Pedestrian mass for dynamic analysis of bridge

[Denial]

I am carrying out dynamic analysis of a proposed pedestrian bridge (natural frequencies, to be followed by a series of harmonic responses). The applied harmonic loadings are from pedestrian footfalls. The controlling cases are those where the "density" of the pedestrians is as high as possible before they begin to impede each other's movements, which I am assuming to be at slightly under 1.5 people per square metre.

My question concerns the mass of the pedestrians. Some proportion of their total mass will need to be considered as rigidly attached to the bridge deck, therefore needing to be defined as added-mass in the structural model. I expect that the proportion that is appropriate for vertical motion will not be the same as the proportion that is appropriate for horizontal motion.

Bear in mind that these mythical pedestrians are walking relatively freely, and so their legs will be more flexible than if they were, say, standing at attention. Also, they are walking randomly, ie with the phases of their footfalls randomly distributed. (I am aware of the possibility of motion-induced synchronisation, and will be investigating it later as a separate exercise.)

Does anyone have a feeling for what these proportions (for vertical and horizontal motions)should be?

 

[Qshake]

Denial,

In assessing seismic induced vibration response of bridges and other structures, it is generally accepted to neglect the mass of any feature that is not positively fixed to the structure. And let say up front that friction nor a terror-stricken grab onto the nearest file cabinet do not count as positively fixed.

For example, only in the most extreme cases of congested bridge traffic will we consider the mass of live load and that is more for the dead load imposed on the foundation than for exciting mass in the horizontal direction.

Simply put, if its not there reliably its simply not there.

 

[PXC]

I agree with Qshake, certainly there is no mass for the horizontal direction and I would think the vertical direction would be very small.

 

[Taro]

A good reference is AISC's Design Guide 11, "Floor Vibrations Due to Human Activity". The principal author is Thomas Murray, who is considered the guru of floor vibration design. Pedestrian bridges are covered as well as buildings. The guide says, "For footbridges... it is suggested that the live load be taken as zero, or at least nearly so."