Which Method of Load Calculation is Correct?
Which Method of Load Calculation is Correct?
(OP)
I need to resolve a question about the use of load combination and/or load factors in the design of a reinforced concrete footing. Say I need to design both the size of a footing, as well as the design the steel reinforcement, both simple and regular enough. Now, sizing of the footing is done using service loads, while the reinforcement is done using ultimate loads.
For Method 1, in my analysis stage, I am 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. In Method 2, 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, and then using the most critical loads from the ASD combination for my footing design, and then the most critical load overall, or the most critical load from my LRFD load combination for my reinforcement design.
Which method is best, or more accurate? Your thoughts, feedback, suggestions and advice, please.
For Method 1, in my analysis stage, I am 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. In Method 2, 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, and then using the most critical loads from the ASD combination for my footing design, and then the most critical load overall, or the most critical load from my LRFD load combination for my reinforcement design.
Which method is best, or more accurate? Your thoughts, feedback, suggestions and advice, please.






RE: Which Method of Load Calculation is Correct?
One thing to note for RAM Foundation, it gets very hairy for lateral columns (they provide 0.09 sq in of steel in the top in some instances), and also when As,min controls.
If I were doing it by hand, your approach of factoring the ASD combination by 1.6 doesn't seem unreasonable (even though it is definitely on the conservative side - a good thing).
I might try to sharpen the pencil a little and get a more appropriate factor (maybe 1.4 if your LL is 50% of the total load or 1.5 if the LL is 75% of the total load.
RE: Which Method of Load Calculation is Correct?
RE: Which Method of Load Calculation is Correct?
RE: Which Method of Load Calculation is Correct?
For the load cases where you have factored wind plus 0.6 dead load, assuming 1.6 dead load instead is unconservative. This can affect the tie-in to the footing, the rebar required for reversed moment, and in some cases, the maximum moment in the slab.
Where you have partial uplift of the footing, internal forces are no longer proportional to external forces and the external forces should be factored prior to calculating the internal forces.
RE: Which Method of Load Calculation is Correct?
RE: Which Method of Load Calculation is Correct?
RE: Which Method of Load Calculation is Correct?
RE: Which Method of Load Calculation is Correct?
J
RE: Which Method of Load Calculation is Correct?