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Load combination 7. .6D+W+H

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Sirs:

 

We have some confusion on the  application of ASCE7 ASD load combination 7, .6D+W+H.

 

ASCE 7 Section 2.1 indicates that the load combinations shall be used for “a particular construction material throughout the structure.”  I understand that the .6D allows for the sometimes overestimating of the dead load and that concrete columns, for example, should be proportioned for this load case as the reduced dead load sometimes leads to the governing condition.  Are these load combinations for the design of the materials only or are they also to be used for the overall stability of the structure itself.

 

Two quick examples:

1)      We do many light framed steel structures and also foundation design for prefabricated metal buildings.  If, for example, the resulting .6D+W load combination results in a 6000# uplift, how much concrete footing weight (along with slab and soil overburden) must be placed.  Is it 6000# or must it be 10000#.  The latter will result in inordinately large footing on a very small building using a factor of .6D on a material weight that is known quite accurately with very little chance of over estimating its weight.  Anchor bolts and any attachments of course, should be designed for the full .6D+W load.

2)      Retaining walls:  Must the retaining wall stability also be designed for this case.  Should there be a case where the unit weight of the wall, footing and soil is reduced 40% by using a .60 factor to resist overturning.  This would even be more magnified where the retaining wall extends above grade and receives wind load (or is all of this ignored and the traditional FS of 1.5 is used for sliding and overturning?  Or is it cumulative?)  Or even just a free standing 8’ high block wall fence receiving wind load.  Should overturning be checked by reducing the unit weights of the wall?  When using a program such as Retain Pro, one should redefine the unit weight of the materials to .6 of their actual values?

Again, I am asking about the overall stability of the structure, not the design of a particular element.

Thanks for reading this and for your thoughts in advance.

 

Joe Eribarne