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1
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JAE
Structural
- Jun 27, 2000
- 15,462
We've been designing a brace replacement for an existing building and came across an issue that created a bit of debate. Here it is:
In most codes (and I'll speak off of the BOCA Code as that's what we were using), the seismic design sections include a limit state to check whether PDelta forces need be considered. In BOCA 1999, section 1610.4.5.2, it provides the typical equation for Theta:
Theta = Px x Delta / Vx x hsx x Cd
Where Px = The total vertical design load at story level x.
Delta = The design story drift occurring simultaneously with the story shear, Vx
Vx = The seismic shear force between levels x and x-1
hsx = The story height below level x
Cd = The deflection amplification factor.
The question is this:
When using this equation, the value of Vx is easily computed as the seismic shear force from our typical base shear (static load method). This is the total lateral force on the story due to the masses within the structure which typically ONLY include dead load and, in some cases, snow (if greater than 30 psf) and storage loads. In our case we had minor snow loading so the snow was NOT included in developing Vx.
In determining Px, however, the code states that it is the TOTAL vertical design load and refers to the standard load combinations at the beginning of chapter 16. This would imply that Px includes the snow weight.
However, this doesn't seem to make sense as the equation is supposed to compare the lateral effect against the vertical effect. Shouldn't Px and Vx be generated from the same vertical load? Or should Px always include dead plus snow or live or whatever, and Vx only include the required masses specifed by the code?
In most codes (and I'll speak off of the BOCA Code as that's what we were using), the seismic design sections include a limit state to check whether PDelta forces need be considered. In BOCA 1999, section 1610.4.5.2, it provides the typical equation for Theta:
Theta = Px x Delta / Vx x hsx x Cd
Where Px = The total vertical design load at story level x.
Delta = The design story drift occurring simultaneously with the story shear, Vx
Vx = The seismic shear force between levels x and x-1
hsx = The story height below level x
Cd = The deflection amplification factor.
The question is this:
When using this equation, the value of Vx is easily computed as the seismic shear force from our typical base shear (static load method). This is the total lateral force on the story due to the masses within the structure which typically ONLY include dead load and, in some cases, snow (if greater than 30 psf) and storage loads. In our case we had minor snow loading so the snow was NOT included in developing Vx.
In determining Px, however, the code states that it is the TOTAL vertical design load and refers to the standard load combinations at the beginning of chapter 16. This would imply that Px includes the snow weight.
However, this doesn't seem to make sense as the equation is supposed to compare the lateral effect against the vertical effect. Shouldn't Px and Vx be generated from the same vertical load? Or should Px always include dead plus snow or live or whatever, and Vx only include the required masses specifed by the code?
