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Re: Designing a remodel - how to treat the existing structure

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The BOCA code and IBC both have provisions in the code for how to handle
seismic design for additions and the existing building to which the
addition connects.  Based upon a quick review of the 1997 UBC, I did not
see any similar provisions.

Basically, the idea of the provisions in the BOCA and IBC code are as

If the addition is completely structurally independent, then addition is
designed for the loads from the code and the existing building does not
need to be evaluated.

If the addition is not completely structurally independent, then the
provisions of the BOCA code state:

"An addition that is not structurally independent from an existing
building shall be designed and constructed such that the entire building
conforms to the seismic requirements for new buildings unless the
following three provisions are complied with:

1. The addition complies with the seismic requirements for new buildings;
2. The addition shall not increase the seismic forces in any structural
element of the existing building by more than 5 percent unless the
increased forces on the element are still in compliance with these
provisions; and
3. The addition shall not decrease the seismic resistance of any
structural element of the existing building below that required for new

The provisions in the IBC are almost identical, except that item 3 is not

With regard to FEMA documents, FEMA 310 (evaluation document) and FEMA 273
(rehabilitaion document) can be used.  However, it should be pointed out
that these documents (at this time) are not code documents.  They are
meant to be standards to be used by building owners (actually, by
professionals hired by building owners) to evaluate and rehabilitate
structures for seismic loading on a voluntary basis.  That is not to say
that someone can "adopt" them at a local level for use, but I don't
believe either document is written completely in "code-speak" (the
official, confusing language of code writers).

Hope that helps,


On Wed, 25 Oct 2000, Dennis S. Wish wrote:

> I have a couple of small remodels on my desk - typically additions to a one
> story wood frame home ranging from 100 to 300 square feet. It is, in my
> opinion, unreasonable to expect full-compliance where the existing structure
> needs a lateral rebalancing due to the addition. In this case what is the
> best approach. Here is my opinion - I would be pleased to hear from others
> on this topic.
> 1. Design the new addition by tributary area in compliance with the lateral
> provision of the new code using ST-12 or the Tri-Country simplified design
> approach AND
> 2. Calculate the existing capacity of the line of shear of the existing
> residence where the addition connects and upgrade that line of shear to
> accommodate the addition shear from the addition.
> This seems like a reasonable approach. Anything more aggressive will, in my
> opinion, destroy the remodeling industry by driving the homeowner away when
> the work becomes much more intrusive than the owner is willing to tolerate
> (not to mention more expensive).
> Are their any code reference to his issue or other documents from FEMA (I'm
> thinking of FEMA 273 which I believe deals with seismic upgrades to existing
> structures) or HUD that apply?
> Regards,
> Dennis S. Wish, PE
> Structural Engineering Consultant
> structures(--nospam--at) <mailto:structures(--nospam--at)>
> (208) 361-5447 E-Fax
> ICQ #95561393