Set modifiers value during drift verification

[ihab sorour]

Dears,
AS per the below paragraph extracted from ACI 318-08 M, is the 1.43 can be used for drift verification of service wind loads and earth quake or it is valid for the service wind load only?

Section 10.10 provides requirements for strength and
assumes frame analyses will be carried out using factored
loads. Analyses of deflections, vibrations, and building
periods are needed at various service (unfactored) load
levels10.37,10.38 to determine the serviceability of the structure
and to estimate the wind forces in wind tunnel laboratories.
The moments of inertia of the structural members in the
service load analyses should be representative of the degree
of cracking at the various service load levels investigated.
Unless a more accurate estimate of the degree of cracking at
service load level is available, it is satisfactory to use
1.0/0.70 = 1.43 times the moments of inertia given here for
service load analyses.

Thank u.

 

[CURVEB]

I've often heard engineers say that there is no such thing as service earthquake forces (using ASCE7). The goal is to make sure that the building is safe, not necessarily serviceable. I've seen engineers put notes on their drawings stating that non-structural items must be designed for full functionality at service-level drifts (wind), and must not experience ultimate failure at ultimate drifts(seismic forces).

Using ASCE7, I am not sure how you can quantify seismic forces at the service level. One might be tempted to use 0.7E as the force, however, given that actual earthquake forces are much higher than E, and the force experienced by a building depends on the structural system and plastic hinging, I don't think that there is a lot of meaning to this number.

In practice, I think it is common to use 1.43I for wind drift verification, and 1.0I for seismic, where I is the cracked section property per ACI.