Steel stair bearing support 2

[cjung]

I have some questions about Diagonal Steel Stair bearing support configuration; fixed at the top and elastomeric bearing at the bottom. I am concerning about the elastomeric bearing pad won’t accommodate the vertical downward component of movement during the service life (Temp+LL+DL). (Due to the diagonal shape, the elongation will have vertical and horizotal component.)

Question:
1. Is it common to have the steel stair fixed at the top and elastomeric bearing pad at the stair bottom?
2. Are there any installation concerns due to the support condition?

(Please see the attachment)

 

[cjung]

anyone has any steel stair design experience, please help me!

 

[BAretired]

cjung,

The connection at the bottom of the stair must be able to slide horizontally in order to accommodate any change in length of the stair. It cannot move vertically because of constraints which have been imposed by the design.

Suppose the initial length of the stair measured diagonally is L. Suppose the height difference is y and the horizontal dimension is x. Then, just after installation x² + y² = L². This can be written x² = L² - y² or alternatively, x = (L² - y²)^0.5.

If the stair changes in length to L* due to the various effects mentioned in your post, then y remains the same, so we have the relationship x* = (L*² - y²)^0.5. The bottom connection must be able to accept a horizontal movement of x* - x.

I see nothing wrong with using an elastomeric bearing pad under the stair to permit movement, but the stair should be fastened to the foundation using anchor bolts operating in slotted holes to accommodate the expected movement.

BA

 

[cjung]

BAretired:
Thanks for your kind explanation.
I think, your explanation is valid under the condition that bottom of stair will be perfectly level all the time. Am I right?
While installing the stair, we may need steel shim plates to make it level. However, I think the bottom will constantly rotate due to nature of loadings, Temp+DL+LL, the bolts won’t prevent the rotation. Therefore, i thought the elastomeric bearing pad won't function in the end. I hope I am wrong.
Is top-fix-bottom-elastomeric typical bearing assembly for steel stairs?

 

[BAretired]

cjung,
The bottom of stair will be close to level, but may not be perfectly level. Shim plates could be used to ensure that stringers are at the same level but otherwise should not be required. Rotations due to LL should not be sufficient to cause concern. If deflection is held to L/360 as required by most codes, end rotation is only 0.011 radians.

Rotation due to DL will be present when installation is complete and will not change during the life of the stair so is not a consideration. Temperature change does not cause end rotation so is not a consideration.

In my experience, top hinged, bottom roller is the typical bearing assembly for steel stairs. I have never specified elastomeric pads at the bottom of stair stringers.

BA

 

[cjung]

BAretired:

Completeness, attached the bearing details, the designer used.
I appreciate your kind responses.

 

[KootK]

As well, elastometric bearing pads can be proportioned to accommodate any anticipated rotations. The suppliers of the pads often provide dsign guides.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

While I agree in theory that allowances should be made for horizontal movement at the landing of the stair stringers, in practicality it is rarely necessary. Most stair sections are not designed with slotted bottom connections and most are not allowed to "slip". There is little or no detrimental result from this.

I've analyzed and designed many stair sections. Most of these have exterior exposure, including direct sun exposure, so thermal movement could be an issue....but it rarely causes issues.

The upper connection of the stringer is usually designed for shear, even though there is actually little shear in the connection. If you think about it a bit, the stair section is actually a ladder....lean it against the wall and make sure it doesn't slip out at the bottom.

 

[wannabeSE]

Most of the time, I see slotted holes at the bottom (slots parallel to stringers) to accommodate some inter-story drift. I've never seen elastomeric bearings for typical steel stairs, only a steel plate bearing on concrete at the bottom. More attention to details is needed for stairs in buildings with big drift (special moment frames in high seismic areas).

Revised post to clarify my comments are about the bottom connection only

 

[BAretired]

Interesting! From the above comments, it appears that there are several ways of dealing with the support of steel stairs.

The upper connection of the stringer is usually designed for shear, even though there is actually little shear in the connection. If you think about it a bit, the stair section is actually a ladder....lean it against the wall and make sure it doesn't slip out at the bottom.

This is an interesting view of a stair. A ladder leaning against a wall which is prevented from slipping out at the bottom has a vertical roller at the top and a hinge at the bottom. With uniform load W, the vertical reaction is W at the bottom and zero at the top. A horizontal reaction H = W.x/2y is required at both top and bottom which must be resisted elsewhere in the structure.

I prefer to provide a hinge at top and a horizontal roller at the bottom. With uniform load W, the vertical reaction is W/2 at each end and H = 0 top and bottom.

Either method is statically admissible but the latter method is more common in my area.

BA

 

[BAretired]

cjung,
Looking at your detail, it appears that you have used top hinged, bottom roller support. I don't see any anchor bolts at the bottom but perhaps they are shown on Detail 3/ST323. I believe that nominal anchor bolts should be provided to prevent the stair from shifting laterally.

BA

 

[cjung]

BA:
Top has embedded bolts as shown.
Detail 3/ST323 attached.

I am working for a contractor as the structural engineer.

 

[BAretired]

cjung,

My concerns with Detail 3/ST323:

1. No keeper bolts to prevent lateral or upward movement during seismic event.
2. Corrosion due to welding dissimilar metals (stainless steel to structural steel).

BA

 

[cjung]

BA:
Vertical movement was my original concern.
you stated that there won't be any vertical movement.

But thanks for helping me out.

 

[BAretired]

cjung;

There won't be vertical movement as a result of temperature change, dead load or live load but in a seismic event, who knows?

In any case, it is not good structural engineering practice to leave a member simply resting on its support with no physical attachment other than bearing. It should be held in position to prevent lateral movement and uplift and should permit limited movement parallel to the stringers.

BA