uising fema 310 and ibc 2006 to analyze an existing building 2

[jdbpe]

I have a 100 year old building (six story) that requires a global analysis. We were going to use IBC 2006 for the loading and model and analyze the structure using SAP2000 version 11. However, the owner recommends that we use FEMA 310 (ASCE 31) to perform the seismic analysis. Does the FEMA 310 analysis generate seismic forces to be inserted into the SAP2000 loading conditions similar to forces generated from IBC 2006. We believe we will be going into Tier 2 evaluation of FEMA 310 to generate this analysis. Is there compatibility here?

 

[JAE]

That appears to be a Pre-standard so I'm not sure how you could use it unless the governing authority allows it. IBC 2006 and ASCE 7-05 do not reference it at all.

 

[Taro]

FEMA 310/ASCE 31 is not really compatible with IBC. It uses "pseudo lateral forces" that are MUCH higher than IBC forces. You will not be able to plug these forces into design programs used for new buildings without quite a bit of manipulation.

 

[JAE]

Taro, a question.

How does a pre-standard like this apply to buildings legally? Technically the building code is the governing document and I always see the FEMA documents as more a source or support to the code. Is this right?

Do engineers actually use FEMA do design by? Bypassing the IBC or whatever code is adopted?

 

[Taro]

The ASCE 31 standard (FEMA 310 is the superceded prestandard) is not a building code. It is a method of evaluating existing buildings to determine if they meet seismic performance objectives such as Life Safety or Immediate Occupancy. Building owners use the results of these evaluations to determine if they want/need to perform a seismic rehabilitation project.

If the owner decides to undertake a seismic rehabilitation project, we typically use ASCE 41 (supercedes FEMA 356) for the design. The IBC sets minimum standards for new construction and is generally insufficient for performance-based seismic upgrades for existing buildings. ASCE 41 is the state-of-the-art for seismic rehabilitation design and is generally considered "deemed to comply" by building officials.

Seismic rehabilitations are usually voluntary undertakings so as long as the existing building condition is not being worsened, most building officials will not have a problem with them. We usually have this conversation up-front so that all parties understand and agree.

 

[JAE]

Thanks.

There is some language in the IBC about existing buildings (i.e. don't reduce seismic load carrying capacity by more than 10% etc.) I think it is in the later chapters of the IBC. How does that relate to the use of ASCE 31 and 41?

 

[Taro]

As you said, IBC Chapter 34 has some very limited information. But not enough to design by, especially when higher seismic performance objectives are desired. Most building officials will approve the use of national consensus standards such as ASCE 41 even if they aren't explicitly referenced in the code. I have used these standards to retrofit many essential facilities such as hospitals and critical national defense installations. The IBC just doesn't address performance-based design except in a token way by using importance factors. So the design in acordance with ASCE 41 usually goes above and beyond the IBC requirements and is deemed to comply with the intent of the code.

 

[msquared48]

FEMA 310os also termed ATC-22 and is titled "Handbook for the Seismic Evaluation of Buildings - A Prestandard". It is prepared by both the ASCE and FEMA.

As previou8sly stated, it is for evaluation of existing structures, not their design. The buildings are evaluated to either the "Life Safety" or "Immediate Occupancy" Performamnce Level.

It further states in the commentary that "Mitigation strategies for rehabilitating buildings found to be deficient are not included in this Handbook." It mentions that additionaql resources need to be used for this.

Mike McCann
McCann Engineering

 

[JAE]

Thanks Eng-tippers!

(Is that a correct phrase?)