Industrial building design 2

[RattlinBog]

Let me know if I should move this to the student section. I work full time at an industrial facility as an owner-structural engineer, but I'm also working on my M.S.C.E. part time. This post is technically about my plan B grad project, but I feel like it's relevant to any industrial structure design, too.

For my grad project, I'm working with my professor to take one of the large structural steel buildings at the place I work and design it from scratch with modern codes as a thorough exercise. Original structure was built in the 70s. My goal is to use the same overall building geometry but analyze and design it for ASCE 7-16, AISC 360-16, etc., and try to cover as many structural systems and components as I can--diaphragm, LFRS, purlins, girts, columns, floors, connections, foundations, etc. I'm trying to do most of the design by hand and only rely on software to double-check things. I don't have a lot of overall building design experience in practice (especially now that I'm in an owner role), so this project has been helpful to understand how individual components act together in a system, especially for lateral loads.

Would it be okay if I used this thread to ask occasional questions as they come up?

If so, here are some questions about diaphragms, purlins, girts, and eave struts. (I've read through and bookmarked several past eng-tips posts about this, but there are still a few questions in the back of my mind.)

Building info:
Ground snow load = 70 psf; basic wind speed = 107 mph
150 ft (north-south) x 456 ft (east-west) x 91 ft mean roof height.
Roof: purlins on trusses @ 24 ft O.C. (north-south)
Diaphragm: horizontal brace system at bottom chord of roof trusses (assuming roof decking attached to purlins is not designed as a diaphragm)
Lateral force resisting system (LFRS): 3 lines of braced frames (east-west); and built-up/braced cantilevered double column tower @ 24 ft O.C. (north-south)
Building is similar to a PEMB in that it's a massive structural steel frame with a lot of open space and no intermediate diaphragms, but all structural members are hot-rolled standard AISC shapes (no Z-girts or rod bracing, etc.)


Questions:
See attached calcs for purlin and girt design.

1. My understanding is cladding/roof deck will provide continuous lateral bracing for the exterior flange of girts and purlins for pressure wind loads. For suction/uplift wind loads, sag rods will provide discrete lateral bracing to the interior flange by preventing twist through a resistant force couple with the cladding/roof deck. Is that what others have seen? (I saw some eng-tips members from Australia and Europe mention bridging instead of sag rods, but I haven't come across that in the buildings I've seen locally.)

2. Is cladding dead load typically picked up by the eave strut through sag rods in tension? My understanding is that girts (assuming channels) are not designed for gravity loads in weak-axis bending and are only meant to resist wind loads in strong-axis. The gravity loads (cladding) are brought up from the girts to the sag rods to the eave strut.

3. Is metal roof deck typically ignored for diaphragm design in large industrial buildings? That's been my assumption so far, and I'm only relying on the horizontal plane bracing in the roof to bring lateral loads to my LFRS.

4. Are eave struts typically designed for both gravity (cladding dead load) and lateral (as a collector for bringing wind loads to LFRS) demands? Or is it better to have two separate members? Seems like two members might be warranted as the girts and LFRS bracing would be offset.

Thanks!

 

[RattlinBog]

Running into a Risa-3D issue (hopefully a simple operator error...)

See attached. In my full 3D model, all of my roof trusses have 0 axial force in the top chord...makes no sense! Made a copy model and deleted everything except one frame and locked into 2D mode. Instead of an area load, I applied an equivalent linear load. Compression in the top chord works just fine in this 2D model like it should. I made no other changes to supports, releases, etc.

Anybody seen an issue like this before? The axial force doesn't even show up if I click on a detailed report for the top chord. Possible Risa glitch?

 

[Eng16080]

Running into a Risa-3D issue (hopefully a simple operator error...)

Did you accidentally click on that member and unselect it?

 

[RattlinBog]

I don't think so--nothing was inactive. There are 20 LFRS frames just like that one spaced 24 ft O.C. They all have the same issue with no axial force in the top chord.

Bottom chord force looks right too, so it's kind of like the top chord force is just hidden/glitching out.

Per beam-truss analogy, truss chord forces should be about +/- 118 kip:

(70 psf * 24 ft) = 1.68 klf
M = (wL^2)/8 = (1.68 klf * (75 ft)^2)/8 = 1181 k-ft
C = T = M/H = 1181 k-ft / 10 ft = +/- 118 k

 

[bones206]

That's weird... what kind of axial forces are you seeing in the roof diaphragm braces under gravity only load?

Did you model a RISA diaphragm at the roof level?

 

[dold]

Running into a Risa-3D issue.......all of my roof trusses have 0 axial force in the top chord

1) if you have a rigid diaphragm associated with the roof level then all of your joints at that level (and truss top chords) will have no relative displacement in the horizontal plane, therefore no axial force in top chords. If you compare your deflected shapes in your 2D vs 3D view it looks like the top left and top right nodes (and all nodes in the plane of the roof) in the 3D model have displaced the exact same amount to the right. But in the 2D model this is not the case. Axial force may still develop in the bottom chords due to the vertical displacement of the nodes of the trusses. Rigid diaphragm does not constrain vertical axis.

2) If you're using an area load over the entire roof make sure:
a) orientation of the load attribution is perpendicular to the trusses. In your 3D model it looks like your area load is spanning parallel to the length of the trusses (brown/tan/orange arrow above the load diagram)
b) all joints defining the area load are co-planar with the top chord of the trusses. To check/validate this (it's been a while), once you solve the model there will be some additional basic load cases (called '....transient load....something') generated for the resultant line loads attributed to the members from the area load (area load * trib unit length). They will be at the bottom of the dropdown list of BLC's. Turn on display loads and select one of these BLC's. Or look at BMD of top chords.

Care to share your model?

 

[RattlinBog]

bones206 & dold - Thank you! I did apply a rigid diaphragm at the top chord level, and that's definitely the problem. I only learned how to use the rigid diaphragm and automatic lateral load features in Risa a couple weeks ago, so I haven't learned all the nuances yet. Before that, I've always just applied lateral loads manually.

I don't even have the rigid diaphragm in the right plane. It should be at the bottom chord truss bracing level (oversight on my part).

I just tried moving the rigid diaphragm to that bottom chord level. While the top chord now has axial forces, as I suspected, the axial forces in the bottom chords are now 0. Is there a way to get around that? Right now it seems like I can only have diaphragm forces in my model, and I might have to check gravity loads in a separate model...

As far as area loads, I have them applied to purlins, so that's why the loads are spanning parallel to the truss chords. The transients loads show up.

My model is attached with the rigid diaphragm at the top chord level--no changes made.

 

[RattlinBog]

Anyone have experience with working around rigid diaphragm loading issues in Risa-3D? See above. Diaphragm prevents relative displacement and causes some internal forces to not act properly (i.e. no axial forces in truss chord, etc.) Wondering if there's a workaround besides applying loads manually w/out use of diaphragm.

Sorry for double post

 

[RattlinBog]

I spoke too soon. Attached is a response from Risa support--I should've reached out to them first. Looks like a workaround might be to move the diaphragm and apply rigid links, but that could be fairly tedious depending on model size. I might give it a try on a smaller trial model.

 

[bones206]

I don't think a rigid diaphragm is needed or appropriate to evaluate this building. It has a horizontal brace system at the roof, which is an explicitly modelled diaphragm. You can just distribute your tributary lateral forces directly to the framing members or nodes, rather than have them distributed out by rigid links.

 

[RattlinBog]

You're right. I think I got excited when I learned about the diaphragm/automatic lateral load generation features that I had to try them and see them in action. The time-savings of auto load generation is really attractive but not if it causes odd/unexpected behavior in the model.

Speaking of braced frame diaphragms, do you folks typically assume that they are rigid or flexible diaphragms (or semi-rigid)? Or do you have to check deflection vs. drift to be sure? From what I've read, I believe a braced system diaphragm would be rigid unless depth-to-span ratios get out of hand and/or if the LFRS is even stiffer than the horizontal braced system. Perhaps the diaphragm could be rigid in one direction and flexible in the other...but I'm not sure.

I setup a spreadsheet to check lateral load distribution based on both a rigid and flexible diaphragm to get a sense of upper and lower bounds, but the differences can be fairly major.

 

[bones206]

This type of building would generally be analyzed with the roof considered as a flexible diaphragm. You could check the deflection vs drift to see where that lands, since this is an academic exercise after all. Considering flexible in one direction and rigid in another is certainly a possibility. The MBMA Seismic Design Guide has some good discussion on this if you can get your hands on it. I'll copy and paste some here:

Approach to Metal Building Roof Diaphragm Rigidity (Flexible vs. Rigid) and Accidental Torsion

Diaphragm Flexibility

Applied forces are distributed within any building in a direct relationship to the rigidity of the structural elements of that building. A significant factor is the rigidity of structural elements that transfer forces horizontally, relative to elements that transfer force vertically. For either extreme of this relative rigidity between horizontal and vertical elements, engineers have developed simplified design approaches to determine force distributions. The two extremes are defined as follows:

• Flexible Diaphragm: The rigidity of the horizontal diaphragm is very small relative to the rigidity of the vertical systems.

• Rigid Diaphragm: The rigidity of the horizontal diaphragm is very large compared to the rigidity of the vertical systems.

Analysis using either of these bounding assumptions produces results that vary in accuracy depending upon how closely the actual structure matches the simplifying assumptions. Although many (perhaps most) structures fall somewhere between these extremes, more accurate analysis can only be done by using complex finite-element models that are generally not practical to use for ordinary building designs.

2006 IBC Section 1602.1 defines a flexible diaphragm as having a lateral deflection of more than two times the average story drift of the vertical elements supporting the diaphragm, and a rigid diaphragm as everything else. This definition requires calculation of diaphragm deflection, which is complex and imprecise for many types of diaphragm construction. Therefore, it is important to be able to select and use appropriate simplified assumptions to obtain rapid structural design solutions.