Braced Frame Second Order Effects 1

[StevenPumphrey]

Hello,

I am doing an analysis of a braced frame for an industrial building. I am confused over the isssue of whether or not I must perform a secondary analysis or not. It says in the new code that generally braced frames are analyzed without secondary effects. Is this true? I performed an analysis on the braced frames with point wind loads, to get the increase in axial load in the columns due to this loading. is this considered Plt when designing the beam-column? When I did this analysis the bracing members were in the frame, so is this really a sway analysis for P-delta effects. I need some serious help and explanations here. Anything will help.

 

[UcfSE]

Second-order effects are related to the change in loading or load effect due to deflection, namely the bending moment for us. There are two types of effects to consider: P-[Δ] and P-[δ], where [Δ] is the global frame deflection, causing additional moment for the frame to resist and [δ] is the local member deflection from its own longitudinal axis, causing additional moment for the member to resist. Whether your frame is sway or non-sway will tell you whether the global deformations or the local will be more pronounced compared to the other.

You should pick up a text or two on this topic. It is very important and too much to cover in a forum. Salmon and Johnson have a good text, and Segui is good also. You may also find some research published by ASCE as well as information and publications on the AISC website. If you are not where the ASCE or AISC are used, there are probably similar banks of information pertinent to your locale. Even older articles will help you to understand the beginnings of the code requirements. To understand some of the code approximations for moment amplification you will need to go back to basics and begin with the governing differential equations and derive what is in the code. There are a lot of assumptions built into all this and it's important to understand the "what's, how's and why's" of what you're doing.

 

[patswfc]

What code are you designing to. In the UK the code gives notation horizontal forces which are 0.5% of the vertical dead and imposed load on each column. These notational horizontal forces are thus loadcase dependent. The frame should be analysed linearly for each set of notaltional horizontal forces and the drift between floors is then compared to a limit which tells the designer wether or not p delta effects have to be considered.

If you are designing a braced frame these are generally very stiff and dont deflect much thus p delta effects can generally be ignored, but this will depend on the size of the building.

Saying all of this if you are analysing the frame in a software package does it have an option to analyse the frame for second order effects. If so you could just run it with this option ticked and that will take care of any second order effects.