Precast Raker Beams-Analysis

[WpgKarl]

Does anyone have any guidance on the analysis of precast raker beams for stadiums / arenas?

My two concerns are:

1. Vibration analysis

2. Accounting for the in-place geometry of the raker beam (the incline of the member, versus a beam that is level and flat) on the statics of the member.

 

[youngstructural]

I have been involved in the design and analysis of a recent stadium project (currently under construction). This work takes a great deal of study and work; I would suggest obtaining a copy of the ATC design guidelines for vibration, and as for the in-place geometry effects they are no different from any other beam.

If you have more specific questions, I will try to address them. I'll keep an eye on the thread.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[WpgKarl]

Thanks youngstructural

For vibrations, I am going to keep the natural frequency of the beams above a certain minimum, and will follow the PCI Design Handbook (and CPCI Design Handbook in Canada).

With the geometry, I wonder if the inclined beam is designed for the vertical loads on the horizontal span, or do you put all the loads into components along the inclined member axis, and perpendicular to the member axis and then design for the diagonal span length?

For the latter case, I guess one would have axial load to contend with and may have axial tension in the beam from the incline? Maybe tomorrow, I'll try to post a sketch - kinda crazy here today!

Working in Winnipeg, under my snow covered home, thinking of New Zealand.....

 

[csd72]

The moment is proportional to the horizontal span but the deflection is proportionate to the actual inclined span(with the perpendicular component of the applied loads).

If axial loads are small then they can be ignored, if large then the whole thing needs to be designed as a column.

Regards

csd

Working in UK, missing the snow already!

 

[youngstructural]

csd72 beat me to the punch... At least for part of the details. Your forces are always parallel with the line of action of gravity, and the sections can be modeled in software as inclined. This gives you the benefit of axial behaviour rather than purely bending, and reduces the effective bending length.

And keep in mind that if this is a large stadium, you're likely going to need prestress or post-tensioning to make the rakers work. Some pretty serious moment redistribution can help a great deal to get the supporting frames to be economical.

As for your vibrations, I'm not actually familiar with either of the texts you're employing, but definitely respect the publishers. I would caution you to make sure you are using recent texts (newer than roughly the middle 1990s) as you don't want to be analyzing a structure like this with foot-fall methodologies (which were created empirically against structures with significant inherent damping during the 1970s).

If you get stuck or would like an authoritative external reviewer, I would strongly recommend David E. Allen, PhD, who is retired, but previously of the NRC and is possibly Canada's foremost vibration expert. He's excellent value and very helpful... Also makes good dinner conversation for business meetings. *smiles*

Keep asking questions as needed! Hope to help as much as I can.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

And keep in mind that if this is a large stadium, you're likely going to need prestress or post-tensioning to make the rakers work.

Not necessarily - there are many stadiums out there that don't use prestressed rakers. It depends upon the available depth but usually mildly reinforced raker beams can be adequate.

The attached drawing shows a raker beam - non-prestressed, used in Texas. With the deep depth, the natural frequency is quite high and thus doesn't resonate with bouncing fans.

 

[youngstructural]

Respectfully, likely does not be definitely... And deep beams were not available to me during our design: Your architect may not allow them either. I was told, in no uncertain terms, that the client and architect would not allow "structural demands" to ruin the aesthetic of the stadium.

Added cost, complexity and stress to the job... But kept the rakers small.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...