Base Shear Calculation fron IBC and ASCE-7 3

[Lehigh2001]

We have a 5-story parking garage in Washington DC area with a dimension of 204’ by 120’. I am in the process of calculating the base shear due to seismic and wind loads.

For seismic loads, IBC 2003 Section 1617.4 and ASCE-7 2005 Section 12.8 were used for comparison purposes.

IBC Equation 16-56, V=1.2*Sd*W/R and ASCE Equation 12.8.2, V=Sds*W/(R/I) give me similar results. The difference is the 1.2 factor which I can understand that IBC equation is a Simplified Method which needs this factor for additional safety factor.

However, ASCE-7 also has Equations 12.8.3 and 12.8.4 which give the base shear an “upper limit”. For my case, since T<TL, Equation 12.8.3 controls but this equation reduced the base shear to half comparing with value from Equation 12.8.2 or IBC results. FYI, the base shear from IBC is 320 kips and the value for using ASCE-7 Equation 12.8-3 is 138.6 kips.

Should I use the lower value for my base shear or should I use the high value just for peace of mind? Thank you for your comments.

 

[JKW05]

If DC currently uses the 2003 IBC, you should be using either the 2003 IBC requirements or ASCE 7-02, as the 2002 ASCE standard is what is referenced by that edition of the code. Use of the 2005 standard would be o.k. for your own peace of mind, as long as the final design, as a minimum, still met the 2003 IBC requirements.

It would appear that the "1.2" factor in the IBC simplified design is simply a conservative importance factor. Regardless, the limitations set forth in 1616.6.1 of the 2003 IBC does not permit the use of the simplified design for your 5-story building. Also, close examination of the ACSE sesimic maps shows that the contours for Ss and S1 have shifted slightly. For DC, ASCE 7-02 has Ss=~.20 and S1=~.06. ASCE 7-05 has Ss=~0.18 and S1=~0.05 (interpolation was used here)

"Equation 12.8.3 controls but this equation reduced the base shear to half comparing with value from Equation 12.8.2"

Something seems amiss here. For purposes of discussion, I'll stick with ASCE 7-05. It appears that in the D.C. area Ss=~.18 and S1=~.05. For Soil Class D SDS=~.19 and SD1=~.08. Assuming T=~0.5, I get:

Eq. 12.8-2 --> Cs = .19/(R/I)
Eq. 12.8-3 --> Cs = .16/(R/I)

The parameters you are using may be slightly different, but a factor of 2 between the two results seems excessive.

Hope this helps...

 

[JAE]

JKW05 is right - DON'T mix codes. The IBC 2003 does not and should not utilize ASCE 7-05.

Check Chapter 35 of the IBC where they list the referenced support documents.

 

[Lehigh2001]

Thank JKW05 and JAE.

I cannot believe I made this stupid mistake to mix IBC and ASCE-7.

However, even though I use ASCE-7 02, the results are the same. Comparing Cs values from EQ 9.5.5.2.1-1 (Cs=0.0229) and EQ 9.5.5.2.1-2 (Cs=0.012), the results are still quite different. I attached my calculations and please let me know what is wrong with my numbers, if any.

Thank you all very much.

 

[WillisV]

Not checking your numbers, but 9.5.5.2.1-2 is the maximum, 9.5.5.2.1-3 is the minimum, so as long as whatever you get from 9.5.5.2.1-1 is within these upper and lower bounds, then it is the value you should be using.

 

[Lehigh2001]

"9.5.5.2.1-2 is the maximum, 9.5.5.2.1-3 is the minimum, so as long as whatever you get from 9.5.5.2.1-1 is within these upper and lower bounds, then it is the value you should be using"

I understand the upper and lower bound limits. However, the value from 9.5.5.2.1-1 is twice more than the value from 9.5.5.2.1-2. It makes me concern because I reduce the base shear by half if I use 9.5.5.2.1-2 as an upper bound limit.

Am I missing something here? Please advice. Thanks.

 

[JKW05]

As WillisV said, they are upper and lower bounds. They are what they are.

I will share a couple of comments, though, on the calculations you posted.

Site Class D is to be used unless you have "sufficient" soil properties ('02-9.4.1.2.1).

Your determination of "R' says it is for an ordinary concrete "building", and you refer to the table in the '05 standard. The only systems with R=4.5 listed in the '02 standard are for "Intermediate steel moment frames" or a "Special steel concentrically braced frames". For your period determination, you refer to it as an "ordinary concrete moment-resisting frame, which would appear to have R=3.

Your original OP said the structure was in Washington, DC, and you note Ss=.155 in your calcs. The '02 standard appears to show the 0.2g contour running through the center of DC, with the nearest .15 contour east of the Chesapeake Bay, in southern MD and DE.

I suggest you may want to discuss this analysis with another engineer in your office.

Regards,

jkw

 

[Lehigh2001]

Thank you, JKW05 for your kind review and comments.

First of all, Ss, and S1 values were obtained from USGS website for a specific zip code. The site is located just north of DC beltway in MD. I believe the values from USGS website should be correct, aren't they?

The R value should be 3 as you pointed it out. The R value is 4.5 for ASCE-7 05 but it is 3 for 02 code.

We do have Geo report but it does not mention what site class is. I changed the site class to D. This chnage actually increases the base shear from 138 kips to 520 kips.

The seismic load rarely controls the lateral forces for buildings in this area. The wind load (180 kips for this case) always controls the lateral load. If 520 kips of seismic load is correct, this is the first building I know controls frame design in this area.

Can someone comment on this? Thank you.

 

[JKW05]

If the 2003 IBC is being used, it is my opinion that the accelerations illustrated in the '02 standard are the ones that should be used, as that is the standard the '03 code references. The USGS website may be based on the updated information. Again, I believe you may be mixing codes and/or standards. In my opinion, it would be reasonable engineering judgment to use the results using the '06 code and '05 standard, as long as they are not less than the adopted building code.

If a geotechnical report has been prepared, I would suggest contacting the geotechnical engineer and request they provide you with a site class. As you have seen, it can make a huge difference.

I did a 5-story flat slab building in College Park about 6 years ago where seismic controlled in the longitudinal direction (the area exposed to the wind is much less than the transverse direction, but the mass stays the same for both directions). It has been my experience that this is not uncommon for concrete frames w/brick cladding and concrete masonry buildings.

You may want to seek some assistance on a more one-to-one basis.

Good luck,

jkw