Progressive Collapse Check of Existing Building - RISA or STAAD? 1

[steellion]

Project is a cast-in-place concrete building built in 3 stages (1942, 1944, 1955). I need to perform a progressive collapse check on it. Out of RISA-3D and STAAD, which program would be better suited to do this analysis on an existing structure? It seems to me that you do not put reinforcement into the RISA model, and you would in STAAD. All things being equal, I prefer RISA, its user interface is just more friendly.

 

[structuresguy]

I don't know of a way to manually enter reinforcing in Staad, so no way to check strength of existing structure directly.

 

[Stazz]

I've done a progressive collapse analysis of a concrete building in Risa. Concrete is good because its so redundant. Depending on the level of progressive collapse protection you need, you might not even need to run the computer analysis. We only did it in Risa just to make a pretty picture for the client.

In the concrete building we did, it was a barracks and everything was an 8 inch wall so we justified the walls acting as very deep beams to transfer the load around a missing wall below. This was done easily by hand.

 

[JoshPlumSE]

When I hear "progressive collapse", I'm thinking non-linear analysis, moment re-distribution due to plastic hinging, that sort of thing. Do I understand your requirements correctly?

To do that sort of thing in a program that is essentially linear elastic (like RISA or STAAD) would be something of a challenge. You can probably do it... It's just more work and book keeping than you would be used to. Essentially, you'd be doing multiple seperate analyses and combining the results on your own (using Excel?) using principal of super position.

This type of analysis requires a lot of book keeping and can get pretty tricky. The programs that specialize in it (Perform 3D from CSI comes to mind) are sort of niche programs and tend to be relatively expensive.

 

[OCI]

Nonlinear adaptive pushover is the way to do this. Take a look at a product called Seismosoft. It's a fiber based RC software for exactly this purpose. It's available for free download for non-commercial and research uses. I'm not sure what the cost is if you are going to use it commercially, but it's state of the art and should be less than proform3d.

 

[OCI]

I just recieved an email from the SeismoSoft company stating that they do not have a commercial licensing policy set up yet and that for now the software may be used commercially free of charge.

 

[msquared48]

Why not Etabs?

Mike McCann
MMC Engineering

 

[OCI]

Etabs would work fine and as far as I know is the industry standard. The program I mentioned uses a fiber based element which has a few advantages (and drawbacks)

- Because the beams and columns are modelled with fibers representing the steel, confined concrete and un-confined concrete, no moment-curvature analysis is necessary to determine the properties of plastic hinges.
- Shear stress and strains are not modelled currently.

I only mention it because it is free and would be interesting to take a look at if a guy had a project to use it on.

 

[steellion]

RISA and STAAD are the two programs that we have in-house. I realize that they do about the same thing (linear-elastic modeling), and I also realize that a non-linear program such as Etabs would be preferential.

But as long as it is possible to do in RISA, even if it requires keeping track of progress separately in Excel, that's the way we need to go. It's a very fast track program, and I don't have time to look into purchasing new software nor to become familiar enough with it for it to be an effective tool where I'm confident with the results.

You can't put in reinforcing, but you can use it as a 3D analysis tool, find the maximum +M and -M and compare it to the capacities of the beams in place.

Thanks for your help, everyone!

 

[aggman]

Aside from the issues of doing a linear elastic verses a non-linear analysis for this structure RISA does allow you to specify a rebar layout for beams and columns. Go to the "Modify" menu and go to the bottom of the list. You will see "Create and Edit a Rebar Layout". Once here you can specify a name for your rebar layout, and the number of bars, their location, and length. You can do the same for the shear bars. Once this is done you can go to your "Members" menu and assign the rebar layouts for shear and flexure to your concrete beam/ column. Then when the program analyzes the beam/ column it will do a code check based on your rebar layout. You could then check when the member forces actually exceed the ultimate capacity and adjust from there. I am not sure what STAAD can do, I have never been a big fan of the program because it always seems buggy, but you can specify rebar arrangements in RISA and it will do the code check based on that layout. RISA has had this capability since version 7.0 or so I believe.

 

[OCI]

To do this without a nonlinear software as JoshPlum pointed out sounds like a challenge. I think you would need to do multiple analysis runs with an increasing load factor ("progressive" collapse), and insert some hinges or short bar elements to represent plastic hinges as they develop. Sort of a "poor mans push over" if you will. Either way I would consider using SeismoStruct to compare my results to.

 

[SteveGregory]

STAAD allows you to do multiple analyses in a single run. You could make a column member inactive for one analysis, then restore that member and make another inactive for the next analysis and so on.

 

[steellion]

You can make column members inactive in RISA as well. As far as I can tell, that's the main difference: in a linear-elastic model, you need to remove the column from the analysis at the very beginning (the so-called "immaculate removal"), while with a non-linear program you have the ability to time-step loads to better simulate a column suddenly removed by blast or other reason.