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RE: ASCE 7-05 Section (#4) clarification needed

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I could not agree more. 
In the ideal world of the future, everything for structural engineering will be contained in the ASCE 7 and the material reference standards.  The NEHRP will still be the lead for seismic, but would just augment the ASCE 7 with new provisions.  The commentary and provisions (after they pass balloting) would all be contained in the ASCE 7.  The IBC would just reference the ASCE 7 for structural provisions. 
That has been the plan for some time, but it takes a while to implement. 

Harold Sprague

From: dennis.wish(--nospam--at)
To: seaint(--nospam--at)
Subject: RE: ASCE 7-05 Section (#4) clarification needed
Date: Tue, 22 Jul 2008 20:51:27 -0700

Thanks Harold

This helps a lot. I just wish it were easier to find these commentaries that should exist in one text rather than knowing if the original NEHRP provision was changed by the 2006 IBC and further modified by the ASCE 7-05. Tracking down the logic should be mandatory for helping engineers make sense out of what is most recent or makes most sense.


Appreciate the help



From: Harold Sprague [mailto:spraguehope(--nospam--at)]
Sent: Monday, July 21, 2008 7:37 AM
To: seaint(--nospam--at)
Subject: RE: ASCE 7-05 Section (#4) clarification needed


This is one of those things that was created in the NEHRP Provisions, adopted in the ASCE 7, and then was put into the IBC.  It's original intent is contained in NEHRP.  And the NEHRP has a commentary to better explain the intent. 
FEMA 450, Provisions Secton
4. In areas where the design flat roof snow load does not exceed 30 pounds per

square foot, the effective snow load is permitted to be taken as zero. In areas

where the design snow load is greater than 30 pounds per square foot and where

siting and load duration conditions warrant and where approved by the authority

having jurisdiction, the effective snow load is permitted to be reduced to not less

than 20 percent of the design snow load."


FEMA 450, Commentary Secton :

"The seismic weight W is the total weight of the building and that part of the service load that might

reasonably be expected to be attached to the building at the time of an earthquake. It includes

permanent and movable partitions and permanent equipment such as mechanical and electrical

equipment, piping, and ceilings. The normal human live load is taken to be negligibly small in its

contribution to the seismic lateral forces. Buildings designed for storage or warehouse usage should

have at least 25 percent of the design floor live load included in the weight, W. Snow loads up to 30 psf (1400 Pa) are not considered. Freshly fallen snow would have little effect on the lateral force in an

earthquake; however, ice loading would be more or less firmly attached to the roof of the building and

would contribute significantly to the inertia force. For this reason, the effective snow load is taken as

the full snow load for those regions where the snow load exceeds 30 psf with the proviso that the local

authority having jurisdiction may allow the snow load to be reduced up to 80 percent. The question of

how much snow load should be included in W is really a question of how much ice buildup or snow

entrapment can be expected for the roof configuration or site topography, and this is a question best left to the discretion of the local authority having jurisdiction."

Harold Sprague

From: dennis.wish(--nospam--at)
To: seaint(--nospam--at)
Subject: ASCE 7-05 Section (#4) clarification needed
Date: Sat, 19 Jul 2008 18:56:22 -0700

I need an interpretation to note number 4 of ASCE 7-05 Section which states:

“Where the flat roof snow load, Pf, exceeds 30 psf (1.44 kN/m^2), 20 percent of the uniform design snow load, regardless of actual roof slope.”


This seems to conflict with itself. My first interpretation is that if the snow load for a flat roof (say ¼” in 1-foot) slope or a shed roof exceeds 30 psf then you would add 20% or 6 psf to the roof dead load. However, the last section of the statement reads “regardless of actual roof slope” in which case I would interpret the 20% of the Snow Load exceeding 30-psf to be added to any sloped roof.


How do others interpret this statement. If the roof is flat should 20% of any Snow Load be considered whereas only 20% of the Snow load exceeding 30-psf be used if the roof is pitched?


Dennis S. Wish, PE


Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253




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