Building Separation/ Seismic Gap 1

[jsp44]

I am designing an addition to an existing building. Both structures are braced frame structures.

Do I need to design the gap between the structures per section 1613.6.7 of the IBC or is there a different requirement for the gap where additions are concerned or gaps in new structures?

 

[DST148]

IBC - 2003 Section 1620.4.5 Building separations. - All structures shall be separated......... Exception - Smaller separations or property line setbacks shall be permitted when justified by rational analysis based on maximum expected ground motions.

 

[hawkaz]

The 2009 IBC also gives exceptions for buildings in seismic design categories A,B and C.

1613.6.7 per the original post is the right section per the 2009 code.

 

[jsp44]

Thank you both for the information. This building is in seismic design category D. I am getting a required separation of about 7.25" and the architect is saying that is way bigger than anything he has dealt with.

 

[PicoStruc]

Have same problem here. The EOR and the architect don't want to hear anything about any gap between both building even if it is explicitly stated in building code that one is required for seismic purpose.

I have already around 7 inch of displacement (caused by torsional behavour) only for the newer building.

So imagine the total displacement if I sum inelastic displacement of both building using SRSS (canadian code)

The general practice of seismic engineering is just bad and fustrate me... don't know what to do !

 

[msquared48]

The separation is large, I would expect something in the 2" to 6" range, but lateral deflection does control the separation, and, depending on the stiffness of the frame systems, perhaps this is needed.

How high are the structures and when was the first building designed?

Mike McCann
MMC Engineering

 

[jsp44]

The structures are about 36' tall two stories with a clerestory. My deflection is about 2" on the addition, but when I multiply by Cd (5) and divide by I (1.25) and rho (1.3) I get a drift of about 6". Assuming the existing building has half that deflection, because it is a bigger building. I get a required separation of about 7". Am I missing something?

 

[PicoStruc]

I think you should consider the inelastic response of the existing building too, NOT elastic of existing + inelastic of new.

 

[hawkaz]

I don't believe rho applies to drift calculations - See ASCE7 12.3.4.1 item 2.

2" of deflection seems high for a standard braced frame. What is the length of the frame- and what type of frame is it? A picture might help

In order to reduce the separation, you may need to upsize the members.

 

[msquared48]

Or fix the base of the frames...

Mike McCann
MMC Engineering

 

[PicoStruc]

These displacement are really possible depending of your lateral stiffness and torsion sensitivity.

Using canadian limit for normal building as reference

36ft * 2.5% drift limit = 10.8 inch. max displacement allowed at upper level !


For a really stiff building, let's suppose 0.5% drift at each story

36ft * 0.5% drift limit = 2.2 inch. inelastic displacement at upper level !

So, if your value is between these, it's quite possible and normal!

If you want to control your displacement, you will need to increase the stiffness A LOT.

 

[jsp44]

Thank you all for your responses. I have determined that the section first referred to in the original post does apply to additions to existing buildings.

I have been able to reduce the gap a little by comparing deflections at the same elevations of the buildings.

Thanks,

 

[DST148]

Per ASCE 7-05, 12.8.6.2 Period for Computing Drift - .....,it is permitted to determine the elastic drifts (dxe) using seismic design forces based on the computed fundamental period of the structure without the upper limit (CuTa) specified in section 12.8.2. May be this would help in reducing the deflections.