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vibrations in a metal stair

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I have a few general questions with regards to the AISC Design Guide 11: Floor Vibrations Due to Human Activity as well as a few specific questions about its applicability to a structure I am designing. 
In regards to the Design Guide in general, could you please explain to me what exactly Po is supposed to represent?  Should it always be those values recommended in Table 4.1 of the Design Guide, or are there situations when other values are warranted? 
Next, with regards to my structure specifically:
First, let me give a basic description of my structure.  It is an indoor stair which has treads made of an non-structural material and these treads do not interconnect so there is no added stiffness or damping from these steps.  The stair spans approximately 45', and is assumed to have fixed supports at each end.  The stair is supported by a single Vierendeel truss which runs below the stair along the centerline of the stair.  The top chord is a large HSS rect. tube (obviously non-composite--no deck), and the bottom chord is to be a pipe section.  The vertical web members are also pipes.  The loads included for vibration analysis are structure selfweight, superimposed dead load, and a portion of live load, which represents a person on every other step (a crowded and heavy situation but one I could easily see happening occasionally in normal service).
I have analyzed the structure in structural analysis software which will solve for natural frequencies and corresponding mode shapes.  The stair's fundamental mode shape is out-of-plane (minor axis) vibration, and this frequency was found to be 6.1 Hz.  The second mode shape is in-plane (gravity and major axis direction) vibration, and this frequency was found to be 10.1 Hz. 
My questions about the criterion's application to my model are these:  Are the vibration criteria included in the Design Guide for indoor footbridges applicable to the similar case of indoor stairs?  Also, do I need to consider the out-of-plane vibration mode if I have no forcing function acting in that direction?  Or, would the energy introduced into the structure from the vertical forcing function still be able to excite a horizontal vibration?  Also, is the Po value recommended in Table 4.1 (=92 pounds for "Footbridge--Indoor") still an appropriate value for my assumed loading condition (i.e. multiple pedestrians on a stair)?
I would greatly appreciate any help you may be able to give me in this matter.
Daniel Kuemmerle