Need Help Regarding Raft Design

[Abdullah Anjum]

I need help regarding MAT footing design in safe. Please read questions mentioned below and comment if any extra step or correction need to be done. This MAT footing is for 4 storey building in zone 2A and bearing capacity would be more than 1.25 ton/sft.

 

1) Does we need to increase the bearing capacity by 33% as mentioned in NILSON book for soil pressure calculation or not if we are taking basic load combinations of UBC including earthquake loads not the alternative load combinations.

 

2) If we want to take reduced live load for footing then is there any way to export in form of reactions from Etabs to safe. In my opinion Etabs does not give Reduced live load reaction, Correct me if I am wrong.

 

3) When we use the basic load combinations which include earthquake load then there is very localized uplift (due to earthquake combo) when we check soil pressure. is it normal or something wrong with my reactions. If it is normal then we always used non-linear basic combinations to check soil pressure and non-linear design combinations for design of raft?

 

4) Does it required to check crack width and settlement in case of raft or not. If required then explain in few words.

 

5) At boundary of raft where there are shear walls/lift core, soil pressure is more than the bearing capacity in these localized regions. what we need to overcome this situation if we cannot increase the area of raft at that side.

 

any additional check  which you think important for design then write in comment

 

your help would be appreciated

 

Regards

[Badar (BAZ)]

1: You can increase the bearing capacity by 33%.But this recommendation is reasonable for dense granular soils, stiff to very stiff clays or hard bedrocks; it is not applicable for loose soils susceptible to liquefaction, or increase in pore water pressure.

3: Yes, it is normal to get uplift. We use service combos for checking bearing pressure and strength design combos for the reinforcement in the raft.

4: If you have provided minimum reinforcement, and satisfied maximum spacing requirements of the code, you don't need to check crack widths. If more than 80 percent of your raft area satisfy the limit of bearing capacity then settlement wont be a problem.

5: One can neglect that localized increase in bearing pressure.

[Ahsan Kazmi]

BAZ how much localized bearing pressure is allowable?

[WR1]

33% pressure increase is not allowed regardless of the soil type for example according to UBC-97 service combinations. If there is an allowance it should be clearly mentioned by the geo tech engineer/report.