Steel Truss Diaphragm High Seismic | Requirements/Tension Only

[bookowski]

Without going into too many details, I am trying to figure out the requirements for using a steel truss diaphragm in high seismic regions. In particular, I am looking at examples from substantial buildings that were done with flat strap structural steel X bracing, visually I'd estimate the straps in the range of 1/2" x 6" straps. The buildings have various lfrs, from special concrete structural walls to special conc braced steel frames. The floor framing and columns are in steel, no concrete slabs and no unfilled metal deck.

I am specifically trying to make sure I understand the acceptability (or not) of tension only bracing in the diaphragm. These are being used where a tension only system would not be allowed in the main lfrs.

I am working on a project where this is being requested, and in researching it I have found very little info but I did find the examples described above which are fairly substantial buildings done by reputable firms using this sytem.

The way that I read AISC (copied below) this is not prohibited, at least not explicitly. The only relevant section that I find is copied below, and states that the diaphragm members should be designed using overstrength. There are two exceptions, but these are allowing to sidestep the overstrength factor, not specifying requirements or prohibitions on tension only bracing. Code aside, I can see an argument that tension only should be used for the same reasons it's not used in the main lfrs. But maybe if the overstrength is applied there is a line of thought that there are not the same issues with buckling/snapback etc.

FWIW I submitted the question to the solutions center and basically got back that it's up to me and not fully addressed.



AISC 341-16 (B5.2) says:
Truss Diaphragms
When a truss is used as a diaphragm, all members of the truss and their connections
shall be designed for forces calculated using the load combinations of the applicable
building code, including overstrength.
Exceptions:
(a) The forces specified in this section need not be applied to the diagonal mem-bers
of the truss diaphragms and their connections, where these members and
connections conform to the requirements of Sections F2.4a, F2.5a, F2.5b and
F2.6c. Braces in K- or V- configurations and braces supporting gravity loads
other than self-weight are not permitted under this exception.
User Note: Chords in truss diaphragms serve a function analogous to
columns in vertical special concentrically braced frames, and should meet
the requirements for highly ductile members as required for columns in
Section F2.5a.
(b) The forces specified in this section need not be applied to truss diaphragms
designed as a part of a three-dimensional system in which the seismic
force-resisting system types consist of ordinary moment frames, ordinary con-centrically
braced frames, or combinations thereof, and truss diagonal members
conform to Sections F1.4b and F1.5 and connections conform to Section F1.6.

Here's the commentary:
Truss Diaphragms
In some structure types, a horizontal truss is used in lieu of a steel deck or compos-ite
diaphragm. In such cases, there is typically an orthogonal grid of beams with
diaphragm-shear deformations resisted by members that are diagonal in plan.
ASCE/SEI 7 does not provide prescriptive direction on how to consider horizontal
truss diaphragms. Although there is a school of thought that diagonal and cross brace
members could be allowed to buckle or hinge as a source of additional energy absorp-tion,
the Provisions requires that these elements be designed for the overstrength
seismic load in accordance with the capacity-limited design approach of the Provi-sions,
unless the exceptions of Section B5.2 are met.
Two exceptions are provided to the requirement in Section B5.2. In the first excep-tion,
the horizontal truss is expected to provide ductility. In this case the members that
are diagonal in plan are treated similarly to braces in SCBF, with the orthogonal beam
system acting as the SCBF beams and columns. Under this exception, the beams
are designed using the overstrength seismic load and the diagonal members for the
basic load combinations. The second exception is for a three-dimensional analysis for
ordinary systems (OMF and OCBF) in which the diaphragm is treated similarly to an
OCBF and the diagonal members are treated similarly to braces.

 

[driftLimiter]

But maybe if the overstrength is applied there is a line of thought that there are not the same issues with buckling/snapback etc.

I want to be sure I'm understanding your concern, I'm going to paraphrase it how I understand it....

If tension only members are used in the 'diaphragm' they are typically detailed to buckle out of plane rather than resist compression loading. The buckling may be a source for non-ductile failure particularly around connections that undergo cyclical seismic loading. An analogy to this problem can be found in SCBF design of brace connections where the connection has to be detailed to achieve a minimum level of out of plane rotational ductility.

If this is the concern I do think its valid, and I'm not sure that simply designing the horizontal truss system to overstrength level loading alleviates this concern.

Some ideas to overcome the potential for failure there would be to provide out of plane brace points as often as possible, the higher the buckling mode, the less rotation occurs at the brace locations. I would try to avoid a super long slender flat plate that only has two support points to buckle about. Another idea would be to investigate the considerations used on a SCBF system brace connection and attempt to provide an equivalent level of ductility at the brace connections. Lastly, detail a tension-compression system and design for overstrength.

When I see these systems in the wild I generally see what looks like a bonafide tension compression system with considerably sized braces. The only exception to that is a PEMB. They use these systems as roof diaphragms and are almost exclusively tension only rod bracing. The detail of a rod brace connection may allow for considerable rotation without contributing to brittle failure though. Basically we have a rod through a hole, it can rotate almost freely as the brace buckles because theres really not much restraining rotation in the joint.

Curious to see how your thoughts on this progress as I'd like to have an approach for this system in my toolbox.

 

[bookowski]

Yes, that but more generally speaking any of the concerns with using a tension only brace in the main lfrs. I was hoping the code would be more clear, so I could either yay or nay it. But it isn't clear. The response from the solutions center said that it's basically up to me and that it does not appear to be prohibited, but at the same time you can see how an argument could be made for potential issues.

Under cyclic loading, if the braces yield and elongate then they have no (or little) stiffness in the next cycle until that slack is taken up. I suppose that if they are designed with overstrength you are theoretically limiting the input that goes into them, i.e. the special structural walls (or other system) limit the input by yielding prior to the diaphragm becoming inelastic. Maybe. The rotation at the connection is another one. I considered bracing these to the framing (16" to 24" o.c. range), but by doing so it becomes less clear that it's actually tension only and will get into trying to show that flat straps work in compression.

I also worry about having a diaphragm that is so discrete, with no redundancy. Normally diaphragms are some form of a plate, and even it gets the crap beat of it there's still a lot going on. In this system if one brace fractures you've got nothing. If it was tension/compression it's still very discrete, but feels like you can push it a lot further and it would lose much of it stifness as buckling occurred but things are still connected.

From nehrp: "tension-only bracing has had relatively poor performance during past earthquakes because the lack of compressive brace resistance leads to inelastic behavior with slack braces that have no stiffness until the slack is taken up. The slack braces may lead to progressively increasing drift and impact loading on the brace, and early brace fracture may occur. Consequently, tension-only bracing is also prohibited for the SCBF system"

You can see an example of this system here: https://seattle.curbed.com/2018/10/25/18023334/modular-hotel-south-lake-union-seattle. This is modular construction, went up in seattle with concrete cores. I have some of the structural drawings from this but not full details (and I can't ask the designers what their thought process was). They have done several of these same buildings in high seismic areas (one in menlo park, CA and another one going in SF).