RISA 3D Plate loading vversus distributed load

[AuEng99]

I have attached two files with the same structure. For simplicity in analysis, it's a flat roof with 2x6's.

One is plate loaded with a dead load = 10 psf and a live load = 20 psf. However, for the plate loaded structure, member 3A, the moment z = 0.275.

The other is a distributed load that is attempted to be modeled the same as the plate loaded one. However for the distributed loaded structure, the moment z = -2.757.

Why are these two moments so far off?

 

[AuEng99]

Here is the distributed load file. I can only attach one per post.

 

[bookowski]

A plate will span over/across members to reactions/stiff members, some of the members will probably take little/no load. Use 'member area loads' if you want a uniform load that is distributed to frame elements (unless you truly have some kind of plate that you need to model).

 

[Splitrings]

Your plate loading is being distributed in both directions, some goes to the beams and some goes directly to the girders. Your member loading is being applied only in one direction, load is applied to the beams and then to the girders through the beams.

Look at using member area loads. You can distribute the loading either two way (as your plate model is) or one way.

 

[Splitrings]

I am not sure what you are modeling but it appears pretty unusual. You have fixed boundary conditions at the top and bottom of your columns and you have release the bending moments at both ends of the columns. Are you sure this what you want?

 

[AuEng99]

Splitrings,

Thanks for the help! I readjusted the file based on your recommendations. For the distribution in member area loads, I selected the direction perpendicular to A-B. Since I was modelling a shed that was supporting 2-2x8 girders by poles in the ground, I removed the fixed conditions for both the member j ends and the joints.

 

[AuEng99]

And here is the adjusted distributed load file.

 

[JoshPlumSE]

The Member Area Loads in RISA-3D using an approximate and numerical load attribution scheme that is highly dependent on the Global Parameters setting for "area load mesh size". In your model this value is set to 144 square inches (which is probably the program default). I tend to recommend using a value closer to the square of 1/10 th the trib width. This would be something like 5.76 in^2. That will make a big difference for the area load attribution for your model.

You can always graphically view the "transient area loads" basic load cases that are created to see how good (or bad) a job RISA did with attributing the area loads. Those BLC's show you exactly what distributed loads are applied to each member based on your area loading. We do this so that you can review them rather than just accept that the "black box" is working properly.

Note:
You are also using an older version of the program (8.1?) that uses a significantly less sophisticated method of area load attribution. So, you are likely to get some variation of the distributed loads between adjacent members and along the length of the members. The latest version (12, but soon to be 13) would likely provide superior area load attribution results to what your seeing.

 

[AuEng99]

Thanks Josh! Splitrings, did I get the member area loading correct by going with perpendicular to A-B?

I have attached a new copy of the file. I resolved it with Josh's recommendation and the suggested members. It came up with some odd shapes like 5x8 and 2x2. I mean who really uses sizes like that? It also recommended different members in similar areas such as where I showed the 2x6's. Does anyone know why it did this?

 

[Splitrings]

My guess without looking at your model is that you have not "told" RISA that your member is braced. It is assuming your member has a high le2/d ratio. Look at page 561 of the RISA General Reference.

 

[JoshPlumSE]

Also, I wouldn't use the Top of Member offset for those beams if I were you. That is causing them to develop some axial force that I don't think you really want them to have. Some in tension, some in compression. I believe that difference in axial force is the reason for the program suggesting different sizes for members with similar moments.

 

[AuEng99]

I have attached a model of a storage building that I built years ago. It has 3-2x8' on top of 10" diameter poles. Above the 3-2x8's there are 2x6's and then 2x4's. I want to know if my model is a good representation of the pdf contained in the next post. Additionally in the 2x6's, I would like to know if I have the Le1, Le2, and Ltop and Lbottom specified correctly on these members?

 

[AuEng99]

Here is the pdf of the storage building.

 

[JoshPlumSE]

Obviously, I can't do a real thorough review of the model. But, there are a few things that strike me as strange. Not necessarily wrong, but items that I would look at closer if I were the supervising engineer and someone brought this to me as their model. Enough things that I doubt whether this model is a good representation of the structure.

1) What's the purpose of modeling in the 2x4's. That's little more than blocking, right? Not sure we should be relying on them to share load in the analysis. That's not necessarily wrong, just not something I would normally do. Plus, it seems like there are more of them in the model than in the drawing.

2) Area load mesh size is probably too small, especially for the old version you're running. It doesn't really make a difference in the version that I'm running. But, I think that is mostly luck based on the errors canceling each other out on each side. I would prefer to see a closer of 5.76in^2 rather than 144in^2 like it's currently set to.

3) Those 2x4's are laying on top of the 2x8 rafters, right? That's probably enough to consider Le_bend-top braced at each 2x4. But, I don't know if it is enough to brace the rafter for weak axis buckling (Le2). And, it is definitely not something that I would rely on for bracing strong axis buckling (Le1).

4) Your load combinations don't seem correct for NDS design. Those are all strength level load combinations rather than the typical NDS service level load combinations that RISA is expecting.

5) You have most of your members with fixed end conditions. That's possible if you have a steel beam with the flanges welded to the column or such. But, I don't know that you can create that type of moment fixity with a wood beam.

 

[woodman88]

I am at home, so I do not have risa to look at the r3d files. But have you check that the total reactions for each of the designs match?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[AuEng99]

Josh,
The 2x4's are nailed to the top of the 2x6's every 4 feet. This is to provide members for attaching the roofing tin with screws. It also provides an easier and safer method for walking on the 2x4's while installing the tin. Also, the ridges on the tin are parallel to the length, so the tin can not run perpendicular to the 2x6's for drainage purposes.

 

[JoshPlumSE]

My point about the 2x4's is not whether they are necessary for construction, but whether they are necessary for the model.

The load is attributed (via the span direction of the area load definition) directly to the rafters. So, why are the 2x4's supposed to be doing in the model? I would think you would design the rafters as if they supported their tributary load with some modification to the unbraced lengths to reflect the presence of the 2x4's.

However, by modeling in the 2x4's you are creating a 2-way bending scenario where the 2x4's pull some load out of some rafters based on deflection compatibility. That might legitimately happen, but it's not something I would usually rely upon if I were stamping the drawings. It's not wrong if you did it intentionally. It's not wrong if you understand the reason why you would model it that way vs the reasons why you might choose not to. Essentially, it should be a conscious design decision on your part.

 

[AuEng99]

Josh,

I included the 2x4's in the model with the bracing because otherwise the le/d is greater than 50 for the UC Mas on the 2x6's.

For the 2x4's, I specified "S" for Segment for Le1, Le2, Lebend top, and Lebend. I don't know if I did this correctly? Does anyone have an example or material with examples of bracing for Le1, Le2, etc?

 

[Splitrings]

Check out page 501 of the Risa 3D General Rference Manual:

http://www.risa.com/documents/risa-3d/General_Reference_3D_V12.pdf

 

[AuEng99]

What I really need now is some projects to challenge me. I am a graduate civil engineer with a PE license. I am interested in creating these models on a part time basis. I feel like I can create the models remotely, so I will not have to be near a client to complete these. Does anyone have any ideas on how I can use the software to create a side income?