Metal Deck to Brace Compression Member

[ChrisNYCEng]

Hi, I am designing a long span structure with a curved roof. The have deep trusses with wide flange chords oriented sideways. Our unbraced length for the weak axis of the chords is equivalent to our panel points. We have provided beams framing into each panel point with 1 infill beam between panel points. These beams span 35-40' between trusses. We have assumed the beams framing into panel points will act as bracing to the top chord member. We have no additional framing/bracing for the infill framing between the trusses. My chord forces are approx. 2000k, therefore, the force required to brace these is around 40k. My question comes in regard to the compression strength and weak axis unbraced length of the members bracing the truss. Since my deck will be rigidly attached to the top flange of the beam, I think all will agree that the beam top of flange is braced. However, the beam bottom flange is free for 35-40 feet. What is the overall unbraced length assumption for the weak axis of this beam? I have already heard multiple opinions from yes, it is braced against weak axis buckling, to no, I must provide additional infill framing as weak axis bracing to the infill beams. I have looked and not really found any papers or solid research from AISC regarding the matter. Any help or educated opinion is welcome.

 

[AELLC]

Please provide a sketch, that will get you a lot more replies.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[ChrisNYCEng]

See the attached. Thanks.

 

[AELLC]

If each end bay is strongly cross-braced, I don't think you are getting 40K compression per beam.

Additionally, since the beams are at 11' o/c, how could you possibly get 40K per beam, period?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[ChrisNYCEng]

Our chord load is 2000k. We assume only beams framing into panel points provide bracing for compression. To provide this bracing we are designing the beams providing the bracing for 2% of the chord force. This equates to 40k load at each panel point.

 

[AELLC]

You have beams at 11" o/c, so isn't that 20K?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[JAE]

Chris,

A number of issues spring to mind here.

1. The erection procedure of this might be very difficult in that the purlin beams are not braced until the deck is on - and the truss is not braced until the purlins are braced.​

2. The metal deck may be able to brace the purlin beams if they are attached in a way that would prevent rotation of the beam (i.e. double attachments to form a rotational couple). The deck would have to have enough stiffness to brace the beam from rotating (rotational/torsional buckling).​

3. What about net wind uplift on the roof? Do you have any? If you do, then the bottom chords of your trusses will go into compression and you have no lateral bracing there.​

4. Without the deck in place you have a house of cards. You might look into using actual horizontal truss framing for lateral stability of such a large system instead of the metal decking​

 

[AELLC]

I was assuming that diagonal bracing would be used. Reminds me of how a former boss would say, "Do not ever assume, it will make an ass of you"

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[ChrisNYCEng]

JAE,

Thanks for the thoughtful response. Regarding your suggestions;
1. We have not discussed with an erector or contractor yet, but I agree with your point.
2. I have thought about this but may be difficult to control deck attachment in the field to have such a pattern... We may insert beams to brace weak axis at every other and attach this framing to deck but this also adds quite a number of small pieces to be erected very high above the floor.
3. There is no net uplift on the long span truss due to minimal wind loading. However, we do have a full layer of framing at the bottom of truss elevation and beams framing at every panel point. There is an entire rigging system and catwalk system, so we created a diaphragm of horizontal framing/trusses at this level - this was omitted in my quick sketch.
4. We do also have a horizontal truss at the curved roof level following the contour of the truss. This is around the entire perimeter of the building. The idea was to have infill beams at the interior of the roof act as collectors to transfer all lateral loads to these perimeter horizontal trusses which then connect to vertical braced frames and all four sides of the building.

 

[AELLC]

3. I don't see how you could have no net wind uplift, especially during construction. If this is in NY area, are there Nor'easters or remnants of hurricane winds?

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[structSU10]

I have used the attached paper in the past. You also would want to consider bracing the bottom chord. I will attached another paper that discusses that a bit, but basically the bottom chord must take all the bracing loads from the web members and span its full length meeting the strength and stiffness requirements of appendix 6 - not very likely - so additional bracing is usually needed.

 

[structSU10]

here is the paper on tension chord bracing.

 

[WillisV]

To answer the original question - the limit state that would be applicable is constrained axis torsional bucking which is stronger than weak axis buckling but someone less than pure torsional buckling - see:

http://www.aisc.org/store/p-2304-to...nal-buckling-tables-for-steel-wshapes-in.aspx

 

[Hokie93]

The Engineering Journal paper link posted by WillisV is a great resource. The topic of buckling about a prescribed axis of rotation is also covered in Chapter 5.6 of "Theory of Elastic Stability" by Timoshenko and Gere (second edition). The same chapter also discusses the case of buckling in a prescribed plane of deflection. Your case is buckling about a prescribed axis of rotation in my opinion.

 

[Hokie93]

ChrisNYCEng: Your 2% brace design force is conservative if you are designing according to AISC 360-05 or AISC 360-10. Appendix 6 (Stability Bracing for Columns and Beams) of both Specifications identifies the required brace strength for nodal bracing of a column (essentially, your situation) as 0.01Pr (1% of the top chord compression load).

In regards to the design of the bracing members, whether they are designed for 1% or 2% of the truss top chord force, another paper that provides an in-depth treatment of columns braced on one flange is "Torsional Bracing of Columns" by Helwig and Yura (ASCE Journal of Structural Engineering, May 1999).

 

[ChrisNYCEng]

Thanks for the paper Willis. Have downloaded and will review.

Hokie, whether the load is 1% or 2% our problem is the same as this is a high KL/r issue. We have decided to provide kicker bracing to provide lateral restraint at center span to the roof beams. This will provide us adequate capacity to support the chord bracing loads.

Also, as was not shown in the sketch, the bottom chords are also braced at every panel point.