Stair Vibration Check

[mijowe]

After checking a monumental stair that i knew would have problems, I got into checking some of the other stairs in a building that fall a little outside of scope of a miscellaneous metals steel stair design. There are some good threads on her about stair vibration, taking that information along with DG 11, I am having trouble getting this stair to work by the numbers and would appreciate some input.

I have modeled the entire two story stair and have taken the analysis back to columns, some of the supporting beams are part of the main floor as well.

The analysis is in RISA, I take the calculated natural frequency, the DL weight, and plugged that into Design Guide 11 equation 4.1. using Indoor footbridges as my design parameters.

The stair has about a 35' x 18' footprint, and at this point I have increased my steel sizes well past the point of stress or deflection controlling the design. And I still have a natural frequency of around 5 and am over the 1.5% acceleration limit.

 

[mijowe]

pdf of the stair

 

[dhengr]

Mijowe:
The various sizes of stringers is kind of perplexing, although no doubt your FEA told you so. A fabricator will not give you a very good price when he has to order so many different sized beams, and different depths will just make the stair soffits difficult to finish easily. Finally, the landings kind of cantilevered off of two or only one beam which is supported, and torsionally loaded can’t be helping your problem. What would happen if you used what some here call a ‘cranked beam,’ a Zee shaped beam? That is, three stringers, same size, for any run that were continuous members; lower landing, sloped run, and upper landing, as a continuous beam from supported beam to supported beam? Many structural engineers seem to think that they must use rolled shapes, they must be better. Maybe your stringers could be built-up members, with a three piece spliced web and two continuous bent flange members. I don’t know what your tread and riser arrangement is, but those could be cut into the built-up web also, in multiples of two from a single plate. To your vibration problem, deeper is better (stiffer), may not need to be a heavier stringer overall. But, more mass in the entire stair system will help also.

 

[271828]

...take the calculated natural frequency, the DL weight, and plugged that into Design Guide 11 equation 4.1.

This is very incorrect. See the following journal paper:

Davis, D.B. and Murray, T.M. “Slender Monumental Stair Vibration Serviceability,” Journal of Architectural Engineering, ASCE, December 2009.

 

[paddingtongreen]

I'll be a little stronger than dhengr, this not a good arrangement. Cranked beam from column line beam to column line beam are the correct way to go.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

Another thought - you seem to be modeling the landing slabs with large fictitious "X" members to model in-plane stiffness. However, there are many other member joints that are not directly tied to the X. I would suggest breaking up the X's into a smaller grid such that every member joint is engaged.