Stability, Bracing, and Moment Frames

[WiSEiwish]

Hello,

I am new to the design of buildings. For the past few years I have been designing components, and chose to make a career change because I wanted to design buildings and other similar structures. I now am working on one of my first projects and had a few questions regarding bracing.

Are there any dos and don'ts as far as how X bracing and moment frames interact? I think that I have heard once or twice that it is usually not good practice to mix the two that much, and it seems like it should make sense, but if someone asked me why I couldn't give them an honest answer.

Is there any common practice as far as how bracing should be laid out in a building?

I am a rookie here and any help is appreciated and if anyone knows of a good reference I would definitely take a look at it.

 

[PUEngineer]

Good reference: Loads and load paths in buildings, authored by Taly.

I'm sure others will provide responses referencing a good mentor and sharing hope that you aren't going through this project without some oversight.

As for your question regarding frame mixing. Take a single bay moment frame and a single bay braced frame, of similar height and width, apply a unit lateral load to each and review the drift results. Should give you some indication as to what would happen if they were tied together with a rigid link.

 

[asixth]

Generally the x bracing will be stiffer and alter the dynamic response. I always think it is best to provide bracing in the corners and ends because this is best not to induce any torsional irregularity. I try to keep the x bracing with an Le/r less than 300. Slender elements can buckle just under incidental loads that they are subject to from construction tolerance.

 

[shin25]

If possible, it may be wise not to mix different load resisting systems (moment frame and braced) in the same direction. Although, having moment frame system in one plan direction and braced frame in the orthogonal direction of the building should be fine. However, if different systems will need to be mixed in a given direction then their relative stiffnesses will need to be used for load sharing for elastic lateral analysis of each system for load cases such as wind. For seismic loading however, mixed systems are called dual systems and relative load sharing between syatems should be based on local seismic code requirements. In USA, it's common to use minimum of 25% of total lateral seismic load to design moment frames in a dual system.

Usually, the stiffer load resisting systems (such as braced frames) should be located close to center bay of the structure to minimize impact of deformation due thermal expansion and contraction.