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# Re: Load Combination, Using Strength Design

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Load Combination, Using Strength Design
• From: fsrahbar(--nospam--at)aol.com
• Date: Wed, 01 Sep 2010 12:57:11 -0400

Thank you.

Farzin S. Rahbar, SE
Vice President
David C. Weiss Structural Engineer & Associates, Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041

-----Original Message-----
From: Josh Comfort <jcomfort(--nospam--at)structuralforte.com>
To: seaint(--nospam--at)seaint.org
Sent: Wed, Sep 1, 2010 8:09 am
Subject: RE: Load Combination, Using Strength Design

The last sentence of section 1605.1 specifies that “Each load combination shall also be investigated with one or more of the variable loads set to zero.”  So not only is it logical to evaluate 1.2D + 1.6Lr (with 0 wind load), it is specifically required.

Josh Comfort, P.E.
Structural Forte, Inc.
7307 N Division St., Suite 304
Spokane, WA 99208
phone:   (509)624-3224
fax:         (509)624-3225

From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com]
Sent: Wednesday, September 01, 2010 4:51 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Load Combination, Using Strength Design

Logically I would run the wind as zero under dead plus roof live if the wind is counteracting. The probability of 1.2D + 1.6Lr + 0.8*W (where W is the Design wind) has to be less than the chance 1.2D + 1.6Lr with no wind or with wind that is some value greater than zero (but still less than 0.8*design wind).

WH

On Tue, Aug 31, 2010 at 1:51 PM, <fsrahbar(--nospam--at)aol.com> wrote:
I have a flat roof. The only wind load on the roof is uplift. I am using eq. 16-3 and set the wind load to zero. Just want to make sure that this is not overly conservative!

Thank you,

Farzin S. Rahbar, SE
Vice President

David C. Weiss Structural Engineer & Associates, Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041

-----Original Message-----
From: Jason Christensen <jason(--nospam--at)wcaeng.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Sent: Tue, Aug 31, 2010 9:43 am
Subject: RE: Load Combination, Using Strength Design
If you are using the ASCE 7 wind loads you should have both an upward acting wind load and a downward acting wind load.

If for some reason you don’t have an upward wind load then IBC equation 16-3 would be the controlling load combination for the roof live load.

Jason Christensen, S.E.

From: fsrahbar(--nospam--at)aol.com [mailto:fsrahbar(--nospam--at)aol.com]
Sent: Tuesday, August 31, 2010 9:46 AM
To: seaint(--nospam--at)seaint.org
Subject: Load Combination, Using Strength Design

List:

IBC 2006, Eq. 16-3: 1.2D + 1.6Lr + 0.8WL

The wind load is always uplift. Therefore, it will actually help the roof live load. In your opinion, do I have to set the wind load as zero in this combination?

If so, why do they also have Eq. 16-2, with only 0.5Lr? Or, why don't they have another equation with just DL and roof live load?

Thank you,

Farzin S. Rahbar, SE
David C. Weiss Structural Engineer & Associates, Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041