Hi,
I have a building with two orthogonal braced frame intersecting, which makes one of the column forms both lateral system in the orthogonal direction. Per ASCE 7-10, section 12.5.3.a, Orthogonal Combination Procedure shall apply (see figure 1 below)
Figure 1
Also, due to the number of bay of braced frame in each orthogonal direction (2-bay in y-direction and 1-bay in x-direction), per ASCE 7-10, section 12.4.3.2.b, I have used rho of 1.0 and 1.3 in x-direction and y-direction, respectively. (See Figure 2 below)
Figure 2
However, when I output the ASD load combination, I found out that, when RAM detected that the load case contains the force in the direction which requires rho of 1.3, regardless of whether it is in 100% or 30% due to orthogonal effect, the entire load combination gets amplified by rho factor. With that said, since I have applied orthogonal effect per code, basically all the load case will contain the x-direction load, regardless of whether it is in 100% or 30%, the entire load case was multiplied by 1.3. But then the advantage of reducing redundancy with 2-bay will be lost.(See Figure 3 below).
My interpretation of the load comb is that 0.91E_9 = 0.7*1.3*(0.3Ex+Ey) = 0.7*1.3*0.3*Ex+0.7*1.3*Ey. However, I believe the correct load comb shall be 0.7*1.3*0.3*Ex + 0.7*Ey.
It would be great if I can get some ideas how to solve this and take advantage of the rho=1.0 in y-direction.
Thank you!
Our default approach is to apply the 1X+0.3Y cases as a separate load cases (at angles), but it can only have one load factor. The alternative is to have loads in the X and Y only, but then generate custom load combos to get the combined forces at the desired angles. You can generate these as custom combinations and then adjust the factors as desired where the rho factor varies.
Answer Verified By: Jian Xiao
Hi Seth,
Thanks so much for your quick response and this is exactly what we are planning to do in the next stage.
A little suggestion is that if the redundancy factor can be automatically incorporated in the load case or the orthogonal factor can be applied in the load combo would greatly help with these situations. Manually altering all load combo defies the intention of having auto generation of load combos, which is a great advantage in the Bentley RAM SS, especially with hundreds of load cases.
Thanks again!