Why in the design of vertical elements (columns and walls) for a structure against lateral loads (such as seismic and wind) we use a slab (or shell) with a stiffness modifiers (such as 0.25 according to ACI Code) to reduce the moment of inertia (example when we design vertical elements in ETABs and Robot), but in the simple design of the slab we do not use these modifiers (and a side question is: why don;t we include the lateral load of seismic and wind in the design of the slab)??

Thank you in Advance.


1 Answer 1


If you read the commentary of the ACI code you you will understand that stiffness modifiers take into account the cracked section properties expected from cyclical loading. Slabs / diaphragms are a critical part of the lateral load resisting system. To ignore the slab design for lateral loads is a major mistake.

  • $\begingroup$ Hi ShadowMan thank you for your reply, so the lateral load is necessary in the design of the slab? if so the recommended shell type is a thick plate and not a thin plate for the design of internal shear in the slab? $\endgroup$
    – J.Daou
    Aug 20, 2019 at 17:39
  • $\begingroup$ I suggest referring to ASCE 7 and ASCE learning modules on diaphragm design. Most of the time the floor design for gravity controls the proportions of the slab, but additional reinforcement is required to transfer lateral loads to the main lateral force resisting system. $\endgroup$
    – ShadowMan
    Aug 21, 2019 at 0:17

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