Cold Rolled HSS stair stringer Q's

[shacked]

I am designing a HSS beam used for a residential stair. It will be a single HSS stringer cold bent to an approximate 4ft inside radius and span from ground floor to 2nd floor. I have never designed a steel beam that is bent in 2 directions like this is, so I have a few questions.

Are any of the steels physical properties changed after bending, such as yield stress?

IF anyone has designed this type of beam what would you suggest that I design each of the end supports for? Fully restrained both ends, pinned one end and fully restrained the other?
Or any information or direction as to where I find information about designing members like that would be greatly appreciated.

Sorry for the very basic plan, but this is what the architect provided

 

[KootK]

Or any information or direction as to where I find information about designing members like that would be greatly appreciated.

 

[KootK]

IF anyone has designed this type of beam what would you suggest that I design each of the end supports for? Fully restrained both ends, pinned one end and fully restrained the other?

I like the following for a system likely being integrated into light frame construction. If you're actually tying into heavy concrete or steel, additional restraint might be plausible based on stiffness and connection competency. Longitudinal and transverse refer to the stringer itself as it enters the connections.

Bottom

Plan view longitudinal = pin
Plan view transverse = pin
Vertical = pin
Rotation all 3 Axes = free

Top
Plan view longitudinal = pin
Plan view transverse = pin
Vertical = free
Rotation all 3 Axes = free

 

[KootK]

What I described above won't actually be in static equilibrium. You'll need some form(s) of rotational restraint I think. I imagine that's some combination of:

a) Strong axis moment top and bottom and/or;

b) Torsion top and or bottom.

Again, much depends on what your supports are. I see steel support beams in your future.

 

[shacked]

Awesome, thanks guys I appreciate the references.

 

[shacked]

So, I reviewed the curved member design guide but the majority of the information is limited to either vertical or horizontal curved members and not spiral curved beams as in a stair stringer.

Since this is a single stringer stair run used in residential I was hoping that I would be able to calculate the stringer(HSS) as a straight beam using the curved length, then design each of the supports for the torsional moment.

Unfortunately since I am strictly residential and most of my projects are relatively small I do not have any finite element software.

Any suggestions?

 

[KootK]

So, I reviewed the curved member design guide but the majority of the information is limited to either vertical or horizontal curved members and not spiral curved beams as in a stair stringer.

Well, yeah. I doubt thatyou'll find much guidance specific to spirals anywhere. You'd have to adapt the the circular stuff to your situation as best you can. Also, in my experience, spiral bends often end up really being compound circular bends anyhow.

Since this is a single stringer stair run used in residential I was hoping that I would be able to calculate the stringer(HSS) as a straight beam using the curved length, then design each of the supports for the torsional moment.

That's something. From a stress and tread deformation perspective, you probably also want to account for torsion / twist in some fashion.

Often these things are governed by vibration. In the absence of FEM, or the kind of mathing skills that I no longer have, I guess just keep the deflections pretty small.

 

[KootK]

In your shoes I would probably be okay with treating this as a horizontally curved member only for strength checking. Maybe one run at the actual radius and another at a radius expanded to match the real length of the member.

 

[driftLimiter]

Review the design guide first to understand the physical limitations of bending.

These limitations will restrict the shapes that the member can reasonably constructed of. You may end up with a section that is controlled by the bending operation and way overkill for strength and stiffness.

Then I would do some strength/deflection analysis of the beam over the 'actual radius' and 'radius expanded to match real length' as @KootK suggests.
Hopefully you can find a section that meets all three of these requirements (limit the deflection to pretty small).

 

[JStephen]

Step No. 1, contact a fabricator and make sure it can be done at a reasonable cost.
You may find it can be done, but have to ship it halfway across the country.
Or could be too tight a radius where it's not easily feasible.

 

[shacked]

Thanks guys, your responses are pretty much what I expected...not in a bad way. I realize there are radius limitations based on the tube section as well as the shops bending process.

I have a meeting set up with the fabricator this week so I suspect some of my questions may be answered then. There is a shop relatively close to where I live so I'm not worried about shipping.

Again I appreciate all the input.