Double bay Braced Frame loads not equal

RE: Double bay Braced Frame loads not equal

Seth Guthrie — Fri, 04 Sep 2020 20:39:35 GMT

When you have a two bay braced frame with tension-only members under lateral load some of the braces drop out as they go into compression and the end result is asymmetric deflection and the brace on the leading end takes a larger force.

Showing the deflected shape with consideration for both vertical and lateral deflections helps to explain why. Here's an example:

RE: Double bay Braced Frame loads not equal

VAEngr — Thu, 03 Sep 2020 14:04:23 GMT

Seth,

I removed the podium level and below to test your theory. I did not change anything else. At bottom level, one of the braces takes all the load, while the other one has zero axial load. see below. under load case +y (wind), I get a negative shear at level 2 brace. See image below.

RE: Double bay Braced Frame loads not equal

Seth Guthrie — Wed, 02 Sep 2020 17:31:25 GMT

I see, your picture does not include the structure below the frames (podium). Since the frame sets upon a slab the relative rigidity of the two points influences the forces in the braces above. In other words, the diaphragm constrains the nodes to move together horizontally, but not vertically, and if the vertical deflections vary then the brace strains are different, hence the stress and force too.

RE: Double bay Braced Frame loads not equal

VAEngr — Wed, 02 Sep 2020 17:16:11 GMT

It has been uploaded.

RE: Double bay Braced Frame loads not equal

Seth Guthrie — Wed, 02 Sep 2020 16:47:40 GMT

Send me your file. Secure File Upload

RE: Double bay Braced Frame loads not equal

VAEngr — Wed, 02 Sep 2020 16:44:11 GMT

The load is axial load in the brace due to wind load. The frame is symmetric and the diaphragm is rigid.

I would expect the loads to be the same as well, that is why I am not 100% sure of the results.

Do you have any suggestions on how to proceed?

RE: Double bay Braced Frame loads not equal

Seth Guthrie — Wed, 02 Sep 2020 16:39:43 GMT

Is this the axial load from a lateral load case? If the frame is symmetric (same dimensions and section sizes) and the diaphragm rigid, then I would expect the results to be symmetric as well.

Note, most users would go ahead and model the beam as lateral here, it makes it easier to do things like pull up an elevation view.