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RE: Existing Bar Joist Capacities and Problems.....

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I would look at the time frame of the construction of the building. Visually you can look at the joist profile and determine some basics. The only real ambiguity is if the bottom chord is constructed from angles or hat channels, the joist could be 36 or 50 grade steel. Contact your local joist supplier to determine the general dates of material usage in your area. It varies geographically. Also many local municipalities keep their old drawings for a long, long time and/or convert them to microfilm.

Strengthening bar joists is a possibility. They are analyzed as simple trusses. Shortening the span can reverse the load in interior web elements. Often times bar joist web members are designed as tension only elements. They will need to be augmented if they carry compression. The SDI has some guidance on welding on bar joists. It may be cheaper to add a structural channel to replace or augment the structural carrying capacity of the joist.

Another possibility is using a load test. Structures generally perform much better than the numbers indicate. Use the procedure for load testing in AISC Design Guide 15, p 219. Provide shoring and deflection gauges. I like to use drums that can be filled with water to provide the load. You can stop applying load if it looks like you are going to yield a member.

Regards,
Harold Sprague


-----Original Message-----
From: Daniel Boltz [mailto:dboltz(--nospam--at)1st.net]
Sent: Tuesday, July 13, 2004 10:17 AM
To: seaint(--nospam--at)seaint.org
Subject: Existing Bar Joist Capacities and Problems.....

I have a copy of SJI's 60-year manual and know the general procedure for identifying the existing bar joist designation. However, it is becoming quite common to identify a bar joist but not be able to determine whether the joist is an H-Series or a J-Series.

1. When no tags can be found to identify the joist, is there another procedure besides testing a sample to see if it is 50 ksi or 36 ksi material?

2. For renovation projects, many times a corridor or office use is required on bar joists that will only support a live load of 40 to 50 psf. What are the proper procedures for strengthening a bar joist to support the IBC 2000 required loads of 50+20 psf for office space and 80 psf for corridor space? Is it acceptable to cut the span in half and build a new joist seat or would the possible reversal of forces cause problems for the "new" joist span?

Thanks.
Dan

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