Applying wind area loads to vertical braces
2

Question

Applying wind area loads to vertical braces

jmurph36 (Structural)

(OP)

4 Apr 19 21:02

I am applying an area load to an open lattice structure. But when I run the analysis and look at the transient loads, it doesn't seem to be considering the chevron bracing between columns on the lattice. I am pretty sure that, even though it is only L2x2x1/4 shapes, it still would be exposed and still needs to have wind applied to it. I have the member type set as vertical brace. When I change the member type to column, RISA applies the transient loads to the braces as well. Does anyone have any solution to this? I would like to keep the type set as vertical braces for seismic purposes.

JAE (Structural) · Answer 1 · 2019-04-04 21:25:58

I built a little mock-up model and got the same thing. Couldn't find anything on the HELP screens.
I think you just need to email RISA and ask them.

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jmurph36 (Structural) · Answer 2 · 2019-04-04 21:33:14

Glad I'm not the only one. I'm inclined to believe though that it may not be necessary to do so since RISA doesn't consider shielding when applying area wind loads onto open structures. The automatic distribution of the load on all faces on the plane will probably still give a reasonably conservative load.

WinelandV (Structural) · Answer 3 · 2019-04-04 21:43:26

It's in the help file, under Members -> Member Spreadsheet. Scroll down to Member Type, it's number 3 in the list:

"3) If you are using member area loads, loads will not be attributed to members defined as Hbraces or Vbraces."

That doesn't help you with what you're trying to do, but at least it's noted.

jmurph36 (Structural) · Answer 4 · 2019-04-04 21:45:34

Well at least I'm not crazy. In that case, does anyone have experience modeling open structures using area loads for wind? What are some tips for getting accurate results?

Celt83 (Structural) · Answer 5 · 2019-04-04 21:46:56

shooting from the hip calling it a brace is probably limiting the behavior to axial forces only which would be why calling it a column then has it pick up the wind pressure since it can then actually develop reactionary forces.

Edit: seems the manual hints at that based on Winelandv's post

Anyway to accomplish similar behavior as a brace with end releases and maybe modified stiffness's?

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JAE (Structural) · Answer 6 · 2019-04-04 22:13:26

Nice find winelandv!!

I'd tend to just leave the braces as "vertical braces" in RISA and manually add a distributed wind load to them.
Not that difficult if you have all similar brace widths and you can select them all and attribute the distributed load to "all selected members" at once.

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JoshPlumSE (Structural) · Answer 7 · 2019-04-05 17:13:39

Note: This was left over from the original design of area loads. There wasn't an "open structure" option originally. So, they were meant for Live load and roof live load applied to floors, or wind loads on horizontal girts. Then it made sense that the horizontal bracing and such shouldn't get the area loads attributed to them.

Then we added in the open structure area loading. It was built as a sub-set of the existing function, so it had all the same limitations. Makes sense for a developer, but not for an engineer using that feature. I'm pretty sure I wrote up an enhancement that would make the open structure area loads behave different from the regular area loads in this regard..... I'm pretty sure that enhancement request has a good number of users associated with it.

So, why hasn't it been done yet? Well, I'm not there anymore, so I can't say. I have two theories based on my guess about how life works at the NEW (but perhaps not improved) incarnation of RISA / Nemetschek.

1) My write up might be a sub-item on a larger feature related to revamping open structure wind loading. i.e. to include shape factors, shielding, only consider one plane, consider an icing or fireproofing area for various members, et cetera. You can see how it could get lost in that larger feature. It would help to ask them about it and see if it could get done sooner without all the other hoopla.

2) While it would be a very easy change to make, if there isn't a revenue based reason to do it (i.e. can they sell more copies by advertising this new feature), then it may get pushed down the line a bit. Remember, the management over there is entirely composed of the old Sales / Marketing staff. They are engineers, or they were at one time. But, most of the senior technical folks were replaced. So, there may not be a real "champion" for users anymore. A lot of good, recently minted PE's there. But, not many with a lot of real practical engineering experienc. that ave the clout to stand up for technical issues.... Or, the ones who do have some clout may be reticent after seeing the folks (like me) who used to be the ones to stand up for such things get pushed out the door. Not saying that's why we got pushed out the door. Just saying that it could have left an impression.... squeaky wheel gets the axe, that sort of thing.

Obvious Caveat: I don't particularly want to get sued by the new version of RISA/ Nemetschek. Clearly I am an ex employee with the kind of biases that an ex employee (disgruntled or not) may have. Also, I now work for one of RISA's competitors. Therefore, any opinion I have stated on the RISA program or the Nemetschek company/ subsidiary have to be viewed with the understanding that I even may have a financial incentive to be unfair. Personally don't believe that I'm bein unfair. But, I have to acknowledge that we are not always the best judges of our own neutrality.

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Applying wind area loads to vertical braces
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