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Waqas Haider

Combined Beam Footing Modeling In Safe

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Umar bhai, I want to model beam footing in SAFE. How to introduce beam along with footing pad?? I form a slab and draw a beam there. then i match bottom of beam with bottob of pad by adjusting insertion points. Is it correct modelling?

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For the first picture(strap type), I would model the foundation and beam in ETABS, get the reactions at foundation and export them to Safe. Beam gets designed in ETABS and foundation goes to Safe with loads representing effect of beam presence. Second and third pictures are easy, you can take average thickness and use them. I haven't used Safe in over 5 years now so. I am not sure if there has been a new feature to accommodate such fdns but I don't see any problem there. Use simple slabs with conservative thicknesses.

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First picture is for the strap beam, you can design it manually (see Nilson book for it) or model it in SAFE. just draw as Umar said, a slab area with equal width to that of strap beam. Dont apply soil supports to strap beam. If you need an excel sheet based on imperial units tell me, I prepared it some time ago.

 

Second picture shows isolated column thickening with basement slab. You can model this as told in first step. The only difference is you will apply soil supports to basement slab too. Just draw isolated column footing with a thickness, and then draw basement slab with its thickness.

 

Third picture shows strip foundation for columns, same modelling procedure will be adopted. Draw strip foundations and apply supports.

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In first picture, my concern is for 2nd one labelled as SLAB AND BEAM TYPE FOOTING. Same is shown in in other two pictures. Slab and beam type or combined beam footings. I am attaching 3dview to have idea what exactly footing i m saying. Regarding Manual calculations, i have no problem for it. I have prepared my own excel sheets. But i m amazed if I have assumed correct behavior of footing components or not.The behavior I assume, goes like this

1) 1st of all, find out plastic centroid of all loads.

2) Then either try to position centroid of footing at plastic centroid or cater this extra moment too produced due to this eccentricity.

3) Then calculate bearing pressure distribution under footing. It may be uniform, it may be trapezoidal.

4) Now Slab will act as Cantilever at both ends of slab (in same manner as a single footing bend in both directions.)  But in this case, slab will bend just in direction perpendicular to beam provided, because beam will act as continuous support for slab in direction parallel to beam (Just similar to wall footing). So footing is designed just for this bending. in other direction, parallel to beam, just provided minimum steel.

5) Now all pressure under footing will ultimately transfer to beam provided, and this load will act as line load at beam. Now beam is designed for this load assuming points, where columns are present, as supported. In short this whole footing and beam is designed as INVERTED both side cantilever slab resting at beam which is further supported by columns and the load acting on slab is area load equal to pressure of soil under footing. 

 

But This all is manual procedure. Kindly confirm it if I assume any thing wrong. More over, i want to check my same model in SAFE for verification. Also provide me with your sheet Waseem bhai so that I may take help of it. 

 

post-1702-0-19298500-1421653469_thumb.pn

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aha you are taling about the stepped footing. okay just draw left part as one slab in SAFE and draw right part too. Then in the center, draw this slab with more thickness. Thats it. No need to adjust by cardinal points. 

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The reinforcement in center part is not like slab reinforcement. The reinforcement provided is similar to beam reinforcement. I am attaching reinforcement detail of this type of footing. 

post-1702-0-49835300-1421663932_thumb.pn

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oho bhai reinforcement type has nothing to do with the type of FEM element you select. You can provide the reinforcement in center part as you have shown in image. But for analysis I would recommend you to model it as slab. Then provide reinforcement details as done in beams but take results from this slab element in SAFE

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