The cantilever part is longer than the design.
4) The roof is super thick and the weight is increased.
5) The formwork removal time is too early.
Treatment measures:
1) Knock off the remaining cantilever beam roots on the wall by 500mm to expose all the steel bars.
2) Saw off the main reinforcement of cantilever beam at 100mm in the wall, and weld the new main reinforcement again.
3) Modify the design, and change the cantilever structure to full cast-in-place.
10, reinforcement error accident
A teaching building in Shanxi is a cast-in-place 10 story frame-shear structure, with a length of 59.4m, a width of 15.6m, a standard height of 3.6m, a ground height of 4 1.8m and a floor area of 95 100 m2. After the completion of the fourth and fifth floors, it is found that the reinforcement of these two columns does not match, and the reinforcement of the inner span column is less.
Accident cause analysis:
In this project, the reinforcement of columns on the 4th and 5th floors is the same, but the reinforcement on the 6th floor is reduced. In the construction, the section of the 6-story column was wrongly used for the 4-story and 5-story, which led to the wrong reinforcement.
Treatment measures:
Reinforcement: The 4th and 5th protective layers are chiseled away, exposing the main reinforcement and all stirrups at the four corners of the column, and reinforced with long steel bars, the diameter and spacing of which are the same as the original design.
1 1, hollow steel bar exposure accident
An office building in Nanjing is a five-story cast-in-place frame, and its plan schematic diagram is shown in Figure 3-90. After pouring two-story frame columns, it is found that six columns have serious defects such as cavities, rotten roots and exposed tendons. Their defects are shown in figure 3-9 1, 92, 93.
Accident cause analysis:
1) column is too thick when pouring.
2) After concrete pouring, leakage or false vibration occurs.
Treatment measures:
Due to the serious phenomenon of void, reinforcement leakage and root rot, the internal quality of concrete can not be guaranteed according to the actual situation on site, so it is decided to dismantle it at one time, bind the reinforcement and re-pour the concrete.
12, beam cracking accident
A project is a mixed structure, and the roof adopts cast-in-place reinforced concrete beam and slab. The beam has a span of 9m, a rectangular cross section, a height of 800mm and a width of 400mm, and the concrete is C 18. The reinforcement situation is as follows: the reinforcement in the beam span is 4: 25, and the reinforcement in the bearing is 2: 18. After pouring, the formwork was removed 14d, and cracks with a width of 0. 1-0.35mm were found on the beam.
Accident cause analysis:
It is stipulated that the strength of beams larger than 8m should reach 100% when the formwork is removed, but in reality it only reaches 80%, and it cracks due to insufficient strength.
Treatment measures:
It is found that there is no obvious crack in the crack, which will not affect the safe use of the structure. Epoxy cement can be used to smear the surface and seal the crack.
13, girder cracking accident
Horizontal fabrication of a workshop 12m reinforced concrete roof beam. After lifting, it was found that 50% of the concrete near the lifting ring was partially crushed, the lifting ring was skewed and the concrete cracked.
Accident cause analysis:
1) upper flange crack: when installing the lifting ring, the stirrup is displaced due to collision and will not be restored to its original state. So there are only two steel hoops for horizontal lifting.
2) Cracking of the main girder web: the lateral stiffness of the web is very small. After the flange cracks, the lateral stiffness of the upper girder is greatly reduced, which leads to the web cracking.
3) Lifting rings deflection: The lifting rings of the two cranes are unevenly stressed, and the residual deformation of the lifting rings with high stress is large, so the lifting rings are skewed.
Treatment measures:
For the oblique cracks at the flange, the concrete within the scope of the oblique cracks shall be chiseled into straight cracks, and then C40 fine stone concrete shall be poured again for maintenance.
14, web beam cracking accident
The span of a forging workshop is 10m, and the roof beam is a thin web beam with double-slope T-section, which is ***4. Its shape, size and reinforcement are shown in Figure 3-42. There is no bent steel bar in the beam, the design strength of concrete is C 18, and the actual strength of test block is 12- 15N/.
Accident cause analysis:
There is no bending steel bar in the original design, and the cross section and quantity of stirrups are insufficient. The measured concrete strength does not meet the design requirements.
Treatment measures:
Because the bearing capacity of thin web beam is insufficient, it must be strengthened. In the reinforcement scheme, reinforced concrete is added to the original thin web beam, and the reinforcement section is shown in Figure 3-43. Increase stirrup to bear the strength of inclined section, and configure longitudinal structural reinforcement.
15, concrete column deflection accident
A cold working workshop in Jiangsu is an assembled reinforced concrete structure with a column spacing of 6m and a span of 18m. The main components are rectangular columns, reinforced concrete roof trusses and large roof panels. When hanging the roof panel, it was found that the 1 column was tilted inward and the top of the column was displaced inward by 50 mm ..
Accident cause analysis:
After the column was hoisted, it was not carefully corrected. When hoisting the roof, it was found that there was dislocation at the joint between the roof and the column, but the reason was not found out in time. The existing tilting phenomenon did not appear until after hoisting.
Treatment measures:
Because the deviation of the column is too large, it must be corrected. There are two rectification schemes: one is to rectify the column after cutting the welded joint between the roof panel and the roof truss; The second is to jack up the roof truss together with the roof panel, and then correct the column.
16, floor cracking accident
A school is a three-story mixed structure with vertical walls bearing loads. The outer wall is 37 cm thick and the inner wall is 24 cm thick. The floor is cast-in-place reinforced concrete ribbed floor. In the decoration project, it is found that cracks generally appear in the upper part of the concrete floor slab on both sides of the girder, and the direction of cracks is parallel to the girder. I dug it and found that the negative reinforcement was trampled.
Accident cause analysis:
1. Building:
1) When pouring concrete, the negative bending moment steel bar in the slab is stepped down, resulting in long cracks near the joint between slab and beam.
2) The concrete dosage per cubic meter is less than 250kg.
3) After the second floor is poured, the specified strength is not reached, and construction tools are piled up on it, resulting in overload.
4) Concrete construction in winter, without any construction measures.
2. Design:
1) floor load calculation error.
2) Beam stirrup spacing is too large.
17, case of frame beam cracking accident
A building adjacent to the street has shops on the first floor and dormitories on the second floor. It is a seven-story cast-in-place frame structure with two longitudinal spans. The floor plan of the seventh floor is shown in Figure 3- 1 1.
When painting indoors, it is found that there are 15 cracks in the top longitudinal frame beams KJ-7 and KJ-8, and the position is 3- 1 1. See Figure 3- 12 for the crack situation.
Accident cause analysis:
1) concrete shrinkage.
2) The additional transverse reinforcement is omitted in the construction drawing.
18, premature form removal leads to collapse.
Accident overview:
A light factory building is a two-story cast-in-place frame structure with precast reinforced concrete floors. After pouring the first reinforced concrete frame and hoisting the first floor, the construction unit will continue to construct the second floor. When hoisting the second floor, in order to speed up the construction progress, the column under the beam of the first floor was removed for decoration at the bottom. As a result, when the lifting of the second floor was almost completed, it collapsed, killing many people on the spot and causing a major accident.
Cause analysis:
After the accident, after investigation and analysis, the main reason for the collapse was that the columns and formwork of the bottom beam were removed too early. When the second-floor precast slab was hoisted, the beam was only cured for three days, and its strength was still very low. Therefore, the weight and construction load of the second-floor frame and precast slab were directly transferred to the first-floor beam by the columns of the second-floor beam, but the strength of the first-floor beam did not fully reach the design strength C20, and only C 12 was determined for the first-floor beam.
19. The main girder was broken due to insufficient anchorage length.
The roof beam of a calcining plant is a T-shaped thin web beam with a span of 12m, which was put into use shortly after the plant was completed. The beam end suddenly broke, resulting in local collapse of the factory building. The collapsed parts include the top beam and the big plate.
Accident cause analysis:
After the accident, the field investigation and analysis show that the concrete strength can meet the design requirements. Judging from the fracture of the beam end, the problem lies in that the anchorage length of the end reinforcement penetrating into the bearing is at least 150 mm, which is actually less than 50 mm, and the distance from the beam end to the outer edge of the column end is 400 mm, which is actually only140 ~150 mm. Therefore, the beam end is supported at the top of the column. In addition, this workshop is a forging workshop, and the dynamic action of the forging hammer has a great influence on the vibration force after the workshop is put into production, which increases the load of the girder to a certain extent. In this case, the girder is broken.
20, steel structure construction is difficult to cause a lot of collapse.
Part 1
In the production workshop of a township enterprise, the girder was placed on the brick column, which collapsed shortly after it was built.
Accident cause analysis:
The main reason is the improper design of beam end support. An anchor bar was added to the original design of cast-in-place beam pad. In actual construction, it is difficult to insert anchor bars into masonry, so the local expansion of concrete cushion blocks is connected with ring beams and poured together. Due to the local enlargement of the top of the brick column, the enlarged part of the brick column is used as the side formwork for pouring concrete. Because the bricks are sticking out one by one, they are not completely consolidated when pouring concrete, so they are very loose. Brick has no bite, and its bonding force with concrete is poor.
2 1, improper handling of cast-in-place beam-column hinge
Causing cracks and damage
The beam and column of a factory building are hinged, and the treatment scheme is in line with the usual practice. However, after it is put into use, cracks appear near the hinge point and some of them are destroyed.
Accident cause analysis:
The original intention of X-shaped steel bars is that they can only bear horizontal force but not bending moment, so as to realize the function of "hinge", but in fact this way has a considerable embedding effect. When the columns on both sides have uneven settlement, the beam end at the joint will have angular displacement, which will make the anchor bar tensile, and the contact surface between the beam end face and the column concrete will be compressed, thus forming a resistance moment. If the bending moment is too large, the joint will crack or even be partially destroyed. When the hinge condition is high, the joint will crack. As shown in the figure, these two kinds of joints are closer to the ideal hinge form, with simple structure and convenient construction. The gap between beams and columns depends on the specific situation and the size of beams and columns.
22, herringbone folding beam calculation error and collapse.
The warehouse is a single-story structure, with a span of10m and a length of 24.5m.. It is supported by brick walls. The roof adopts herringbone folding beams with a spacing of 3.5 meters. Prestressed reinforced concrete purlins are placed on folding beams, with 3 purlins per meter and 30 workers. Purlins are paved with 85cm*60cm*5cm precast slabs. The structure and reinforcement of herringbone roof truss are shown in the figure.
Accident cause analysis:
The herringbone roof truss is planned to be used in this project, which looks like an arch in form, so 8% 18 steel bars are evenly distributed at the beam concentration. In fact, the project has no tie bars and no tensile thrust structures at both ends. In fact, it is a folded reinforced concrete inclined beam with serious insufficient strength and bearing capacity. In addition, the tension reinforcement at the corner of the folding beam is placed smoothly, which is extremely unfavorable to the tension at the corner, which is prohibited by the code.
23, accidents caused by improper steel binding.
The roof of a teaching building is a TIC-tac-toe beam floor with a plane size of 10.8× 14.4m, a beam section of 25×70cm, and 3% reinforcement. After the concrete pouring is completed and the formwork is removed, a large number of cracks are found at a distance of 2.5m from the bearing. See figure 4- 12.
Accident cause analysis:
After the accident, after investigation and analysis, it was found that the accident was caused by improper steel binding. According to the design drawings, the reinforcement is 3 ~ 22. During the construction, because there is no material with the length exceeding 10cm in ￠22 steel bar, the stressed steel bar is cut off at the same section at 2.5m at both ends of the support, and 1 655438+09 and ￠ 22 are overlapped and welded, resulting in six sections being welded at the same time. When pouring concrete, it is impossible to ensure the concrete protective layer around the steel bar, and the adhesion between the steel bar and the concrete is lost, and the lap joint of the steel bar is useless. After the formwork was removed, the beam was severely cracked at the lap joint.
24. Collapse accident caused by wrong support of cantilever beam formwork in reinforced concrete structure engineering.
Four-story internal frame structure, with 80cm high cast-in-place reinforced concrete sun visor in the first floor window of the external wall.
In the process of casting the sun visor, the local external wall suddenly collapsed. The collapsed objects, including all the walls between the sun visor and the window, fall outdoors, and the collapse line basically occurs along the feet, hands and eyes. After the collapse, most of the diagonal bars of the hanger have serious buckling deformation.
Accident cause analysis:
1) The collapse accident is related to improper support. The sun visor is formwork supported by hangers: each window wall is provided with a hanger, and the hangers are connected by wooden mats. The bottom formwork of the sun visor (4-6) is supported by 10×5 cm square timber, and the diagonal braces (10×5 cm) are supported on the windowsill wall (Figure 4-7).
2) Through the stress analysis and bearing capacity checking in the construction of the window partition wall, it can be known that the direct cause of the collapse accident is that the newly-built window partition wall cannot bear the tilting moment transmitted by the hanger during construction.
25. Insufficient bearing capacity of brick columns leads to collapse accidents.
The teaching building of a school is a two-story brick-concrete structure, and the project is drawing to a close. Indoor plastering suddenly collapsed, causing many deaths.
Project overview:
The plan, elevation, section and main dimensions of the building are shown in the figure. The teaching building is a two-story brick-concrete structure, the foundation is a rubble foundation with cement mortar, the wall thickness is 180 mm, and the deep beam in the middle of the head classroom is a cast-in-place reinforced concrete beam. Three months later, the bottom brace and formwork of the beam were removed, and the decoration began to find that the wall was deformed greatly, so the workers beat the protruding wall back with a hammer. Continue the construction. On the third day, it was found that the window wall of the big classroom had a horizontal crack about 100MM wide under the window sill in the city, with a width of about 20 mm. The whole house collapsed and two floors were stacked together. All the workers who failed to evacuate in time died.
Accident cause analysis:
There is no formal design drawing for this project, and the user directly entrusts the construction unit to build it. According to the site situation and referring to the general brick-concrete structure, several sketches were drawn for construction. The construction team consists of bricklayers in the village, and there is no technical management system. After the accident, the grade of the brick was determined to be MU0.5, and the mortar strength was only M0.4 after the formwork was removed the next day. No emergency measures were taken, which led to a major accident.
I believe that after the above introduction, everyone has a certain understanding of the detailed explanation of the quality accident cases of reinforced concrete structures of beams, slabs and columns. Welcome to Zhong Da for more information.
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