Question: I am designing a structure that has a number of horizontal cantilever members, which require the application of ASCE 7-10 Section 12.4.4. This section states: “In structures assigned to Seismic Design Category D, E, or F, horizontal cantilever structural members shall be designed for a minimum net upward force of 0.2 times the dead load in addition to the applicable load combinations of Section 12.4.” I have a few questions on this provision:
1. Is the required 0.2D load supposed to be a factored strength level load? If not, what load factor should be used in the load combinations?
2. In which load combination should I include this load?
Answer: This provision is intended to protect elements that would otherwise be vulnerable to net uplift as a result of the vertical acceleration. In the expanded commentary to ASCE 7-10 seismic provisions, the following can be found on this provision.
“In Seismic Design Categories D, E, and F, horizontal cantilevers are designed for an upward force that results from an effective vertical acceleration of 1.2 times gravity. This design requirement is meant to provide some minimum strength in the upward direction and to account for possible dynamic amplification of vertical ground motions resulting from the vertical flexibility of the cantilever. The requirement is not applied to downward forces on cantilevers, for which the typical load combinations are used.”
In other words, 0.2D is a net upward force, derived from an assumption of a 1.2D upward load resulting from vertical acceleration minus the dead load of the member itself (1.0D). This load condition is in addition to, and separate from, all other load combinations of ASCE 7 Section 12.4. Since this is a seismic load case, and ASCE 7-10 specifies all seismic loads at the strength level, it is assumed this 0.2D load is also a strength level load.
So, the following seismic load combinations need to be considered for horizontal cantilever members:
Basic Combinations for Strength Design:
a. (1.2 + 0.2SDS)D + ρQE + L + 0.2S
b. (0.9 – 0.2SDS)D + ρQE + 1.6H
c. –0.2D
Basic Combinations for Allowable Stress Design:
a. (1.0 + 0.14SDS)D + H + F + 0.7ρQE
b. (1.0 + 0.10SDS)D + H + F + 0.525ρQE + 0.75L + 0.75(Lr or S or R)
c. (0.6 – 0.14SDS)D + 0.7ρQE + H
d. –(0.7)(0.2)D