Steel Framing Residential Construction

[jreit]

Hello. I am looking at a set of draft residential plans with an engineer friend just to grow my knowledge.
He's handling everything professionally - I'm just trying to expose myself to a different field since I work on bridges mostly.

The plans show a new floor being constructed over existing construction. Typically, wood framing would be used but due to the rising lumber prices they're using steel framing.
I'm a little concerned of the steel members being used.
The HSS 6x6x3/8 seems underdesigned.
Roughly speaking for a 40 psf residential LL and assuming ~15 psf DL, we get a 55 psf total load.
For ASD and a trib width of 23' for the highlighted 21.5' beam in the attachment, we get a uniform load of 1.27 KLF.
Moment Demand ~ 75 kip-ft
Zx ~ 16 in³
Design Moment Capacity with the 1.67 reduction factor and Fy of 46 ksi ~ 37 kip-ft

Am I missing something?

https://files.engineering.com/getfi...7b8-45b3-91e5-d9e334cae0be&file=Document1.pdf

 

[Enable]

Maybe I am a bit batty but to me it seems like the TS 6x6x3/8 call-out is referring to the column. In the cut section the girders are W-sections so I would assume the other section would be the same (the one you are concerned about). The plan also shows square columns go go with the HSS call-out.

You sure the beams are HSS?

 

[GC_Hopi]

Ditto.

 

[jreit]

Yes you're right, that makes sense that those would be the column callouts.
That section would probably work for the columns since it's just axial loading.
Also, what would be the purpose of the all the steel stud framing? Is it just to help with vibrations?
The main steel beams would still need to be designed for the load and deflection assuming the studs aren't there correct?

 

[canwesteng]

Steel studs support drywall in the walls. Do you mean what are effectively the joists (or would be called joists in timber construction, but in steel I think are called infill beams)? The girders will need to be designed for the load, but unless you have a concrete floor, you'll need all those infill beams to support the sub floor.

 

[jreit]

Yes, I mean the joists/infill beams. Ok that makes sense.
Any issues that could be caused by mixing and matching steel framing and columns one side with a concrete wall on the other side? Does the unequal stiffness matter?

 

[Enable]

Could there be? Yes, of course! But is there likely to be in residential construction? Nope. We mix steel and wood and concrete all the time in residential. Ever wonder how when you walk into the basement of a home you find spans nearly twice as large as any other room in the place? You have steel beams and more than likely some intermediate posts (people hate the posts...they always want to remove the posts). We also sometimes have concrete foundation walls on one half of a structure and wood framed on the other half if it's built on a steep slope!

The issue with stiffness, as you know, is that it'll attract more load relative to other less stiff members. But that doesn't mean the load gets there. If the distributing members (or diaphragm) are wet noodles are they going to be able to distribute the load? Not a chance in hell. They will go limp before it gets to the support. In other words, if you have a flexible diaphragm loads will be distributed based on tributary area rather than in proportion to stiffness. In residential, with OSB sheathing and sub-flooring you typically have a flexible diaphragm.

For example

For wind design, ASCE 7 permits all diaphragms constructed with wood structural panel sheathing (e.g., plywood or OSB) to be automatically idealized as flexible in Section 26.2

If you are interested in learning more about low-rise buildings / residential then you should get your hands on the following books:

Fundamentals of Building Construction by Edward Allen & Joseph Iano: Perhaps the single best text I've seen on construction means/methods and materials. I have been struggling with this for nearly a decade seeing as the authors are Architects (us building folk don't tend to like their ilk...so it's hard for me to accept that this book is so good given that it is written by architects!)

Structural Design of Low-Rise Buildings: An okay book but covers more details on the design aspects of residential structures. It also reviews a bunch of different material types. A rather good introduction to general design philosophy for low-rise buildings.