Strength Design Methodology

California, as most of you know, has adopted the 2006 IBC and ASCE / SEI 7-05 in the form of the 2007 California Building Code. This new code requires the user to obtain a copy of the current ASCE 7-05 to find many of the tables and the revisions of the IBC that are contained in the ASCE document. For example, the redundancy factor (ρ) for seismic design is covered in the 2006 IBC but is not mentioned except in one short reference in the ASCE document. The Design coefficients and factors for seismic force-resisting systems tables are not located in the IBC but are only available in the ASCE 7-05 as Table 12.2-1. Furthermore, it is not always clear that when determining the lateral load distribution V, that the IBC and ASCE default to the basic Strength Design Methods which must be adjusted within the analysis for the design method (Ultimate Strength, LRFD, or Allowable Strength Design (ASD)). The following discussion was held on our local e-mail distribution listservice. Please note that while some of this may seem obvious to you, we are in a first time learning curve on the ICC in California that was previously handled with clear consistency (at least in regard to the design methods) from the UBC.

“Dennis,

It is still my interpretation that the “design base shear” is V = C_s • W (Eq. 12.8-1), not V = ρ • Cs • W. I do not see “V” defined as “ρ • Cs • W” anywhere in ASCE 7-05. The factor, “ρ” (“rho”), is used only in Eq. 12.4-3 and as a load factor in the load combinations in Section 12.4.2.3.

CBC 1603.1.5 (Item 7) requires that the “design base shear” be included as part of the earthquake design data on our drawings. If this data is going to be of any use at all, it must be clear as to what we are calling the “design base shear.” Currently, I am showing it as V = C_s • W, not V = ρ • Cs • W. I would be interested to know what others are showing, or plan to show, on their drawings. I feel it is not a matter of “right” vs. “wrong”, but one of consistency.

Dave”

Dave,

Let’s put Rho aside for the moment. One lead I found on page 10 of the ICC “ Seismic Design Manual “ for Problem 1A. It is note in parenthesis that states: “(Note that design base shear in the ASCE/SEI 7-05 is a strength design basis.)”

Below this line continues a brief explanation:

“All tables in the IBC for wood diaphragms and shear walls are based on allowable loads. All tables in the NDS-05 supplement Special Design Provisions for Wind and Seismic (SDPWS) are nominal values and must be adjusted for both strength and ASD loads. It is not known how much longer the IBC will publish ASD values since the trend is toward having all values in strength of nominal format. Since ASD is still predominantly practiced, it has been decided to have this design example in ASD format. In addition, all the manufacturers of metal hardware connectors only publish ASD values.”

On page 12 of the same problem, the equation 12.4.2.3 from ASCE 7-05 is adjusted for ASD design:

“D + L + 0.7 ρ Q_E Eq. 12.4.2.3

Vn-s = 0.183 W

Therefore: Vn-s = 0.183 (102,500 lb) = 18,750 lb”

I can’t account for why the ICC is not clearer on the fact that the code is written for strength design methods to allow the user to choose the method (i.e., LRFD, ASD, Ultimate Strength etc.). However it is clear in the design guides as noted above. Furthermore, the 0.7 factor is the same conversion factor (approximated here) dating back before the 97 UBC. Go to section 1612.3.1 in the 97 UBC and look at the load combinations for Allowable Stress Design methods. The seismic lateral load is divided by 1.4 and the reciprocal of 1.4 is 0.7. The only difference between the two codes 97 vs. 2006 is that the ASD design method was much more clearly defined in the code and used more often than other design methods. LRFD was barely making the scene in wood in 97 and had been used in Steel with much of the industry opting to stick with ASD or Ultimate Strength design of Steel than to use the LRFD version of the Steel Manual.

This is the same in the 2006 IBC but with more provisions. For ASD design, the basic load combinations will be found in equations 16-8 thru 16-15 in section 1605.3.1.

Now let me add one comment for Rho. In many cases Rho will be equal to 1.0 in residential design. I have to opt out of this and check Rho with each project, but in my case I choose to apply Rho uniformly to the entire structure rather than to only the floor levels that meet the 35% or greater percentage of the base shear in each direction (where it would be equal or 1.0). On a one story building Rho must equal 1.0 but depending on the height of diaphragms above fixity, the factor would be applicable on the first level (second floor diaphragm) of the two story building because the majority of shear is moved up to the roof as the example I gave you with 10’ story heights and showing that 70% of the shear occurred at the roof and 30% occurred at the second floor diaphragm (100% at fixity). I’ll be conservative here until I feel that the shears I’ve been evaluating are not significantly lower than the shears we used since I started practicing in the middle 80’s.

Convince me! I don’t want to just argue the point as I think it is important to agree or find a level of agreement or the writers of the code can be attributed with creating an ambiguous code.

Thanks for your replies

Dennis

Please feel free to respond to the discussion and to add your thoughts by adding your comments below. Comments will be reviewed prior to posting them to the blog. Thank you!

Dennis S. Wish, PE

Administrator

~ by structuralist on March 9, 2008.