Modeling floor bracings in RISA 3D 1

[Jatfuentes]

Hi !

Some of you guys, knows how to avoid that the floor bracing
take loads from the area load in RISA 3D.
Obviously bracings don´t work in bending, but my results in RISA 3D, shows bending moments due to the area load I´m applied to my floor. What can I do in order to avoid this problem ?
Thanks
jatfuentes

 

[Ron]

I'm only marginally familiar with RISA 3D, but it sounds like you have some joints/nodes restrained that shouldn't be. Check the releases and see if that helps. Also animate the model to see if it is moving as expected.

Also, don't forget that your modeling doesn't dictate the reaction of a structure....the structure will react based on its loading and connections.

JAE??? You're the RISA Guy....

 

[JAE]

If the diagonal "diaphragm" braces that you placed in your floor are at the same vertical elevation as the floor members, and you add an area load, the load will be automatically transmitted to these diagonals...its simply part of the program code.

I'd think you might be able to try a few options:

1. Model the diagonal braces with their proper area, A, but model them with a very very small Ix such that they simply may not take as much load. I'm not sure this will work because I think the area load is apportioned to members, not based on relative stiffness, but simply on their tributary area.

2. Model the braces at a slight (0.25") lower elevation. This will require double nodes at the ends of the braces and some sort of fictitious rigid member to tie the braces and the floor framing together. This may allow the load to "skip" the lower braces and proceed as you want.

3. Don't use the area load generator - simply apply distributed loads on each member manually. This takes a little extra time - but not that much - RISA is very fast to use and input data.

4. Finally, call RISA direct - or email them. I've found that they are very helpful.

http://www.risatech.com

Ron calls, I answer :-0

 

[Jatfuentes]

Thank´s JAE and RON

JAE, I think the third option is more easier.
I will apply the load directly on the beams in the places where I have diagonal floor bracings.
RISA could improve this problem that come´s since version 4.5. Ex " IF the element is a bracing floor,
THEN Ignore. "
Thanks again .

Jfuentes

 

[Ron]

Hey JAE...I knew you'd be able to help as always!

I'm going to upgrade my structural software this summer and have narrowed it to RISA 3D, STAAD-Pro, and GT-STRUDL, with RISA holding the lead at the moment.

 

[JAE]

Ron - I've used RISA, of course - and in a past life used Mcdonnel Douglas Strudl-Pro - which was bought out by GT-Strudl back in the late 80's. Strudl Pro was one of the first graphical-input programs that structural engineers could use. We also used STAAD along side Strudl-Pro on a large stadium project to compare results. We ended up with a LOT more trust in the Strudl program (at the time we only had RISA 2D). Both Staad and Strudl are, or were, batch based programs and used essentially the same root solver (essentially both use the original STRUDL code).

I don't know much about GT-Strudl and Staad as far as today's versions go - just a little long-ago experience that perhaps jaded me against Staad.

 

[Ron]

Yeah, similar experience with STAAD. I did use the first version of STAAD Pro a few years ago and found it to be much improved over the older versions, but still buggy.

Have played with the RISA 3D demo and it looks like a pretty good program. Easy input and plenty of options for output, though it currently doesn't do aluminum code check and that's about 80 percent of my structural analysis. That's OK though, 'cause I'm not much on code checks by software anyway. Still prefer to do a fair amount of manual post-processing...shows my age!

Sorry Jatfuentes....didn't mean to hi-jack your thread!

 

[JAE]

Ron - with RISA its really easy to copy/paste their output into spreadsheets if you have aluminum design Excel templates already made up. Even if you don't, you can cut and paste into a spreadsheet to use for hand calcs.

 

[Ron]

JAE...thanks. I'm really leaning that way. I was impressed by the demo ability.

 

[mrMikee]

Ron and JAE,

I'm not an experienced spreadsheet user, but isn't it fairly difficult to program a spreadsheet to do code checking when there are many load cases. Just curious, I too have been reluctant to do much work with built in code checking, and instead continue to use my own calculation program.

Regards,
-Mike

 

[JAE]

Mike,

I've written many spreadsheets that cover multiple load combinations. For some RISA models I've made, to strictly satisfy the code, there can be a LOT of combos to check -

Plus or minus Wind in one direction, in the other orthogonal direction, with roof live, without, with crane loads, without, plus/minus seismic, plus/minus seismic with overstrength factors, etc. Its amazing how many get generated. But a spreadsheet can be set up with multiple columns to check all of them if needed.

I also wrote a nice big spreadsheet that allows me to model a concrete frame in RISA and copy/paste in a moment envelope of axial, shear, and moments along the span of beams or joists and then determine all the reinforcing for that span. Concrete makes multiple load combinations easier as you can design for an envelope rather than having to check each and every combination like in steel.

 

[mrMikee]

JAE, thanks for the comments.

Over the years I've written many programs in BASIC, then QuickBASIC, and now VisualBASIC, the result being that I never learned spreadsheets. Recently I have become interested in Mathematica and while extremely powerful it is sometimes awkward in my opinion for engineering type programming. I occasionally wonder if I should have done some of this in a spreadsheet.

I agree about how easy it is to end up with many load combinations. I just design elevated bins now, but analyze these structures for either 22 or 26 load combos depending on whether orthogonal seismic is required. If you put two bins on a common structure and consider that one may be full while the other is empty, or both may be full, will at least double the load combos. Because of load combos even simple structures can be difficult to do by hand.

Regards,
-Mike