STAAD's footing design

[ABCDesign]

I tried using staad.pro's footing design but the results seem to be unbelievable. I got one footing with a dimension of 20m x 2om. My model is for two story residential bldg. only with all loads minimum. Can anybody please help explain why is this so?

Thanks in advance.

 

[lippie]

Are you using the STAAD.etc component? If so, did you set your min and max lenghts to be in the range you were looking for?

 

[KrisSTAADSupport]

There are 2 footing design facilities within STAAD.Pro :

1) Through the basic analysis engine. This uses a command syntax "START FOOTING DESIGN ... END FOOTING DESIGN".

2) Through the link/interface with STAAD.etc

Method 1 has a limitation in the sense that it requires the entire footing area to be in contact with the soil. Partial uplift of the footing bottom surface is not incorporated for this method.

As a result, if P is axial load, A is area of footing slab, Mz is moment about global Z axis, Mx is moment about global X axis, Sx and Sz are section modulii of slab, the above condition can be put into the following mathematical equations :



Maxim. bear Pr. = (P/A) + (Mz/Sz) + (Mx/Sx) <= Allow. bearing pr.

Minimum. Bear. Pr. = (P/A) - (Mz/Sz) - (Mx/Sx) >= 0.0


Usually, if P is small and Mz or My is large comparatively, the footing dimensions will tend to be large in order to satisfy the second equation. Another aspect of this approach is that the maximum bearing pressure resulting from such a large footing slab will be small.



In Method 2, this limitation does not exist. The STAAD.etc module finds the actual footing contact area for axial load + biaxial bending allowing for partial uplift of the footing. This may lead to a much smaller footing size than the one that Method 1 determines. The portion of the slab in actual contact is also displayed using a base pressure contour. Detailed hand calculations are provided as part of the output for step by step verification by the user. There ought to be no doubt in anyone's mind as to the accuracy of the results in this approach.

 

[MPHenry]

One point I would also add to Method 1 above. The foundation design allows a fixed value of 10% of the total axial load for the weight of the footing. The footing weight is not automatically calculated by the program. This is another reason why the second equation can lead to large foundation sizes.

 

[structuresguy]

Well, ViperCE, don't take this as insulting but, are you sure you input the allowable soil bearing pressure in the correct units? Are you sure the program is using the same units to calculate the footing area? ie, allowable soil in N/m^2 but loads in kN.

Staad is not the best at handling units, particularly when the normal units change from loading (kN) to resistance (N/m^2). At least when working in Imperial units, this problem comes up a lot. Loads in kips, but soil resistance in psf, and soil subgrade modulus in kip/in^3. If not very careful, a simple units error can obviously throw the results out the window.

I always double and triple check the data input, because, as the saying goes, &quot;...garbage in = garbage out&quot;.

 

[ABCDesign]

Thanks for the note structuresguy. I usually use the same unit system, anyway I will double check there might be lapses along the way. KrisSTAADSupport thanks for your reply. And to lippie, I will try using staad.etc and compare the results, it might help.

 

[namare]

I found the same problem. Checking the results, I found that the footing design was made for seismic or wind load ONLY (load case that produce a low P).

To solve this I'm using the Load List command to include only the combos in the footing design.

Regards

 

[structuresguy]

Good point namare. Definitely need the load list command to limit staad's design functions to only the load cases you are interested in.