0.6DL+0.6WL load combination for overturning and sliding case

[SELVANAGARAJ]

Dear Gents,
I have one question using ACI-318-11 code. using ACI 318-11, there is no footprint in the overturning and sliding, the load combination explained only for ultimate strength condition and not for serviceability case, in the other hand using ASCE-7-10 describes load combination using 0.6DL+0.6WL, here the load reduction is almost 40percent, is this acceptable, kindly guide me, because in my project specs says it has to maintain F.O.S for overturning 3.0 and sliding condition 1.50, what will do, kindly any one guide me


Nagaraj.S

 

[CELinOttawa]

Your specification and your code aren't speaking the same language. If I were faced with this I would proportion my structure using specified loads and the dictated FOS a la 1970s, and then do all my design to Limit States (US equivalent would be LRFD). If the LRFD work shows the wall to be insufficient, iterate while all the while knowing you have already satisfied the client's spec.

 

[Lomarandil]

There are two potential "40% reductions" here, I'm not sure which you are talking about.

Are you referring to the 0.6 load factor on wind load?

This is because ASCE 7-10 provides wind loads at a factored (strength) level. When using these loads in ASD (service level), the 0.6 factor is the translation required so that your loads and your design method are "speaking the same language" as CEL put it. It's still the full design load, not a reduction.

Or another way of putting it, 0.6 of ASCE 7-10 (strength) wind load is equal to 1.0 of ASCE 7-05 (service) wind load.

Or are you referring to the 0.6 load factor on gravity load?

ASCE 7-10's load combination of 0.6D+lateral load (provided for use with global overturning and sliding checks) already includes the typical FOS (1.5) against these failure modes. So if you are required to maintain a FOS of 1.5, and your structure maintains unity when 0.6D+lateral is applied, you're OK.

Older versions of ASCE 7 described the load combination as 0.9D+lateral load, and expected you to apply the 1.5 FOS after the fact. The 10% reduction in DL was meant to encompass uncertainty in DL (maybe the concrete only weighs 140pcf, etc).

 

[sponton]

Originally the load combination for overturning was
0.9D + W [ASCE-7 05 ASD]

So the original text said you needed to make sure you had a Safety factor of 1.5, following edition of IBC/ASCE-7-10
modified the equation and it became

0.6D[0.9/1.5=0.6] + 0.6 W [0.6 because the wind tables are now given in LRFD rather than nominal as in previous versions.

The approaches for sliding were not changed, somebody might shed more light on this since I'm not that well acquainted with the changes in the sliding provisions.