LAVA automatically applies the Omega factor to beams and posts that support shear walls.
However, it is typical that the governing Load Combination does not contain the Omega factors. When a beam that is carrying a shear wall above and large portion of roof and or floor area, Dead load plus Live load will govern instead of the seismic load combinations. You can toggle between Load Combinations in the Beam dialog to see all of the results per load combination described further below.
Checkout a video showing this in action: Seismic Load Effects on Beams
The Load Groups shows the type of loads applied and Special Seismic is checked on by default.
.png "image(163).png")
The Omega factor is shown in the Project Definition tab- Criteria- Seismic tab. The Omega factor is editable by the user.
.png "image(120).png")
The Load Combinations used for the Seismic Load Effects are shown in the Database- Loads- Load Groups.
Note: The Omega factor is applied by noting it as Em in the Load Combination in the last term.
.png "image(165).png")
Per ASCE7 2.4.5, there is an allowable stress increase factor of 1.2 used with the seismic load effect. It is applied in LAVA as the reciprocal on the load application and shown as 0.83 in the Sizing tab of the beams and posts.
Omega Factors on Posts
In LAVA, Posts refer to the beam supports and the Shear Wall end posts. You can control whether Omega Overstrength factor is applied to the Sole/Sill Plate Bearing calculations
.png "image(166).png")
Omega Factor on Beams
In LAVA, the Report and the beam calculations display the governing load combination in order to keep the report concise and to follow engineering convention. However, you can review all the load combinations that are analyzed from the Sizing tab in the Beams dialog.
Just click on the Load Combination drop-down and select a LC with Em listed. Click on the .png "image(123).png")
button to Solve that LC and review the results on the right side screen.
.png "image(121).png")
Example Beam Problem
Let’s review an example with seismic load with Omega governing, with Omega = 2.5, and Sds = 0.8:
This model has a shear wall (SW.RF0.2) on 2nd level as screenshot below. The seismic reaction at shear wall end post is high-lighted, E = 1204.39 lbs.
.png "image(156).png")
There is a 8’ long beam (BM.FL1.2) on 1st level below the shear wall shown in screenshot below. The reaction of SW.RF0.2 lands on the center of this beam (4’ from supports).
.png "image(157).png")
The loads on this beam are shown below:
Self weight D = 5.88 plf
Shear wall end post reaction W = 732.15 lbs
Shear wall end post reaction E = 1204.39 lbs
Governing load combination is D±0.7Em. D = 5.88 plf; 0.7Em = 0.7 x (Omega x E ± 0.2Sds x D).
.png "image(161).png")
Per ASCE 7-16 2.4.5, Allowable Stress Increase for Load Combinations with Overstrength: allowable stresses are permitted to be increased by 1.2. Therefore 1/1.2 x (D±0.7Em) = 0.83 (D±0.7Em) can be used to calculate design moment, shear or deflection. This example verifies the maximum positive and negative moments.
Distributed load; w = 0.83 x D (1 ± 0.7 x 0.2 Sds) = 5.43 plf or 4.33 plf
Design moment due to distributed load; M = 1/8 x w x L2 = 43.44 lbs-ft2 or 34.64 lbs-ft2
Point load; P = ±0.83 x 0.7 x Omega x E = ±1749.38 lbs
Design moment due to point load; M = 1/4 x P x L = ±3498.76 lbs-ft2
Design positive moment = 3498.74 + 43.44 = 3542.18 lbs-ft2; design negative moment = -3498.74 + 34.64 = -3464.1 lbs-ft2.
The result by LAVA is high-lighted below in the Report.
.png "image(162).png")