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Re: Engineering judgment

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For the footing, there is more than just the bearing pressure to be concerned about.  Can the strip footing handle the shear from a 4.5 k point load?  What about bending of the strip footing?  Even if the bearing pressure is fine, is there a concern about settlement or differential settlement issues?  Keep in mind that bearing pressure is not really a "this is the safe load it can support" value but more of a "this is the load it can support with acceptable settlement/compression of the soil".  Thus, the point load could cause some sort of differential settlement that might cause issues else where.

For the shearwall, again is it not just the capacity of the shearwall.  What about uplift forces at the ends?  How will those uplift forces be transmitted down to the foundation?  Can you achieve realistic connections from the end posts of the shearwall in tension to something at the lower level and then from the lower level into the foundation?  Same for compression post forces at the ends.  Can you design the diaphragm and/or collectors to fully get all the load into a 6 ft length of wall?  And as someone mentioned, what about construction quality issues (those shearwall code capacity values assume pretty much "perfect" construction)?

My point is that based upon the limited information your provided there seems to be things that you may not have looked at that could be why your boss changed things. 

As someone else suggested, ask your boss if he can explain why he wanted the changes that he wanted.  Phrase it not as if your are questioning what he did but that you are asking so that you can understand why the changes where needed so that you can learn from this and maybe "do it right" (so to speak) next time.  Not everyone tends to treat such situations as ways to teach stuff unless you "force" them to...they tend to focus more on "getting it done".

Regards,

Scott

On Mar 10, 2010, at 10:59 PM, <erik_g(--nospam--at)cox.net> <erik_g(--nospam--at)cox.net> wrote:

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.