Tributary width of the bottom track 1

[Trainee339]

Hi All,

I am designing a bulk head as show in the sketch. I am bit confused about the tributary width of the load , bottom track needs to be designed for. Please share your understanding.

 

[r13]

If the panel below the pin supports is non-deformable, the bottom chord will not be stressed, the upper supports take 100% of the load.

 

[Trainee339]

It is light gauge steel. And the bottom chord is in the air. The above supports are in to the concrete soffit (pin supports).

 

[r13]

Yes, I understand. My previous response was for the gravity load case. For lateral load, it is a beam (the panel) supported by pin on top edge, and elastic support (the tension rod/member) at the lower points. Hand calculation can be quite tedious, I suggest to analyze by FEM method.

 

[jayrod12]

For the bottom track it isn't that complicated to need an fem analysis. It's have the length of the studs plus half the height of the window below. If nothing below, then just half the height of the studs above.

 

[r13]

This is a hanger type loading system. There are two probable conditions for gravity load:

1) Infill panels are fit in the frames without fastening. The bottom chord member takes 100% weight.
2) Infill panels are fastened to the frames. The pin supports take 100% weight.

My suggestion on PC analysis stands.

 

[jayrod12]

Design the bottom track for the full gravity load of the panels above and half the height of the panels for wind (my previous post was indicating wind load design trib width only, that's my fault for not being clear).

No fancy analysis required. Bottom track designed in under 5 minutes. Waste of time to create a FEM analysis for every situation. What did they do 20 (30, 40....) years ago before every answer was "That's what the model shows".

 

[r13]

Try both - hand calc and FEM, you'll see the difference and learn.

 

[jayrod12]

I'm afraid you're missing the point.

It's not necessary. Nor do most employers budget for that level of analysis on a steel stud design job. Most steel stud design is a delegated design item, and therefore the fees are fairly cutthroat. If you have to whip out a computer program for every small detail, you will not get the job or at a minimum you will continually lose on those jobs until you stop taking them.

The client (the steel stud contractor) doesn't usually care if it is 18 Ga track instead of 20 Ga. But they care about getting the design yesterday. They're paying you to do it fast. You think they want to spend $150 for you to build your fem model and do the analysis instead of increasing the bottom track thickness costing them a couple of bucks. *Caveat: There are exceptions to every situation, I'm talking in general and this detail I'm seeing here is an extremely common detail that requires no more than a quick calculation.

The advice we provide here should be intending on teaching young engineer's practical design and not pushing them towards software right away. And I'm a young engineer by many people's standards. I hate having to open up my software if not necessary. It's a waste of time and a larger source of error in my eyes.

 

[r13]

This is a necessary step, if you are new and started solving an unfamiliar problem with hand calculation, to verify the results, and firm up your conviction/understanding. I too believe in simple is the beauty, but experience is required to make things simple. If you are experienced, this question won't be raised. If you are new, do both ways.

 

[DaveAtkins]

FEM is not necessary, but a 2D computer analysis would be helpful. The diagonal kicker induces compressive or tensile force into the vertical stud, which must be accounted for.

DaveAtkins

 

[r13]

Trainee,

If you are a fresh engineer, try solving simple problem using more than one approaches, and always verify the results against each other. It will accumulation your experience, and consolidate your knowledge/understanding.

For present case, other than the pin supports, you should pay attention to the light gauge panels, the vertical and the inclined members, as your design most likely be governed by the lateral load. Since the panel is light weight, the size of the vertical member could be just be good for the bottom chord member.

Don't let the name "FEM" intimidates you, nowadays all analysis programs are finite element based. I suggest to do a 3D analysis, though you can simplify the model down to 2D with valid assumptions. Do it later if you wish to learn how to reduce 3D problem to 2D.

 

[Trainee339]

I would like to thank all of you for such a wonderful discussion.
For Vertical loading, bottom track, I am designing for gravity load due to the lining (minor axis) also checking for the uplift and suction for the adjacent soffit connected to it.
For lateral load bottom track, I was confused about using half of the height of plenum as the pressure are bit higher and needs a bracket to connect the droppers with the track. I was trying to reduce those reactions to avoid bracket.

For dropper stud and diagonal members, i checked for the axial forces and respective moments and shear forces.

I decided to take the half of the tributary width of the total plenum to the bottom chord.

Thanks

 

[r13]

I think either one of the two suggested modifications will make the distribution of lateral load much clear.

 

[Trainee339]

Yes I think option 1 is a good idea.
Unfortunately option 2 is not possible.

 

[r13]

Me too, like (1) better.