Philosophy of Lateral Bracing

[charliealphabravo]

Hi guys, steel frame grasshopper here.

I am doing some design for a tower concept and I am wondering whether either of these bracing patterns is better or more conventional than the other. One has the bracing symmetric in the bays. The other has the bracing spiral.

I have the same types of questions when setting diagonal webs on trusses. I know there are famous "named" truss styles but what are the benefits of each? As long as the truss/frame is stable and has the desired redundancy/indeterminacy what difference should it make to the analysis?

Thanks in advance

 

[SteelPE]

I don't think it really matters from an analysis stand point as long as your tower is sufficiently braced. Now, others may chime in saying one way is more economical than another because of the design of the connections, or because one way makes fabrication/erection easier.

In regards to the truss styles. Again, from an analysis stand point some trusses are a little easier to design by hand than others. Where the benefit comes in is with the design of the web members and the material required resist the forces in the truss. A member will resist more load in tension than compression (due to buckling). Also, some trusses may be more acceptable than others architecturally.

Others may have more opinions and this is just my $0.02

 

[BAretired]

The arrangement of bracing is usually selected on the basis of economy, not for simplification of analysis.

BA

 

[rb1957]

i guess the only difference is that the LH sketch will always have some twist reactions in tension (fatigue issues rather than compression buckling/crippling) whereas the RH sketch will have the bracing in compression or tension depending on the direction of twist.

Quando Omni Flunkus Moritati

 

[charliealphabravo]

Ok, let me put it this way.

What if the braces were completely random? I suppose that could result in irregular load concentrations for a very tall slender structure, but that should still show up in the frame analysis and be accounted for in the member sizing and connection design I think. As BAR says...not efficient or simple...but still functional I believe.

 

[JAE]

I don't think there's much of a difference - the connections from brace to column/beam all occur in their own areas - I can't see any spatial conflicts.

Usually multiple member connections all at one point where they would geometrically conflict with each other causes problems with detailing, design and cost.

(also note that you are missing a brace between levels 3 and 4 on the Right Hand example.)

 

[McSEpllc]

I agree with SteelPE, that from an analysis point of view it does not matter: Each member will still have the same maximum tension or compression force, making the members and connection the same.
I also agree with BAretired, that economy usually control design.
Personally I would use a bracing layout that is symmetric, with the braces on opposite sides of walls oriented the same. While the elongation or shrinkage on opposite’s side would be the same for the same loads and member sizes, compression members would also experience some amount of buckling (hence the stricter requirement of slenderness ratio for compression members.) With an un-symmetric layout, where the brace on one wall is in tension and the other in compression, I assume there could be torsion induced into the building.


Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[charliealphabravo]

Yes Thanks JAE, you have a sharp eye. I just threw the second example together for this post.

 

[dhengr]

From the structural standpoint, I have always been a big fan of symmetry both from the analysis and the construction point of view. Repetition has merit out in the field, even if we can FEA the hell out of these things. Furthermore, when you have beams. columns, and several braces framing into the same connection, you resolve and transmit the loads right there; you are not chasing them all over the place and forgetting them. And, while JAE’s point about detailing complexity, if you have too much going on at one joint or location is well taken, there is also something to be said for laying down one main member and doing a bunch of layout and fab’ing in multiple directions, but at one location on the member. When every member has a few different connections or details, at different locations, in different directions some of that efficiency is lost. And, the same goes for erection, when each corner, or each face, at a given level is different, this would seem to add some confusion 40' in the air with three guys hanging there. There would also be something to be said, in arrangement and detailing, if two opposite faces at a given level could be assembled on the ground and lifted into place and then only needing bracing btwn. them at that level. Maybe two adjacent faces would be more efficient and stable more quickly.

I think your spiral bracing scheme on a tall slender tower will lead to some unintended torsional problems, whereas those unsymmetrical conditions might be easier to deal with on a lower huskier structure. Look at the design of any existing type of tower, those aren’t designed and detailed that way to be clever. Time and experience has proven that to be a fairly efficient way to do it. So, you better have a reason for trying to reinvent the wheel.

 

[NS4U]

The one on the right is very bad from a seismic perspective, if one brace buckles you will create a huge torsion that needs to be carried by the remaining braces which will overload them in compression and cause failure. You need to make sure some braces are in tension. The Scheme on the left is definitely the way to go