Appropriate IBC Classification to Determine Seismic R Value 1

[msquared48]

I have a project I mentioned previously in the recent past where an 8" brick "historical" wall has to be temporarily supported both vertically and laterally during a building demolition and subsequent erection. I have been tasked to re-design the shoring system.

A previous design by another firm was undertaken under the 1997 UBC and wind and seismic values calculated and posted on the drawings. The "Rp" value used for the seismic force was 3.0 (Table 16-O, item 1.A(2)), which corresponded to Section 1632, Lateral Force on Elements of Structures, Non Structural Components, and Equipment Supported By Structures". The net seismic force shown was just under 4 Kips. If I run through equation 1632.2, it appears that the original design force should have been closer to 19 Kips, not 4 Kips. Hmmm.

Well, OK, that's one problem. I have to re-design this thing per the 2006 IBC and ACSE 7-05. In reading the IBC, I am confused here. Should I be designing this frame, a space frame in reality, under Chapter 15 of the IBC - "Seismic Design Requirements for Non-Building Structures"? If so, which table to determine "R", Table 15.4-1 for Non-Building Structures Similar to Buildings", or Table 15.4-2 for "Non-Building Structures Not Similar to Buildings"?

To keep things simple, I would like to use an Ordinary Steel Concentric Braced Frame system, but the R factor is 3.0, pushing the seismic force to about 15 Kips, quadruple the original posted design, but nearer to the "what should have been used" design force. For that degree of difference, I think I could also be missing something here too. The approximation of Section 15.4.2 gives about the same V. If I go to a Special Concentric, the force drops to about 8 Kips, still double the original posted force.

Or were the original design forces wrong? I think that I just have to scrap the original design, assuming it underdesigned, and use the forces I am generating, unless I have missed some code loophole.

Sorry for the long post. It's not like me. I'm just momentarily confused, I think.



Mike McCann
MMC Engineering

 

[ishvaaag]

Not a direct answer, but I would look FEMA 273 and 274 as well that deal with seismic rehabilitation of buildings. Extracted from FEMA 274 what seems to be a cautionary note for anchor points

"Following the 1971 San Fernando earthquake, the Uniform Building Code adopted requirements for positive direct connection of wall
panels to diaphragms, with anchorage designed for a minimum force equal to ZICpWp. In this equation, the quantity ZICp represents the equivalent out-of-plane inertial loading on the wall panel and typically had a value that was 75% of the effective peak ground acceleration for the site. This section of the Guidelines
imposes design provisions based on observations made following the 1994 Northridge earthquake."

This note when even loosely integrated all over the wall even with some kind of modal-like reduction doesn't seem to fit with low forces for the overall design. So in not finding further confirmation, I would go for the safer within the thought permissible, and some that would respect in design terms drift limits for damage of masonry.

 

[ishvaaag]

FEMA 356 prestandard for rehabilitation seems to be consistent with what said in my previous entry.

 

[woodman88]

You might read the attached. Also this one

http://www.masonrymagazine.com/9-04/bracing.html

The biggest problem here is that the building codes applies to the built structure (IBC section 101.3) not the construction ways and means.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

Mike - have you checked out ASCE 37-02, "Design Loads on Structures during Construction"?

It has recommendations for seismic design - a couple of points:
1. Design per ASCE 7-95 (seems old but that is what it says)
2. All structures treated as Category II
3. Seismic can be ignored if Aa <= 0.15 (per map 9-1)
4. If Aa > 0.15, seismic can be ignored if the horiz. wind loads are > 0.2Aa times the weight to be braced by the temp system.

I'd get me a copy of ASCE 37 (latest edition) and rely on that.

 

[msquared48]

JAE:

Thanks for the lead.

If you have used this publication in the past, did you find the seismic/wind loads generated more or less than "typical" IBC loads for buildings in your area?

Mike McCann
MMC Engineering

 

[JAE]

Mike - let me take a look at the load combinations tomorrow when I'm back in the office - The above info is the basis but I don't recall if it uses 1.0E or not. It doesn't have specific R values, etc. but as above - relies on the ASCE 7.

 

[JAE]

Mike - there is also mention in ASCE 37: "The mapped values for Aa and Av may be multiplied by a factor less than one to represent the reduced exposure period, but the factor shall not be less than 0.2..."

R is indicated as being 2.5 max. unless the system is detilaed in accordance with ASCE 7-95.

Only strength requirements of ASCE 7 are required. (i.e. no drift requirements).

I'd get the book - but it doesn't give you specific numbers relative to reducing the seismic Aa and Av factors based on duration of the construction. There is most likely information out there on how to actually justify a reduction factor on the seismic load derivation.

The load combinations are typical combos - with 1.0E as the factor on seismic.

 

[msquared48]

Thanks JAE. Looks like the loads might wind up being lower.

Mike McCann
MMC Engineering