Often, aluminum is not the main lateral force resisting system of a structure, but rather serves to transfer loads to the lateral force resisting system (for example, curtain walls). In those instances, the R factor for the aluminum members is less of an issue. ASCE 7 chapter 13 provides R factors for architectural components.
In those structures in which aluminum does comprise the lateral force resisting system, wind is often the governing lateral load rather than seismic, since the mass of aluminum structures tends to be much less than that of other structures of other materials. This reduces the effect of seismic loads, but is a disadvantage with respect to resisting overturning from wind. If wind governs even when R is taken as 1 for the structure, then the R factor for the aluminum frame is moot.
The R factor is not only a function of the material used, but also the type of lateral force resisting system - for example, a braced frame or a moment frame. The article that Ron Ziemian and I wrote and that appears in Structure magazine this month was based on a study of an ordinary moment frame.
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Randy Kissel] P.E., M.ASCE
managing consultant
Trinity Consultants
Durham NC
919-493-8952
TGBPartner
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Original Message:
Sent: 08-18-2018 18:03
From: Pedro Munoz
Subject: Seismic Design R Factor for Aluminum ASCE 7
The recent article of Kissel and Ziemian in the August 2018 issue of STRUCTURE magazine certainly opens the discussion for appropriate R values for Seismic Design of Aluminum Structures, but ASCE 7 is still short of providing some specific values in the Standard. This might be a fertile ground of new and innovative research of Aluminum Structures under Seismic Loading Conditions. Perhaps a Joint Committee of ASCE, Aluminum Producers, and IBC might foster some type of specific research that can result into incorporating R values for Aluminum Structures in the ASCE 7 Standard. At this time the ASCE 7 Table 12.2-1 provides R values for Steel, Reinforced Concrete, Masonry, Wood, Cold Formed Steel, Timber, and refers to "All Other Materials", one could see some values of 2 (A-17), 2-1/2 (B-24), and perhaps 3 (H), so one may be tempted to be conservative and use a lower bound value of R=2 for starters, until ASCE 7 and Aluminum Producers could come up with a meaningful documented and tested value of R for Aluminum. Any other suggestions?
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Pedro Munoz Ph.D., P.E., M.ASCE
Principal
PRM Engineering, LLC
Methuen MA
(978)7388001
Original Message:
Sent: 08-17-2018 11:56
From: Brett King
Subject: Seismic Design R Factor for Aluminum ASCE 7
I'm not certain you'll easily find any answers to correct R values for aluminum structures. Very little research has been done it seems. Here's a very new article you may have already seen that gives insight although not solutions. It seems that others struggle with the same questions.
I can say that I've designed a couple aluminum frames used in aircraft control tower cabs. We selected relatively low R values 2.5 or 3 as I recall and then paid attention to the detailing to prevent non-ductile failures like fractures etc. by using a capacity protection approach.
Seismic Design of Aluminum Structures
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| Seismic Design of Aluminum Structures |
| Earthquakes impose such large and infrequent forces on structures that building codes permit seismic damage if the structures do not collapse. This damage often takes the form of inelastic, permanent deformation of members and connections. |
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Brett King P.E., M.ASCE
Senior Structural Engineer
GHD Inc.
Lake Oswego OR
(971)925-3743
Original Message:
Sent: 08-16-2018 14:14
From: Pedro Munoz
Subject: Seismic Design R Factor for Aluminum ASCE 7
In looking at ASCE 7-10 Chapter 12 Table 12.2-1. One can notice that there are no Recommended Values for R (Response Modification Factor), Omega (Overstrength Factor) and CD (Deflection Amplification Factor) for ALUMINUM STRUCTURES. Aluminum weights roughly 1/3 of steel, has high values of Yield, elongates more than steel, E of about 1/3 of steel. Any recommendations from ASCE 7 to introduce some reliable values for Aluminum so that it can be used more confidently as major supporting structural members, particularly for Grid Shell tubular structures or other similar. Can anyone share some specifics on Aluminum Seismic Design Parameters? Is an R value of 2 to 3 reasonable to be used for Seismic Design of Aluminum Framing?
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Pedro Munoz Ph.D., P.E., M.ASCE
Principal
PRM Engineering, LLC
Methuen MA
(978)7388001
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