ASCE 41-17: Q(E), Fp, and C1C2*(J) Questions 1

[KelleyA]

Currently lost in the code a little bit and had some questions about how all of the items in the title relate to each other.

So we are trying to solve two things for a tier 2 check withing ASCE 41-17:

1. If the existing connection between a concrete wall and the diaphrapm is adequate to resist the forces.
2. If the top chords of existing members can resist the out of plane wall forces plus forces applied to them by sub diaphrams.

The primary question we have is, can we substitute Fp in the Q[sub](E)[/sub] in equation 7-35, and if so, what are the limitations to that. ASCE 41-17 states the following:

(Eq. 7-35): Q[sub](UF)[/sub] = Q[sub](G)[/sub] + X*Q[sub](E)[/sub] / (C[sub]1[/sub]C[sub]2[/sub] * J)

The definition of Q[sub](E)[/sub] under (Eq. 7-34) states that Q[sub](E)[/sub] is "Action caused by the response to the selected Seismic Hazard Level calculated using either Section 7.4.1 or Section 7.4.2;".

The issue here being that the Fp force is calculated within 7.2.11 and not either of the sections listed. However, if you go to just the defintions section of ASCE 41-17 Q[sub](E)[/sub] is simply defined as "Action caused by the response to selected Seismic Hazard Level, Eqs. (7-34) and (7-35)," which has no reference to 7.4.1 or 7.4.2. There is no clear guidance as to how to apply the Fp forces to the Acceptance Criteria equations.

One final point of concern is that we found a design guide by FEMA that states the following in one of their examples:

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"4.7.4 Acceptance Criteria for the Out-of-Plane Masonry Wall Design:

The acceptance criteria for the out-of-plane masonry wall design stipulated in ASCE 41-13 § 11.3.5.3 and § 7.2.11 require the walls to be considered forcecontrolled actions. When evaluating the behavior of force-controlled actions, ASCE 41-13 § 7.5.1.3 requires that the lower-bound component strengths, QCL, be used. Per ASCE 41-13 § 11.3, the lower-bound strength of masonry walls is permitted to be calculated based on the strength design procedures in TMS 402-11, Building Code Requirements and Specification for Masonry Structures and Related Commentaries (TMS, 2011), with a strength reduction factor, ϕ, taken equal to 1.0.

The acceptance criteria for force-controlled actions using linear analysis procedures are outlined in ASCE 41-13 § 7.5.2.1.2. This section is intended to be applicable to components that are resisting the pseudo seismic forces from ASCE 41-13 § 7.4.1.3, and is not applicable to Fp forces from ASCE 41-13 § 7.2.11, so the C1, C2, or J factors in ASCE 41-13 § 7.5.2.1.2 should not be applied to the Fp forces. The acceptance criteria for force-controlled actions with Fp forces should be as follows per ASCE 41-13 § 7.5.2.2.2:"

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Is that second paragraph correct? I'm unable to find anything within ASCE 41-17 that backs up the inability to use C[sub]1[/sub], C[sub]2[/sub], or J. ASCE 41-17 7.5.2.2.2 is just the DCR equation and gives no additional information on how F[sub]p[/sub] should be used. Additionally, this design example is for the out of plane strength of the wall, not it's connections, but it is still using a force controlled method. They also substitute F[sub]p[/sub] directly in for Q[sub](E)[/sub], which has no code reference.

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QUF is the combination of gravity and seismic loads, as follows:
QUF = QG ± QE (ASCE 41-13 Eq. 7-35)
where, per the load combinations in ASCE 41-13 § 7.2.2:
QG = 1.1(QD + QL + QS) (ASCE 41-13 Eq. 7-1)
and:
QG = 0.9QD (ASCE 41-13 Eq. 7-2)
QE = Fp

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Thanks for taking the time to read this and hopefully for the help.

 

[chris3eb]

It could be a little more clear, but Equations 7-35 and 7-37 provide the acceptance criteria for force-controlled actions for LSP or LDP. Similarly, equation 7-38 provides the acceptance criteria for NSP or NDP. These only apply to forces from respective their analysis procedures. The out-of-plane wall force and out-of-plane wall anchorage checks exist outside of the four analysis procedures. The forces don't come from your model, and you are using the same equation regardless of the analysis procedure. The C and J factors are corrections for the linear analysis procedures (since the linear forces will be higher than actual forces). For the oop wall stuff, the intent is to check the specified force directly (note also that there is no R factor for the oop wall forces in ASCE 7).