Simple Load Combination questions

[LearnerN]

I have 2 newbie engineering questions. First, when using LRFD load combinations for dead and live (vertical) and wind loads (horizontal), is the resultant factored load only vertical for doing concrete/steel design? Basically, do load combinations need to be run separately for vertical loads and then for horizontal loads?

Second question. For the overturning factor of safety calculation, is this calculated using service loads? Thank you.

 

[frv]

1. No. LRFD or ASD both address both gravity and lateral cases (see W or EQ cases in the load combinations in ASCE 7).

2. This question often causes confusion. This calculation can be done in either LRFD or ASD, but is commonly done in ASD because you end up with a service level load that you can then use to check foundations (geotechnical reports almost always sepcify "allowable" loads). The confusion comes in when, in addition to the load combination (0.6D+WL in ASD, or 0.9D+1.6WL in LRFD), engineers apply a "safety factor" of 1.5 to the overturning resistance. This, as has been discussed many times on these fora, is redundant. In other words, you only need to comply with the load combinations; there is no need to add an "additional" safety factor beyond that.

 

[RFreund]

I will add to point 1 as sometimes, I think, confusion arises when doing frame analysis and using approximate 2nd order analysis techniques (i.e. B1, B2 method). Atleast it did for me when I started.
When performing a frame analysis using a simplified approximate second order analysis (such as the AISC's B1, B2 method) the gravity loads and lateral loads are typically 'separated' during the analysis 'phase' so you have a gravity only 'run' and a lateral only 'run'. Then the results are combined when designing the members. This however, does not change the load combinations just that there are additional analysis 'runs' (i.e. instead of putting both the gravity and lateral loads on the model then 'running' the analysis, you do two 'runs').
*If your software performs forms a rigorous 2nd order analysis then the two runs (or the approximate method for that matter) are not required.

*=this is not entirely true in that it does not need to be rigorous but I don't want to get into that now.

EIT

http://www.HowToEngineer.com

 

[JAE]

I posted earlier but for some reason the post is gone.

Load combinations are not directionally specific. The loads (dead, live, snow) are typically vertical downward while wind/seismic have both vertical and horizontal effects.

The combinations are really combinations of effects, not loads. So for a vertical dead load, you could have vertical axial, shears, moments, etc. that can go in all sorts of directions depending on the geometry of your structure.