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RE: Residential alterations, is upgrade for wind required?

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Jeff,

For the most part the loading codes are based more on economics and risk
rather than life safety issues. So depending on how pedantic the building
officials are, you can try using the extra detail in the commentary to ASCE7
to gain reduction in wind loading. For example terrain exposure averaging,
if exposure is varying on approach to building. Also consider variation of
wind speed with direction.

Then there is risk and life expectancy. The 50 year mean return period wind
speed is some what meaningless relative to the performance of the building,
it is otherwise factored. Whilst the ultimate strength limit state wind
speed with mean return period of 500 years, is the speed at which expect the
building materials to yield and permanently deform, such that after the
design event the building is no longer serviceable. It is based on a 5%
probability of exceedence in 25 years, or roughly 10% in 50 years. (for 5%
in 50 years need a 1000 year mean return period. Which we had here from 1989
to 2002, but housing industry complained.)

An existing building has a lower remaining life expectancy than a new
building. Therefore maintain the risk at say 5% and reduce the life
expectancy to suit some estimate of the desired life of the building after
the renovation.

Alternatively try working backwards, you know the resistance of the existing
structure, calculate the wind pressure required to fail it, then convert to
wind speed and then convert into mean return period, then from mean return
period and risk of 5% calculate the life expectancy. When the answer comes
out at say 2 years, see if owner wants to avoid upgrading resistance for
wind loading. One problem is convincing the owners the mathematics is valid,
when building has been there for far longer in the first place.

Another problem: unlike AS1170.2 I don't believe ASCE7 provides the wind
data or mathematical models for varying return periods.

In any case the approach is useful for assessing existing structures.

Also if the building lacks resistance I don't understand the need to remove
plywood sheathing and replace. There should be some way to add some extra
resistance: steel strap cross-bracing on inside face, or timber diagonal
struts between walls studs, and then there are various proprietary steel
bracing systems (eg. Mitek: Posi-Brace). These may not be ideal, but I
assume the existing structure has most of the resistance required. But then
I don't design diaphragm boxes so I may be on the wrong track: I'm assuming
the plywood sheathing only goes on the external face of the timber frame and
that discrete braces could be added from the interior. Also steel strap and
brackets are typically accommodated on surface behind plasterboard.


MiTek:gangnail
http://www.mii.com/australia/



Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia
 



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