Where to appropriately apply lateral forces? 1

[AmirZamanian]

Why do we assume that the roof lateral forces act at top plate level and not at the center of gravity of the roof?
Since the center of gravity is somewhere above the top plate level, it seems logical to assume that this force acts with a moment arm that corresponds to its center of gravity (above top plate). However, every set of calculation that I have reviewed calculated the uplifts by applying the wind or seismic load at the top plate. This might be insignificant when the building has a relatively flat roof or when it is amply fortified and conservatively designed. On the other hand, when the roof is steep and the design is lean, applying the force at center of gravity would produce significantly different outcomes.
Why is it that municipalities and engineers check building structures with a loading condition that underestimates the uplift forces? Would someone tell me what am I missing?

 

[apetr26542]

I might be mis understanding you, but I've always used the "mean roof height"

 

[kxa]

The shear load is applied where the support is and that is the top plate or the ceiling diaphragm. As far as the moment goes, you might even have different loads over the vertical projection of the roof and you should always use the appropriate moment arm for each load. One more note about the ceiling diaphragm to point out is that you should also include half the wall height lateral load from the wall below.

 

[AmirZamanian]

Thank you apetr26542 (Structural)
Given what you stated, what do you mean as the mean height? In other words what do you chose as the moment arm for a one story building with top plate at 10feet and mean height of the roof at 8 feet above the top plate?
Lets say the weight of roof is 5 times the weight of that portion of wall to be applied at roof level.

Lets say:
Length of shear wall= L

And let’s say:
item Shear Height
Roof 5V 18
Wall V 10

Then:
Is Uplift = (5V*18+v*10)/L, or is Uplift = (5V+V)*10/L?

 

[apetr26542]

I use the vertical projection of the roof surface, my wind load componenet is applied at mean roof height, my horizontal wall component is applied at top plate, your first equation, mean roof height is the average height of the roof from top plate to ridge. If, by your numbers above, your mean roof height is 18ft then your ridge would be at 26ft.

 

[AmirZamanian]

Looks like that what I have been doing all along is right. The answers above seem to confirm that we need to keep the roof vertical projection into account when calculating uplift. Funnies that it looks like that some of our colleges seem to differ on this practice.
I have lost two good clients to those of my fellow engineers that don’t consider the vertical roof projections when estimating uplifts. Obviously, their uplifts are lower than mine are. That results into higher uplift forces and consequently heavier foundations. My client uses a number of engineers and he has picked up on this issue he has asked me why I come up with bigger hold owns and larger foundations. He does not care about my reasoning and he reverted to other engineers.
I am caught between a rock and a hard place. Do you have any suggestions for me? What are my options? Has any one face this and if yes what has been the cure?
thanks

 

[kxa]

Many times. Clients always want to redefine engineering to their advantage at our expense (liability). You just have to do the right thing and not be influenced by them. If they know better, why do they come to us in the first place. They are just looking for somebody who does what they want and assumes the full liability.

As long as there are engineers out there that are willing to do things for the money or just simply don't know any better, there won't be any cure.

Bottom line, try to reason with them and educate them when possible. If that does not work, you are better off not dealing with those client.

 

[apetr26542]

Agreed, kxa, it is not worth loosing sleep at night. Not every client is a great client. Amir Z, are you considering the dead loads of the shear wall and and partial floor dead loads, 0.9D+W. This would reduce your overturning moments.

 

[AmirZamanian]

Does any one have any suggestions for dealing with this? Do I go to the building departments and ask them if they sanction the practice that my competition engage in? Do I just let my clients go off to those that are exposing themselves as well as my client to potential liability? Am I obligate under the board rules to alert authorities to this practice? Do you suggest I take the bull by the horn or pretend that I don’t know what I suspect?

There is a wide spectrum of avenues this issue can be approached and dealt with. The way I see this, I would be unfairly competed against as long as there are other engineers who are willing or unaware that they are assuming the liability associated with shorter moment arms for wind and seismic case as discussed above. I may point out that this practice expose everyone involved to potential risks. In support of exposing this practice, I have an ethical dilemma and continence to deal with. I may also choose to underplay this and take the complacent approach. This wouldn’t rock many boats and wouldn’t pull chains.

On the other hand, my predicament cannot be a unique and I cannot be the first engineer to come face to face with such dilemma. I am hoping that I can draw on someone’s experience in similar position.

I appreciate any comments and suggestions on how to deal with this seemingly no so unique situation.

Thanks
Amir Zamanian