gymnasium & running track deflections

[ehjelle]

Hello,

I've got a 120'x108' gymnasium. The roof joists span 108'. There is also a 10' wide running track on the inside perimeter of the gym hung from the joists every 21'-8" to 24'. The joists are spaced at 12'o.c., with each end where the track is parallel with the joists, the joists are spaced to match up with track below.

When the joist is parallel with the track, the track is hung from an individual (or double) joist(s) at 4 equally spaced points along the joist. When the track is perpendicular to the joist, the support is about 10' in from the building's perimeter.

Not having run dealt with this situation before, I think my primary concern is deflection of the track, especially where the joist and track are parallel as the track will be influenced by the full deflection of the joist.

So, my thought is that I should determine what amount of movement I will allow in the track due to live loads, then translate that in to an allowable deflection for my joist. I was originally thinking of about 1/2 inch in the track, equating to about L/2160 in the joist which seems a bit extreme for a 108' joist. Is this general approach valid? Is 1/2 live load deflection too conservative?

I'll be happy supply additional details, though right now I'm thinking about this problem in more general terms. My thinking goes that if I determine a realistic deflection for the track, the question of joist layout will solve itself.

Thanks,
Erik

 

[BBruins]

I've designed a number of buildings as you've described. I double up the joists where the track runs parallel with the joists and hang the track from each joist where the track is perpendicular. I support the track at approximately 10' on center in both directions (12' shouldn't be that much difference). The 10' on center allows me a place to connect a continuous handrail around the track at each hanger location. I've never had a problem with excessive track deflection or roof deflection. It sounds to me like you are on the right track.

 

[COEngineeer]

I just look at it seperately. Start with the running track with Live load/total load deflection you need. then you will have a point load at the roof joist consisting dead and floor live load. Then design the joist/beam with that point load+roof dead and live load.

 

[Lutfi]

Few points to ponder:

1. I like to caution you to check the vibrations from human impact due to running. SJI does not require you to check vibrations on roof joists while they require it for floor joists!

2. The deflection at the track will be calmative. You will need to add he track deflection to the joist deflection!

3. I read that the joists will be spaced 12 foot on center! Or is that only spacing for rood joists that will support the track? If the roof joists will be spaced at 12 foot on center, you need to select proper deck.




Regards,
Lutfi

 

[ehjelle]

Lutfi - You read correctly, joists are at 12'oc. I'll be using 3", 18 gage deck. Per your other comment, the cumulative deflection is what concerns me - a small deflection for the joist is not small for the running track.

 

[COEngineeer]

ahh.. now I kinda understand it now. You only have to add the live load deflection of the roof to the track right?

 

[ehjelle]

COEngineer - yeah, actually 42psf snow, but that's correct. Live load on the track will cause some deflection, and snow load on the roof will cause some deflection.

 

[SlideRuleEra]

Two years ago I was involved in the evaluation of an existing structure similar to yours. Here are general background documents that I found useful:

Modern Steel Construction, March 2001

http://www.aisc.org/ContentManagement/ContentDisplay.cfm?ContentID=910

MSC, September 2001

http://www.aisc.org/ContentManagement/ContentDisplay.cfm?ContentID=987

MSC, April 2003

http://www.aisc.org/MSCTemplate.cfm...Management/ContentDisplay.cfm&ContentID=21998

MSC, April 2004

http://www.aisc.org/template.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=25918

Institute for Research in Construction

http://irc.nrc-cnrc.gc.ca/pubs/ctus/ctu22e.pdf

and AISC Design Guide 11

http://www.SlideRuleEra.net

 

[DaveAtkins]

I just finished the design of a building similar to yours. I did a vibration analysis of the channel at the edge of the track, as well as the steel beams I put in the roof to hang the track from. If you do a vibration analysis, you may find that you need stiffer members than some deflection criteria would indicate (I ended up with C12X20.7 suspended by rods @ 12'-6" oc from W18X60 beams at the roof--these beams span 25' between pre-engineered steel frames).

DaveAtkins

 

[mitchelon]

Have you considered designing the running track with a camber? From a visual standpoint, a camber is more desirable than an excessive deflection.

 

[youngstructural]

If I remember right, when designing atheletic facilities, cambering out the dead load is always recommended. A very good idea from Michelon...

About the vibration issues: I've dealt with alot of vibration issues, and enjoy keeping up to date on the procedures. Whatever you do, make sure you are using a walking/runing resonance analysis method. SlideRuleEra's suggestion of AISC's Guide 11 is bang on. It is THE reference for your situation, and has easy to follow design examples in both Imperial and proper metric units (sorry guys, couldn't resist!).

You will be designing against an acceleration limit, and I suspect that DaveAtkins is right, but only being too mild. I would hazard a guess that you absolutely will have to stiffen your system to a much greater extent than that required for deflection criteria. A good primer for vibration issues is Dr. David Allen's article: Allen, D.E., Pernica, G., “Control of Floor Vibration”, Institute for Research in Construction – National Research Council of Canada. It's available as a free download. If you can't find it, let me know... I'll either post it for you, or find the link.

ALSO: Do keep in mind that specifying your OWSJ and steelwork should include the resultant requirements with respect to vibration. I worked for a very good structural engineering firm back in Canada that has adopted Guide 11 as a standard on their structural notes for OWSJ load plans. Basically I would say that you should speficially dictate that OWSJs be provided which have been checked against Guide 11, and stipulate your requirements to be met. The Guide will fill in the gaps for you.

Please let me know if you want any help. I love vibration problems, but really appreciate those opportunities to prevent them, rather than spend time trying to get them to go away once they've come up.

Regards,

YS

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

 

[ehjelle]

Thanks, everyone, for many helpful tips & discussion.

One other question...considering 2000 IBC loads (table 1607.1), I'm applying 100 psf live load to the running track. The 100psf LL equates to 1400+/- really big persons (250lb) on the track. I realize that with running, impact forces are greater than static loads, being cautious about vibration is important, etc.

For purposes of calculating deflection only (I will design strength for full 100psf), and also realizing that IBC 1607.9 doesn't allow live load reduction for public assembly occupancies, am I going outside the bounds of engineering judgement if I consider a lower live load for purposes of deflection?

Put another way - I don't consider it realistic that the track can be fully loaded, more like 140 people instead of 1400 people, BUT, I can't shake the feeling that I am cheating if I consider such a drastic reduction in live load to deal with my deflection concerns. Any comments would be welcome.

Thanks again,
Erik

 

[civilperson]

The track can be and will be fully loaded at a pep rally with the entire student body crowded on that upper level and jumping up and down to the pep band as the varsity is introduced at the Homecoming Spirit Rally. (Seen it and had to vacate the track when failure seemed immanent).