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Sudden collapses of roof systems without beams – what do they have to say? The news of further collapses of such roof systems concerns many professionals in the field. Why have there been so many sudden collapses of roof systems without beams in recent years? What is the safety factor of a beamless floor structure? Several senior experts expressed their views: http://img.civilcn.com/d/file/zhishi/jggc/2019-08-30/edaaf1c82cb2e3e0a172caa521a58ac8.png The chief engineer advising a design institute said the following regarding the reasons for the collapse of beamless floor systems: 1. The concrete codes overestimate the shear resistance of slab-column joints by about 16%; this is because the experimental studies conducted in China to date have focused on slabs supported on all sides and subjected to loading at their centers, with no actual tests on slab-column joints carried out. 2. The double-layer, two-direction reinforcement in the slab of the slab-column joint should not be too small; the reinforcement in the lower layer of the slab has a significant impact on the slab’s local bending strength. According to my research, the local bending strength of slab-column joints without reinforcement in the lower layer is approximately 10% lower than their shear strength. The national standard drawings specify that the diameter of the reinforcement in the lower layer of the slab should be no less than 10 mm, with a spacing of no more than 200 mm. When the thickness of the joint slab is large, the reinforcement ratio is far below the minimum required value. In such cases, the slab-column joint functions essentially like a foundation, and it is not surprising that local bending failure occurs. 3. The concrete code includes in an appendix the formulas for simultaneously transferring punching shear force and unbalanced bending moment; as a result, some designers consider this loading condition unimportant and fail to perform the calculations. 4. The soil load during construction backfilling is a local load; together with the mechanical load, it should in principle be considered as a live load. Considering points 3 and 4, when half of the area around a slab-column joint is filled with soil and the other half is not, the allowable load of the soil in the half-filled area is only one-third of the allowable value when the entire area is filled with soil. Summary: For a beamless floor design that meets the conditions mentioned above, its safety margin will be only 0.85x0.90x2/3=0.51 of the value required by the standards. An experienced expert said: The abuse of beamless floor systems represents a regression in structural design principles, or a typical example of \"structural surrenderism\"! I. Qualitative analysis of the structure 1) Analogy: If wooden or steel structures are used in buildings with short spans, it is theoretically possible to create beamless floor systems (plywood must be used for wooden structures). However, if such a building were actually constructed, it would undoubtedly be an example of poor design (highly unreasonable). Why is that? If only it were easier for laypeople! So, is it reasonable to replace the material with (reinforced) concrete? 2) Those who have studied mechanics of materials have a clear understanding of component cross-sections: the cross-sections of flexural members should preferably be I-shaped or H-shaped ； Metal structural members commonly use thin-walled sections with cross-sections in the shapes of L, I, T, or O, or are constructed as hollow beams or trusses Never use solid rectangles! 3) Practice has shown that the misuse of beamless floor systems inevitably leads to problems; strict control and reliable structural measures must be implemented! 4) Better alternative methods: shaft-type floor slabs, cast-in-place hollow slabs, steel-concrete. II. Comprehensive consideration of the design plan: A good overall design plan may not be the best option for each individual specialty, but none of the specialties should go below the minimum standards; instead, they should strive to exceed those standards as much as possible, avoiding extreme approaches. 1) Architects must be a bit romantic, creative, imaginative – thinking of things like castles in the air or mirages... Although they should make full use of their imagination, they ultimately need the support of structural engineering. 2) The structural engineer must work diligently and steadily, silently bearing the pressures and hardships from all sides, while fully supporting the architect’s creativity and inspiration. But when it is discovered that the architect has become obsessed, every effort must be made to pull him back and prevent him from falling off the cliff. One must never lose oneself, forget the principles of one’s own field of expertise, willingly let others control oneself, or even go to great lengths to justify unreasonable situations – it’s truly incomprehensible! 3) To accommodate the special needs of other disciplines, unreasonable structures are not necessarily unusable ； But on the other hand, just because something is available does not mean it is justified, and it certainly cannot be abused everywhere at will!