Steel Web Joists & Girders 2

[ashhafPE]

My client does not have blueprints for two commercial businesses he was retained to replace A/C units. Old units are approximately 2400 lbs and new units are 3000 lbs. Several field inspections were performed but no luck indetermining the type of steel joists used (J, H, LH, DLJ...etc). Field Measurements of the Top & Bottom Chords do not match some of the tables on-Line. My analysis of the joists revealed higher reactions and moments than shown on some of the tables that could or match the type of joists. I double checked my work. I do not want to take chances and assume. Any suggestions?
Any idea's?

 

[Vesvictus]

you couldn't find a joist tag on the joist?

 

[UcfSE]

Can you pick the smallest joist with a depth equal to or less than what you measured? For instance if you measure 24" pick 24k4, 23" 22k4. Check the joist seats also. K series have 2.5" seats, LH and DLH have 5" seats except for sections 18 and 19 that have 7.5" seats. That's all a very rough estimate and assumes you can figure out what was done within reason. If they had any kind of special detail requirements then who knows what's there. If you can figure out what's going on well enough to be comfortable I would definitely reinforce the joists instead of adding more.

 

[jike]

Get joist tags. They will usually have the sizes and manufacturer. Contact the joist manufacturer and request the specific. info. on the joists (including complete layout, all memeber sizes, yied strengths and welds. Sometimes you will be able to get this info. depending upon who the joist manufacturer was and how complete his files are.

If that doesn't work then you must completely measure the joists including all panel point locations and member sizes. Try to get the yield strength info. from the manufacturer. Note that the web strength might be different than the chords. Analyze the joist as a truss using the SJI specification for the year that the joists were manufactured.

One word of caution: Sometimes existing joists are overstressed in their current state so don't just say well we are only adding 600#......

I hope this helps!

Good luck!

 

[jike]

One other thing.....

Make sure that you personally verify the existing loads in the field both below the roof and above. Things often change from the design drawings. Roofs getted re-roofed (sometimes heavier, sometimes lighter). Owners add equipment. Ceilings change (sometimes lighter, sometimes heavier). Buildings get additions (sometimes creating drifted snow).

I think that you get the idea!

 

[jike]

It is important to look at the whole problem in a realistic way! One never knows, until you dig into it, whether the existing was underdesigned or overdesigned but certainly it will change the approach that you must take. Rarely, does something need to be "condemned or ripped out". It just may need additional reinforcing that the Owner was not counting on paying for.

It is your professional responsibility to fully analyze the complete situation.

This has nothing to do being "stress free"......but that is a whole different subject!

 

[Lutfi]

I just went trough hell on similar project. Fortunate for me, the joists were built in the early 60 (H series for those who recall them). I added a roof top unit that weighed 3800 pounds. I had to evaluate the effects on the new concentrated load plus the wind overturning moment.

I called Vulcraft, John Hancock Joists and their engineers, I must say, were very helpful. The concern was stress reversal due to the shift in the vertical shear.

I recommend that you get a copy of SJI’s 75 YEAR MANUAL 1928 - 2003. I won the previous edition. This is a valuable reference if you get involved in renovation work and need to evaluate capacities of steel joists. You can get a copy from their web site, for modest fee. Here is the web site:

http://www.steeljoist.org/publications/

As a last resort, I would as-built the joist members and run a 2-D model using STAAD or similar software and had check the stresses using the joist specifications that are published by SJI. You may want to confirm the period that they were built. I would also search for the tags, as previously mentioned. The tags hold good information that may be of good help.


Lutfi

http://www.cdeco.com

 

[TFL]

has anyone ever had any luck with joist tags? i have never seen a size on a joist tag... only a piece mark and a joist mnfr job #. i have caled vulcraft several times nd they have always told me they do not have the data that is over say 10 years old.

 

[ashhafPE]

Thanks for the responses. I found vulcraft tags, however the vulcraft office I contacted had no record of having done the project in that location.

 

[LPPE]

tfl is right on - if your building is more than 10 years old or so, forget it - joist tags are worthless. Tags older than that were basically used for erection purposes (J1 goes here, J2 goes there, J3 goes over there, etc...).

 

[bjb]

I have had the same problem with joist tags. If you can't justify the existing joists for the new loads, another approach would be to build a roof top platform. That is the approach I recommend when I can't determine what the existing joists are. There are lots of unknowns, and joists are not known for their reserve strength or tolerance for abuse. If you don't know what the existing joist is to start with, I don't see how you can reinforce it, unless you tried to do it in such a way that the reinforcing caried all of the stress, not just the stress from the proposed loads.
If you do a computer model of the joist based on field measurements you can get a reasonable estimate for the member stresses. In order for the joist to be ok the stresses in the welds also have to be ok. I question if it is possible to accurately field measure the welds that tie the joist together.

 

[jike]

I have had luck with joist tags from many different manufacturers. Make sure that you look at both ends of the joist and perhaps at several joists. I have run across multiple tags (metal or paper)on joists. Some of them are painting tags that don't usually have any useful info. on them.

 

[dasman]

I had mixed results with joist tags depends on the manufacturer. I've actually had the opposite experience of LPPE -- in our area it seems that the older buildings will usually say on the tag what the joist is -- the newer buildings usually list the code/piece number. It probably depends on who the predominant manufacturers were for the area at the time of construction.

I agree with the rooftop platform -- we do that often on existing buildings.

Also keep in mind that many older buildings proprietary (non-SJI) joists were popular -- Macomber Allspan and Kalmantruss are two that come to mind off the top of my head...

 

[TpaRAF]

Are the same number of joists being loaded? Could cross-members be used to distribute the loading to more joists (hence lowering the load per joists)?

 

[ashhafPE]

All members are different sizes. Here is one scenario: 22"x4" joists at 5.5'spacing and spanning 37'with 2(1-3/4" x 1-3/4" top chord) & 2(1-1/2" x 1-1/2" bot chord). The main girder supporting those joists is spanning 50' and measured 28"x9" with 4"x4"x3/8" top & bot chord.

 

[teero]

ah Vulcraft. that is thier standard answer. they hardly keep any records. I have always taken the conservative approach to joists. In Canada thay are exposeed to the design live laod and I have seen many problems. They are designed very sharply. I avoid overstresing them more than 2%. Reinforce them.

 

[steeler709]

The age of the joist is very important. There are still joists around that were made with A7 Steel with allowable stress of 18KSI. Also lots of J-Series with allowable stress of 22KSI. You may be able to extract a coupon for analysis without destroying the joist, possibly the end of one of the bottom chord angles. Once you are sure of the chord material, you can measure the cross section of the Bottom chord and assume that the angles are fully stressed. Calculate the force in the Bottom Chord and work back to moment and then to uniform load. Look in the appropriate SJI Load Table for the derived load at the span of the joist you are working with, this should point to a joist designation. Once you have the desingation, you can check for existing and proposed loads. All of this assumes you have a "standard" SJI joist.

 

[steeler709]

I just remembered. The approach I offered will work for any K-Series joist. For S,J,or H-Series, it will work as long as the span of the joist under consideration is greater than the critical span. The critical span of pre-K-Series joist is denoted by a heavy horizontal line in the load table. (At spans below the critical span the bottom chord is not fully stressed under the load-table load.)