High Loads under Columns and Bearing Capacity Failure

[muneeb1213]

I m designing a 4 storey building on a poorly graded sand with SPN value of 5 and allowable bearing capacity of 0.5 tonn/sq.ft..... for the mat foundation, i provided beams of 18 x 36 and mat thickness as 24 inches ... however the soil pressure are still above safe bearing capacity.. My question is should i assign line spring property to the footing beams or not???

The area of footing is also increased but the stresses are still high...

Edited July 2, 2017 by UmarMakhzumi
Changed topic title to make it more relevant to the discussion.

[Ayesha]

You can do any of the following things muneeb.

1) Go further deep to get a better bearing capacity. It would increase the excavation, foundation and project cost.

2) Use deep foundations like piles to support the superstructure load.

3) Increase number of columns so that your load per column gets less and so does force on foundation. 

4) Verify loads and manually check load development to make sure that you are not over-estimating any loads.

5) Change roof design so that superstructure load reduces on foundation. Reduce slab thickness if possible  or provide a steel truss if that is a possibility instead of concrete slab.

6) Discuss with architect and try to remove masonry walls and partitions and use lightweight equivalents.

[UmarMakhzumi]

I will just add that adding beams with raft/ mat foundation wouldn't reduce the bearing pressure. With or without beams, the same load would get transferred to exact same area (as bearing area stays the same). The only difference that beams would make is that they would reduce the thickness of mat and eliminate punching at columns. 

Thanks.

 

[EngrUzair]

AFAIK, assigning springs to beams is not required.

In additon to what Ayesha has suggested, it might be better to check back from your current (and if possible, also from some other) geotechnical investigator about the usable bearing capacity for raft foundation, because 0.5 TSF bearing capacity appears to be very low value for a sandy soil. IMO, it should be more. So, better get it reviewed.

Moreover, increasing the number of columns will not reduce overall load or required area of raft. However, if possible, it might reduce required raft thickness due to punching shear.

You may reduce overall raft loading by reducing thickness of raft  itself, in the areas where column punching does not affect. This may be done by providing thicker raft in column strips, or in even in the form of thicker pads under the columns only, provided other structural requirements are satisfied.

Regards. 

[Ahmed1]

Hi Muneeb,

The N value you provided is at the surface or average N over the influence depth?

For raft foundations on sandy soils, bearing capacity from shear failure point of view is not a problem.  If you see the bearing capacity formula q=cN_c+0.5*Unit Wt*B+qN_q the middle term has B, which could be quite high for rafts.  Also empirical formula for mat foundation is 20N (kPa) (if I remember correctly from Meyerhof).

Therefore for your case the bearing capacity is in the rage of 100kPa (As Engr Uzair suggested bearing capacity value you are using is quite low for sandy soils).

Important thing for you would be to check settlements.  Please refer to Bowles book for settlement calculations.

If settlement are high then you can consider using piles (screw piles would be a good option in sandy soils) under columns.

 

Regards,

[UmarMakhzumi]

7 hours ago, Zia said:

If settlement are high then you can consider using piles (screw piles would be a good option in sandy soils) under columns.

Zia has summed it nicely. My comment is only on the use of screw/ helical piles. One of the common risk associated with the use of screw piles in sand, that you should raise with stakeholders is hitting a boulder. I have experienced some pile refusals in similar situations. Generally I hold a constructability meeting at 60% design review to discuss the pros and cons and decide if client wants to take that risk. With driven piles, you can drive through boulders most of the time. With screw, refusals are more common as helix plate wouldn't drive down if obstructed by boulder. You will need geotechnical input, pile driving records of area (if available) and construction input to make a recommendation to the client.

Thanks.

[Waqar Saleem]

Muneeb, here is my view though seniors have explained above nicely.

1.what is the value of subgrade modulus ? in some cases using subgrade modulus with 2" settlement solve the issue of over-stressing under foundation. As normally value for the SM given is with 1" settlement when we calculate by the formula given in Bowles, as Geotech to provide you the value or simply multiply the value by 2 which is calculated from the bearing capacity 0.5*3*12*2204*2 lb/ft^3 . 

2. if you have basement then assign stiff to the foundation under rcc basement wall.

3.check your applied loads, make them realistic and compare with reactions, check your load combinations; sometimes load combos are assigned greater factors mistakenly or by abrupt system shutdown induce some error in them.

4. increasing thickness of raft also decreases bearing pressure, try.

Regards

[muneeb1213]

On 07/07/2017 at 0:04 PM, Zia said:

Hi Muneeb,

The N value you provided is at the surface or average N over the influence depth?

For raft foundations on sandy soils, bearing capacity from shear failure point of view is not a problem.  If you see the bearing capacity formula q=cN_c+0.5*Unit Wt*B+qN_q the middle term has B, which could be quite high for rafts.  Also empirical formula for mat foundation is 20N (kPa) (if I remember correctly from Meyerhof).

Therefore for your case the bearing capacity is in the rage of 100kPa (As Engr Uzair suggested bearing capacity value you are using is quite low for sandy soils).

Important thing for you would be to check settlements.  Please refer to Bowles book for settlement calculations.

If settlement are high then you can consider using piles (screw piles would be a good option in sandy soils) under columns.

 

Regards,

Hello zia. Sorry for late reply.. The N value of 5 is taken as average. The SPT was carried out upto 32 feet depth and N value ranges from 4 to a maximum of 7.. I have extended the footings upto 9 feet from each side however the results havent altered much.. 

[muneeb1213]

Should i assign the stiffness modifiers to the raft (0.25 * I) ?? Will it be a realistic approach??

[UmarMakhzumi]

8 hours ago, muneeb1213 said:

Should i assign the stiffness modifiers to the raft (0.25 * I) ?? Will it be a realistic approach??

Please check this thread for the answer:

 

[Ahmed1]

Hi Muneeb,

I am not really into structural design.  Other experienced structural engineers on this forum can suggest you regarding raft stiffness.

I understand that the N_avg is 5 blows/300mm penetration with N ranging from 4 to 7.  This value is still good for sandy soils supporting a raft foundation.

What I understand that the pressure per storey on a raft (covering the entire footprint of the building) will be in the range of 12.5 to 15kPa (Foundation Design by Tomlinson).   As you are constructing a four-storey building the contact pressure should be in the range of 50kPa to 60kPa.  Therefore I am not sure why you need to extend the raft  by 9' (2.7m) on each side.

Regards,

Zia

[muneeb1213]

Attached is the soil pressures distribution contours. the total bearing capacity is 1102 Ib/ft2 however here the maximum value comes out to be 1288 Ib/ft2 and it is concentrated in the middle as raft is extended beyond the column lines..

[muneeb1213]

On 7/13/2017 at 6:19 AM, Zia said:

Hi Muneeb,

I am not really into structural design.  Other experienced structural engineers on this forum can suggest you regarding raft stiffness.

I understand that the N_avg is 5 blows/300mm penetration with N ranging from 4 to 7.  This value is still good for sandy soils supporting a raft foundation.

What I understand that the pressure per storey on a raft (covering the entire footprint of the building) will be in the range of 12.5 to 15kPa (Foundation Design by Tomlinson).   As you are constructing a four-storey building the contact pressure should be in the range of 50kPa to 60kPa.  Therefore I am not sure why you need to extend the raft  by 9' (2.7m) on each side.

Regards,

Zia

Hello Zia

60Kpa comes out to be 1253 Ib/ft2 however the total bearing capacity is 1105 Ib/ft2 .. I have attached the soil report which states the allowable bearing pressure for the footings.. please have a look and suggest.. thanks