Mat Foundation Design w/ STAAD PRO 2

[STR04]

All,

Has anyone designed a mat foundation with STAAD PRO and not pay the additional money for their Foundation Design module? I've been through the examples, but feel like I need a little more assistance (research) before I tackle the design.

TIA

 

[structuresguy]

I have designed several mats using Staad without any additional modules. I lay out the mat using plate elements set to the thickness of the mat. Then I create spring supports using the plate mat support type, which is located under the foundation tab. Make sure you define the springs as tension only in the vertical direction. To define the spring, you will need the Modulus of Subgrade Reaction for the soils. Then it is simply a matter of adding loads and running the analysis.

I have found it yields smoother results if I divide up large point loads, say from a column, and place them at the four corners of the column, or some other similar arrangement, depending on geomtry.

I have used this method for mats which support as little as two individual columns, combinations of columns and shear walls, and even large mats with multiple columns and very large uplift forces.

The first few times we used Staad, we were somewhat sceptical, so we ran hand calculations using standard mat analysis methods to compare against the Staad results. Based on this comparison, and our own gut sense feel for what it should be, we were quite satisfied with the results from Staad.

In the last 5 years or so, I have probably designed 12 or 15 mats of varying size using this procedure and have not had any problems with any of them.

That said, my mats usually range in the 2-3 foot thick range, and are loaded by up to 10 story structures. Much thicker than that, I can not say how accurate this method stays. I would love to hear from some one who has compared Staad to Mat3D.

 

[structuresguy]

Oh ya, forgot to mention, since about Staad 2003 I think, you can view soil pressure distribution curves for the mat. This way you can design both your soil bearing pressure, and concrete bending stresses using the same model, just need the right set of load combinations. I always check punching shear by hand, as I don't know of a method of doing this in Staad using plate elements. (The foundation design utility does check this though)

I also check lateral loads by hand as well, though I suppose you could set up horizontal springs to simulate the passive resistance of the soils.

 

[STR04]

http://www.reiusa.com/Asp/Webhelp/P...nds_and_Input_Instructions/Section_5.27.3.htm

1. When you say tension only in vertical direction are you saying define at 'DIRECT Y', not as 'DIRECT YONLY'? This allows the spring to slack (lift off)?

2. Which mat option do you prefer; Footing, Elastic Mat or Plate Mat?

3. Do you include your concrete walls as a stiff element to keep bending out of your mat?

4. I'm using a subgrade modulus of 75 pci. My input value converts to 129.6 ksf/ft. Stupid question, but I cannot afford to miss this one.

5. Are you modeling your plates 2-3 ft thick? And by what length? Are your plates 3-noded (triangular) or 4-noded (quadrilateral)?

TIA

 

[structuresguy]

Well, lets see if I can answer all of these:

1. "Direct Y" is what I use, which tells it that the spring support is in the Y direction. This then pins the footing in the X and Z directions. "Yonly" literally only means Y direction only. So if you have any shear defined, the mat will fly off to infinite, so to speak. I use the command "Spring Tension" to tell it that the springs are tension only. This way the soils wont be modeled as capable of holding down the mat. With the tension only command, it will iterate until it converges. However, it might not ever converge, which will tell you your mat is not large enough.

2. I use Plate Mat. One of the tutorials I believe it is, describes the use of plate mat vs elastic mat.

3. Whether you include the walls for the stiffness is up to you. I would say it depends on the geometry. I have done both, though not on the same mat, so I can't compare the effect.

4. You can change the input units for that you enter your value of 75 pci, rather than converting to KSF/ft. This is what I do, as I have a feel for pci, and none for the other units.

5. Yes, I have modeled plates this thick. I think I have gone as high as 4 ft. Like I said, we were spectical at first, but after running some verifications, we felt satisfied with the results. I strongly encourage you to do the same. At the very least, it can give you an indication that you have modeled the mat and springs correctly. I ususally try to keep my elements about the same width as the depth. THough I do add more in tricky areas. I have used both 3 and 4 noded, again to get a feel for it. I usually use 3 noded, as that is what the autop mesher creates by default using the new parametric plate modeling function in 2004. This function is great as you define the corners of the mat, and any internal nodes you need for loads, walls etc. Then it auto meshes for you, including all the internal points you defined.

Hope this helps clarify a bit.

 

[STR04]

Thanks a bunch!!! I really appreciate your help.