To Use Load Combinations Or Load Factor?

[kk88818]

I have a question about the use of load combination and/or load factors in the design of, say, a reinforced concrete footing. I will need to size an adequate footing, as well as to design the steel reinforcement. Now, sizing of the footing is done using service loads, while the reinforcement is done using ultimate loads.

In my analysis stage, in Method 1, should I just use the load combinations using allowable stress design to get my service loads, and THEN apply a load factor (1.6, say) to get the ultimate loads for my reinforcement design? Or should I use Method 2, where I do my analysis using all the possible load combinations using BOTH allowable service design, as well as strength design, or load and reistance factor design? Your thoughts, feedback, suggestions and advice, please.

 

[JAE]

Use the ASD load combinations to design for soil bearing (i.e. footing size).

Then use LRFD load combinations to design the concrete and reinforcing.

Two separate sets of combinations are required.

 

[rday]

Go with method two. Speaking from experience, it will cause you less headache in the long run.

 

[SeanMD]

I use method 1. If the DL/LL ratio is high I multiply the service loads by 1.5 to get an ultimate load. If the DL/LL ration is small I multiply by 1.6. I almost never run two sets of load combs.