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Re: Engineering judgment

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While I don't disagree with Alex's and Irv's comments, I think you should probably do a quick check of shear and bending on the continuous footing.  

Do you know that there are 2- #3's in the footing for bending resistance?  Is that enough for the add'l moment due to the concentrated load?  I think it would check out.  Otherwise, add on to the footing from one side (or both sides) to make an "isolated" footing.

As for the shearwall, yes, the 6-foot length is asking a lot for an upper floor.  Aside from collectors along the header/plate line to drag the force into the shearwall, you also have to deal with the uplift and footing load transfer.  Double check the redundancy factor, rho, but the 6-foot wall should still work if you make it with a capacity that's about 25% higher than the demand.

Possibly some of your boss' concerns relate more to quality of construction...  The design loading, your structural detailing, and Simpson's catalog, assume ideal and perfect construction.  The thing is that any knucklehead can swing a hammer, and many times they miss...

On Thu, Mar 11, 2010 at 6:06 AM, IRV FRUCHTMAN <ifaeng(--nospam--at)yahoo.com> wrote:
Eric,
It seems to me that you need to be conservative when dealing with existing construction. Do you really know concrete strength or if soil has eroded beneath the footing, etc etc. without spending a bunch of $$? Also you might calculate the footing's shear and moments and the resulting soil pressure especially if the point load is significantly off center.
Irv   

--- On Wed, 3/10/10, erik_g(--nospam--at)cox.net <erik_g(--nospam--at)cox.net> wrote:

From: erik_g(--nospam--at)cox.net <erik_g(--nospam--at)cox.net>
Subject: Engineering judgment
To: "seaint" <seaint(--nospam--at)seaint.org>
Date: Wednesday, March 10, 2010, 10:59 PM


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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David Topete, SE