Seismic load. Can waffle or flat slabs be use when large seismic forces are present?

Flat and waffle slabs usually have edge beams, defining their perimeters and any openings, and so do not have the load bearing task flat beams in joist floor slabs do. Therefore, the use of flat and waffle slabs is not recommended in zones where the design acceleration > 0.16g.

There is no mention in any design code, of these slabs not being able to be used when these seismic conditions are present. Some do state that flat and waffle slabs will be treated as having low ductility (mu = 2) when calculating forces and displacements. This will induce greater displacements and forces than if the structure were to be calculated with high ductility (mu = 3) or very high ductility (mu = 4).

Therefore, designing a structure with a flat or waffle slab cannot be discarded because it is designed with low ductility, as this precisely is more unfavourable for the structure; it is as if it were exposed to a greater horizontal force.

To continue talking on this topic, if the user wishes to have a structure with greater ductility even though it was designed with low ductility, the program allows for the column and beam reinforcement to be more ductile, regardless of the ductility that was selected for the force analysis.

This reinforcement ductility criteria (increase in the amount of longitudinal and transverse reinforcement, anchorage lengths, reduction in stirrup spacing, etc.) is selected in Job > General data > Criteria of reinforcement by ductility.

It is especially important to select the criteria for reinforcement due to ductility for columns as well as checking that the correct reinforcement table has been selected when a high seismic acceleration is present as this contains the correct reinforcement arrangements for the specific seismic conditions.

It is also very important that the column reinforcement has been selected to overlap at mid-span.

By saying the structure has low ductility, more unfavourable results are provided when designing against forces than if it were to be designed with high or very high ductility. Therefore, it is not necessary to apply a reinforcement criteria due to ductility. However, it may be applied and the structure will have an even greater applied safety factor.

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