Let the basic theory "support" the mold standard GB "Safety Technical Specifications for Construction Construction Fastener Stee
In the year before, the No. 4 plot of Beijing Xixi Project resulted in the death of eight people due to the collapse of the formwork; in April last year, Dalian lost six people due to the collapse of the template. The mold support accident has become a difficult problem and has attracted the attention of the state. The moldwork accident is not only a problem of a certain enterprise or a responsible person, but also a problem that construction workers need to ponder. Analysis of the mold frame accident can not be a brow and a beard, otherwise it will shift our sights to the branch problem and ignore the real "culprit" of the accident, and ignore the control of the key problem. For example, the steel pipes currently supplied on the market are almost all smaller than the standard, and the error can reach more than 10%; the fasteners may be worse. Xinhua News Agency published the "three red-headed documents can not control a murderous fastener" comments, in fact, the steel pipe fasteners are not up to standard is easy to grasp, and the error caused by steel pipe fasteners may only be a percentage In the tens of decades, theoretical mistakes will likely lead to more serious mistakes, which is more worthy of attention. The author believes that the theoretical error that is easily overlooked by people is the "culprit" of the collapse of the formwork.
"Theoretical environment" is seriously polluted
The promulgation of the national standard "Safety Technical Specification for Fastener-type Steel Pipe Scaffolding for Construction" (JGJ130-2001) took 18 years, and the national standard has taken into account the calculation theory and construction method of the template support. In the past twenty years, the application calculation theory of fastener steel pipes can be described as a hundred flowers: many scholars have proposed different calculation theories and methods according to their own understanding, which also brings great difficulties to the unified national computing theory in the future. Many fierce debates during the preparation of the national standard delayed the time and main energy of the promulgation of the national standard. After the promulgation of the national standard, some of the comrades' "Building Construction Calculation Manual" was not adjusted according to the new norm, but claimed that "this manual is all written according to the new national norms and standards," and some of the errors caused by this will inevitably lead to risks. . In addition, there is negligence in the national standard JGJ130-2001. The national standard "Safety Technical Specification for Construction Fasteners for Steel Pipe Scaffolding" provides for the specification of scaffolding and also for the formwork bracket. Although there are only five relevant clauses involved (three for template support calculation and two for construction), it is still calculated. It plays a standard control role, and there are quantitative controls in the construction regulations. Formwork construction measures are an important means of guaranteeing the theory and safety of calculations. Unfortunately, the regulations do not adopt the provisions of mandatory enforcement clauses. On the other hand, the omission of key words in the narrative of the text poses a threat to the safety of the formwork.
From local regulations to future formwork construction
Shanghai has taken the lead in the preparation of the “Safety Technical Regulations for Supporting Systems for Horizontal Forms of Steel Pipe Fasteners” (DG/TJ08-016-2004): This procedure is similar in principle to the national standard JGJ130-2001 (computation theory and construction method); Several important words of “induced load”, “safety load” and “installation deviation load”. The appearance of these words was not accidental: the British had tasted the bitter fruit of the induced load; the Japanese had also collapsed the template because of the inexplicable force, and had to add a 2.5% horizontal safety load. Therefore, these two additional loads will become an important topic in the future theoretical research of formwork calculation. As for how to take the value, it can be further discussed. The Shanghai Regulations clearly state in the General Regulations: "In addition to complying with this regulation, other relevant current national and Shanghai standards and regulations should also be observed." The Shanghai Regulations did not specifically quantify the various scissors and failed to be in the national standard. Omissions are noticed. The Shanghai Code also provides examples of calculations for stresses generated in the uprights under lateral forces (wind, 1% erecting error loads, and 2.5% safety loads), but unfortunately there are still some clerical errors.
The dynamic impact and induced load problems are important issues worthy of attention in the design of the formwork. All structural failure processes are not dynamic processes, but currently the world's structural engineers and even structural specifications are based on static calculations, and the dynamic impact effect is compensated by the dynamic load factor at best. Due to the structural engineer's lack of evaluation of the dynamic impact effect on the power student. The dynamic impact effect is often as many as several hundred times hundreds of times the static effect. For example, we can easily cut a round steel that can withstand more than ten tons of shear force with a hammer of less than ten kilograms. In the construction practice, we also ate the pain of a variety of power shocks. In addition to the stringent requirements of the above construction, the formwork bracket should also consider the dynamic damage during construction. For example, the vertical splash of concrete and the horizontal vibration of the cloth machine or the cloth pipe are important issues to be concerned. Special attention should be paid here to the fact that the stress of a neighboring member is rapidly increased by the instability of a certain rod, and the process is only a few seconds. To this end, the top layer should be reinforced in the safety strategy: whether in the horizontal or vertical direction, it is more important to avoid placing the weak joint on the top and directly bearing the power impact (the deep buried lower part can disperse the impact on the joint of the rod).
The 2.5% horizontal safety load in the Shanghai regulations actually solves the difficult horizontal impact load from a static point of view, but its value remains to be discussed. The horizontal impact load is external energy and is not necessarily directly linked to the plate weight. Depending on the actual situation, it must also rely on actual tests. This problem is a concern and unavoidable problem in China's formwork and even in all countries. The Shanghai Regulations also stipulate that 1% of the error horizontal load should be set. This figure can be reduced to 3‰ when the mold frame is set to meet the erection quality, because the verticality of the pole is required to be ≤3‰ in the regulations, and the deviation is statistically observed. It can't be unidirectional, and the value is safe.
As for the calculation of wind load, there is no need to pay too much attention to the calculation of the structure. People will not carry out the construction under the extreme value of the wind (the calculation of the windy theory once in a few decades), so it is not necessary to combine with the construction load; On the one hand, if we have considered that the 2.5% horizontal safety load is generally greater than the wind, and the two do not occur at the same time, it is not necessary to consider the calculation of the wind load, and the special case is different.
In the future direction of the formwork structure, it should be noted that the theory of formwork calculation in the national standard or local regulations has ambiguity and even less stringent structural problems. The "Safety Technical Specification for Bowl-Button Steel Pipe Scaffolding" in the compilation attempts to construct the frame as a geometrically invariant system to change the current state, but there are also many problems worthy of study and further improvement. Even if the terminology of the formwork design is correct, it is difficult to imagine that the three-dimensional structure of the formwork can be expressed only in terms of language. If the drawings (flat, vertical, sectional or even axial projection) are not fully utilized, it may cause an accident: The diagram should be used as the main engineering language when the specification is compiled.
The template support accident is controllable, which requires us to carefully study the theory and construction methods and objective facts of the formwork. At present, in the process of development and reform, the phenomenon of neglecting basic theoretical research due to economic interests needs to be paid attention to. The industry should study the "support" model frame with basic theory, and beware of the recurrence of the collapse of the template. Xiong Yaoying