Design load distribution member! 1

[dcredskins]

What is the proper way of designing size of HSS load distribution member, as shown in attached sketch? Do we need to check the thickness of HSS for localized bending effect? If so, how? I would appreciate your help.

 

[H57]

I do not have a tremendous amount of experience in light gage construction, but it looks as though you are using the tube similar to a top plate in wood construction. I would check it for stresses/bending supporting a stud and joist bearing midspan between studs supporting the tube. I would also check the lower stud for compression based on an upper stud and joist directly above it. I'm sure others who are more familiar with this type of construction will chime in with input as well.

 

[msquared48]

From the loads you show, not knowing the unsupported lengths or any lateral load requirements, I would assume a minimum of 16 gage studs above, and more than likely 14 or 12 gage for the lower studs. At these loads, I would strongly recommend the use of 3" diameter pipe columns with a small wide flange over the top of them to rest the metal joists on. The lower steel stud wall could then be much lighter.

The detail you show with the tube spreader doesn't feel right to me, and the loads seem too concentrated to accomplish what you want. Plus, the spreader is being loaded on its weak axis which is inefficient.

Mike McCann
MMC Engineering

 

[dcredskins]

Mike and H57,

Thanks for the response. Let me make things little bit clearer. The studs above and below are already designed (just DL and LL, not lateral load). I have designed shear walls for lateral loads. My concern is the design of HSS load distribution member. The detail I had attached previously is the most common detail in load bearing CFM stud walls design.

H57, yes, I have already checked the HSS section as a whole for bending. Worst case scenario, I designed HSS for simply supported beam of 16" span and point loads from Hambro joist and stud above (11,5000 lbs total). That turns out to be okay. Now I checked HSS member for the plate thickness (horizontal portion of HSS tube) of 1/8" (thickness of HSS member) supported by two vertical sides of tube (3" deep in our case) for localized effect of point load on HSS tube, it did not work. However, I am not sure this is the proper way of designing the load distribution member or not.

 

[dcredskins]

Sorry, my bad. The total load is 11,500 lbs.

 

[H57]

When you check the top "plate" of the tube how did you distribute the load and what length (parallel to the tube)did you distribute the load.

Conservative way: 11500# load concentrated at middle, designed as simply supported. I would use a width equal to the width of the joist.

I would check it this way first as a quick check. If it worked move on - if not review more closely.

The 11500# is really distributed over an area decreasing the moments in the plate. The load would distribute laterally as well so the effective width will be greater than just the joist bearing width. The joist seat will also distribute the load and provide rigidity to prevent bending of the top plate. The edges really aren't pinned so they are somewhat fixed.

I guess what I'm saying is there are alot of areas that I would start to look at. Not saying that I would use all of the above, but I may consider some of them depending on individual cases. This really gets into engineering judgement at individual connections.

 

[csd72]

AISC hollow structural connections manual covers this type of thing.

 

[lkjh345]

JMHO:

I agree with msquared.

I would either put a small pipe or tube under the joist, or at least double or triple up the stud below the joist end. And call out on the drawings that this double or triple stud must be directly below the joist end.

I may be mistaken here, but to assusme that the load from the joist will be distributed out evenly to the studs below may require a fairly stiff distribution member. Off the top of my head, I am not sure you can get there with only 2" of depth to work with. I think the stud under the joist end ( or closest to the joist end) will likely see most of the load from the joist. And very little of this load will distribute to the adjacent studs.

Could be wrong on that. Been a while since I have done deflection vs. stiffness compatibilty analysis.

 

[jheidt2543]

I've seen something similar to this for the support of precast concrete roof plank where, instead of the HSS you show, a 6x6x1/2" angle was used with the leg down and welded to the steel studs to distribute the load. Admittedly, the precast is a more uniform load, but it is pretty heavy stuff and while I haven't run the numbers, I have seen the detail used. You might try talking to the Stud Manufacturer's Technical Services people.

 

[swearingen]

My question is are the lower 4" CFM's designed for the 11.5k? When a joist is over them, there is no redistribution - it all goes to the single stud below it...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[youngstructural]

I do quite a bit of CFS design, and the difference in stiffness and base material thickness between CFS and rolled steel usually has two important consequences:

1. Must specifically consider shear, as well as combination involving shear (such as bending & shear).

2. Members fail rather than "deflect" and allow redistribution.

IF I were dealing with a typical construction in timber, I would often be comfortable taking 1/2 of the load comming from the wall & OWSJ above and put it into the adjacent studs, simply because the centre post will deflect and allow this....

ie: __ <-- 1/4
|
|
TOP 1 -->|__ <-- 1/2 BOTTOM
|
|
|__ <-- 1/4

However, CFS has no plastic capacity... They also have little to no reserve capacity. You need to design for all of the load going into the stud below the OWSJ:

ie: __ <-- 0!
|
|
TOP 1 -->|__ <-- 1 (full load) BOTTOM
|
|
|__ <-- 0!

It's the relative stiffness & inability to redistribute any load or permit deflection that prevents the previous assumption.

Please note: I don't know if this is supported or contracdicted by any research, however it is my best opinion and how I carry out my work when dealing with CFS.

Potential Solution: Faced with this problem, I would simple use a double stud (back to back CFS) at each OWSJ support point, presuming a double stud suffices.

Let us know how it goes,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Of course at ULS you could also consider:

ie: __ <-- 1/2
|
|
TOP 1 -->|__ <-- *FAILED* BOTTOM
|
|
|__ <-- 1/2

Or use a header (spreader) that is stiff enough to actually distribute the actions)... Something like a W-section (I-beam), but this would be horendously inefficient!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[lkjh345]

Agree with YoungStructural.

YS: Good Post!