I've run to 20 modes just to see what all is happening. The second mode shape is vertical movement of the entire stair assembly at 4.93 Hz. I can probably eliminate this mode since there is cross-bracing between the wall assemblies that I haven't modeled. The first mode shape appears to be lateral movement of the wall hanging assembly and corresponds to 4.54 Hz. I've modeled these because I feel, if detailed properly, these will help stiffen the stringer for vertical movement - think some vierendeel truss action. As a design note, there are HSS tubes hanging from the stringers to carry a wall. Although my modeled angles aren't as often as I'd have risers.ģ. The angles modeled basically eliminate these modes and I feel this is reasonably accurate because I'll have a closed riser and pan assembly that creates an angle anyways. I consider these extraneous to what I'm doing. I can run the model without these but I get a lot of local mode shapes for the plates themselves. I've modeled small 元x3 angles between the stringers at intermittent locations matching the plate nodes. I've modeled flat plates between the stringers to capture the deep beam action for horizontal sway.Ģ. Short of modelling every pan, I've done the following:ġ. I'm trying to capture the actual (or reasonably close) frequencies for this in RISA. I used the simplified models from the new AISC DG11 for stairs but they are requiring some very large stringers. Strength and deflection aren't really an issue - but I'm concerned about vibrations. The pans will be at least 3/16" thick and will have closed risers. I've got some relatively large HSS stringers on each end (HSS16x4). It's one span with no opportunity for posts and is about 7' wide. The stair spans about 33' measured on the flat and has a rise of 18' total. I'm looking at a monumental stair design.
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December 2022
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