AISC LRFD and Deflection Calculation
AISC LRFD and Deflection Calculation
(OP)
Does anyone know the rationale behind the LRFD method using service loads for deflection calculations. What I have found is that LRFD typically is the same size or a slightly smaller beam selection than ASD from flexural calculations. However when checking deflection, the LRFD beam deflects more due to the smaller moment of inertia and so I go back to the ASD size anyway. Has anyone else come across this issue. Any tips will be greatly aprreciated.






RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
This isn't necessarily true. LRFD placed a higher level of load factor on loads that have higher variability, such as live loads.
So if you have a high live load condition (relative to dead load) your LRFD beam would tend to be larger than the beam determined using ASD.
Conversely, a low live load condition presents a smaller beam under LRFD than with ASD.
Deflections are checked as "actual" deflections. In other words, you use the true loads to determine the true deflections. LRFD load factors are a means of applying a safety factor against failure, so deflections should never be based on factored loads.
RE: AISC LRFD and Deflection Calculation
The factored load cases represent extreme cases. As such, using factored loads to come up with deflections is very conservatve as they do not represent the every day use condition that deflection limits based on.
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
Respectfully, I disagree with that. There are all sorts of conditions where strength does govern over deflection. Both strength and serviceability should be checked.
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
I agree with JAE. It is dangerous to dismiss checking strength provisions.
On a side note, I think some people get way overworked about deflection limits. If the code states a total deflection limit of L/240 governs, and you're coming up with L/230 or L/220, you are well within engineering judgment to allow that deflection. Conversely, depending on your situation, the code deflection limit may not be suitable; you may require much more stringent limits for your particular purpose. I emphasize judgment and experience.
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
Depending on the job specifics, LRFD may give a smaller, larger, or same size beam. However, the designed beam should be checked for deflection using service loads (real loads).