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

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Dennis,

Section 105 of the 1997 Uniform Code for Building Conservation (UCBC) gives
rules for additions, alterations and repairs.

To summarize, the structural portion of the addition must comply with the
Building Code; the addition or alteration must not cause the existing
building to become unsafe or overloaded; height limitation rules are stated;
new non-structural and non-fire related materials may match the original
construction.

My approach for the lateral system is to design it to brace the new
construction and tributary portions of the existing, and to avoid increasing
the lateral demand on any existing bracing element.

I'm currently designing the structural system for an addition to a
single-story '20's era unreinforced, ungrouted, concrete block residence.
I'm requiring that the walls of the addition be reinforced concrete block so
that I know that the rigidity of the new construction matches or exceeds the
rigidity of the existing.  I plan to detail the interconnection of the new
and existing so that there is a designed load path to the new bracing system
from the existing construction for loads for which the bracing path is
indeterminate, and the loads may be shared between new and existing bracing
elements.

The roof diaphragm is not bolted to the tops of the existing masonry walls.
I have recommended that bolting of the roof framing to the tops of the walls
be included in the project.  The Architect has said that that is not in the
budget (the house has a tile roof that is not to be removed and bolt
installation, if it is to be done, should be done through the roof), but it
is to be presented to the Owner as a recommended option.  I don't believe
that there is a Code requirement to bolt the roof to the walls provided the
lateral forces between the walls and the roof are not increased.

Your proposed approach seems reasonable to me.

I don't believe FEMA 273 is applicable -- it deals with seismic
rehabilitation of the existing building (which you are proposing not to do)
and would only consider the addition as it relates to the upgrade of the
seismic strength of the existing structure.

Nels Roselund
Structural Engineer

-----Original Message-----
From: Dennis S. Wish <dennis.wish(--nospam--at)gte.net>
To: aec-residential(--nospam--at)polhemus.cc <aec-residential(--nospam--at)polhemus.cc>
Cc: SEAINT Listservice <seaint(--nospam--at)seaint.org>
Date: Wednesday, October 25, 2000 1:22 AM
Subject: Designing a remodel - how to treat the existing structure


>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