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- To: "seaint" <seaint(--nospam--at)seaint.org>
- Subject: Engineering judgment
- From: <erik_g(--nospam--at)cox.net>
- Date: Wed, 10 Mar 2010 19:59:47 -0800
have a question about engineering judgment or better yet, engineering comfort.
Let me explain what I mean with 2 typical examples:
A small residential addition where the owner is adding a new roof. The new
ridge beam is supported by a new post onto the existing continuous footing. A
soils report was conducted and the allowable soil bearing pressure was
determined. You know the existing continuous footing size (15" wide x 24"
deep) and based on the allowable soil pressure of 2000psf you have
determined that the new 4.5k point load onto the existing ftg will be no
After your boss reviews your work he tells you to use
a new pad ftg under the existing ftg at the location of the new post.
You ask why and he tells you that 4.5k is a lot of load
for a continuous ftg. You are left scratching your head because by your
calculations as the 4.5k spreads out at a 45 degree angle from the top of the
ftg the increase in soil pressure will is only 900psf.
Another residential remodel, but this time the architect is removing a lot of
the existing exterior shearwalls in order to get "the view." You conduct a a
full lateral analysis and you determine that a 6ft section of shearwall
located on the exterior of the upper floor of a 2 floor home is
adequate to resist the designed seismic force. Since the force on the shearwall
is close to the maximum allowable shear force for the lowest rated shearwall on
your shearwall schedule which is based on the current CBC, you upsize the
shearwall and specify a higher rated shearwall.
Now your boss reviews your work and he says that he
doesn't like the fact that there is only a 6ft section of shearwall along the
exterior of this residence, but you show him your numbers and everything checks
out. He insists that the architect give up a couple more feet of shearwall,
or provide another shearwall along that line.
Both situations I am dumbfounded because my numbers
make sense and in these cases I do not understand his
I have enough experience to know that just because
the numbers show one thing doesn't mean that the solution is practical, but in
these 2 situations I just can't see the logic.
Any comments would be appreciated, and sorry for the long