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Re: Engineering judgment

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Erik,

As a general comment, I see a lot of young engineers do all sorts of fancy calculations to prove out the stresses but often forget about member deflections and especially foundation settlement.  Highly loaded foundations with questionable soils can easily settle which can cause all sorts of distress in the structure above and any after the fact fix is extremely expensive.  You have to look at the potential disaster you might cause if you are wrong with your calculations.  I have that case right now on a project where I told the engineer to max out all the member stresses and soil bearing because in one structure it is not critical (i.e. a free standing concrete wall) however in another case I have told the engineer to use 50% of the allowable soil bearing because any settlement in a particular structure would be enormously expensive and possibly shut down some critical operating equipment.  It is certainly a judgement call based on experience and proportional to the number of times you have been involved in a lawsuit.

Thomas Hunt, S.E.
Fluor



<erik_g(--nospam--at)cox.net>
03/10/2010 07:59 PM
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Engineering judgment





I have a question about engineering judgment or better yet, engineering comfort. Let me explain what I mean with 2 typical examples:
 
1. 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 problem.

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.
 
2. 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 reasoning.
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 winded email.
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