Overturning safety with buoyancy

[nrjdjjs]

Need help with determining the correct factor to use to calculate the resisting moment when a foundation is subjected to buoyancy. Here are some quick sample calcs to illustrate the point:

Footing dimensions:16'X20'X2'
Bottom of footing below grade = 4'
Soil cover = 2'
Water table at grade
Concrete unit wt = 150 pcf
Soil unit wt = 100 pcf
Total overturning moment = 200 kip ft
Load Combination self weight factor = 0.6

Case 1:

Resisting moment due to footing weight along 16' length at one edge of the footing = 0.6*16*20*2*0.15*16/2 = 460.8 kip ft
Resisting moment due to soil cover =0.6*16*20*2*0.1*16/2 = 307.2 kip ft
Resisting moment due to buoyancy = 0.6*16*20*4*0.0624*16/2 =383.4 kip ft

therefore net resisting moment = 460.8+307.2-383.4 = 384.61 kip ft
Safety Factor SF in overturning = 384.61/200 = 1.923

Case 2:

Resisting moment due to footing weight along 16' length at one edge of the footing = 0.6*16*20*2*0.15*16/2 = 460.8 kip ft
Resisting moment due to soil cover =0.6*16*20*2*0.1*16/2 = 307.2 kip ft
Resisting moment due to buoyancy = 1*16*20*4*0.0624*16/2 =639 kip ft

therefore net resisting moment = 460.8+307.2-639 = 129 kip ft

SF in overturning = 129/200 = 0.645

Based on the calculations above, the variation in SF is significant between the 2 cases - 1.92 vs 0.645. The only difference between case 1 and Case 2 is the load factor considered for buoyancy - Case 1 uses 0.6 whereas Case 2 uses 1.0.

Section 2.4.1 of ASCE16 discusses the use of factor 0.6 vs 1 for H type loads - buoyancy loads. Any comments on the correct factor to use for computing buoyancy would be highly appreciated.

Thank you

 

[Settingsun]

You've assumed the soil above the footing is a monolithic solid which is conservative. The buoyancy force only acts on the solid part, not the voids.

Use reduced densities for the concrete and soil when calculating the dead load to account for buoyancy. Then there isn't any separate buoyancy load so the question of what factor to use goes away. If you need a code reference to justify this, treat buoyancy as an 'F' load (well-defined fluid load) which will give the same result. This is like your case 1.

 

[HTURKAK]

2.4.1 is LOAD COMBINATIONS FOR ALLOWABLE STRESS DESIGN
H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials

The code is IMO , self explanatory.. in this case,The buoyancy should be assumed ground water pressure and the code states ;


-If the buoyancy is favourable for the subject loading combination , include H with a load factor of 0.6 ,
-If the buoyancy is unfavourable for the subject loading combination , include H with a load factor of 1.0 ...

In your case, buoyancy is unfavourable ( reducing the OT resistance ) ..Use factor 1.0 Another approach could be, the ground water density could be more than (10 kN∕m3) and the dry unit wt of the materials could be overestimated.. In this case, a factor 0.9 could be adopted but never 0.6.

You are expected to look 6.9.1 Buoyancy .

Use reduced density (( submerged unit wt- water density ) as suggested by Mr. steveh49 or case 1 solution.