TYPE OF OPEN WEB STEEL JOIST?

[chicopee]

I am having difficulty identifying a joist which is at least 35 years old. Top chord consist of two channels (estimated at 1.5" deep by 1/4" thick). Bottom chord consists of two angles (leg size estimated at 1.5" by 1/4" thick). I have no engineering data and info on the type of joist. Current material on internet does not provide much help.

 

[JAE]

Was there a joist tag on the joist?

 

[chicopee]

JAE, I dont know. Where underneath the roof would you suggest to look for the joist tag? And would each joist be tagged? By the way the two channels are 2.5" deep and not 1.5" as originally stated.

 

[samdamon]

Look for a small piece (say 3/4" X 6" roughly) of stamped sheet metal tied with wire near one end of the joists. The wire is usually wrapped around bott chord or bottom of a web member. Not every joist is always tagged, you may have to look at a number of them, at both joist ends, to locate one. Are you adding new loads on the joists?

 

[chicopee]

Samdamon, yes to the "added load" question. All calculations are done except that the type of joist was assumed to be a 24k4 serie. The depth of the roof joists is 24", web rod 3/4" dia and width of the bottom chord is 3-5/8", consequently my assumption puts me in a ball park but I am still off the mark because current literature describes K series having angles for top and bottom chords. I could be dealing with KCS serie but so far I have not been able to verify this either.
With my current assumption, the added loads are not significant to the roof support system, however I can not submit what I have to the buiding inspector until I can identify the type of joists.

 

[apetr26542]

35 years old would put you about 1970, you are more likely into a J or H series, J series have a max allowable tensile of 22 ksi, H series have 30 ksi, K series did not exist. All J,H, and K series have a 2 1/2" bearing depth,the long span joists have a 5" bearing depth. Verify the web diameters in a few spots as this can vary at the ends. If your load does not work assuming J series, you may have to have a sample tested for tensile strength. There really is no other way to tell if you can't find the tag.

Try to get a copy of the Steel Joist Digest, they have come out with the 75 year Digest 1928-2003 and covers all joists manufactured with specifications and load tables.

 

[samdamon]

Chicopee, I believe KCS type joists are a relatively recent innovation and were not available until well after your orig construction date. Shapes used as top and bott chords are not any help in narrowing down your joist size because unfortunately different joist fabricators were allowed to use different shapes to achieve the required propeties. There was no standardization in the shapes used. Your best bet is still to find a tag if you can.

 

[apetr26542]

Chicopee, are you adding point loads? What percentage is the bending moment being increased?

 

[connect2]

Tags r ok and for the most part u can find them .... related more to erection/placement and fabrictor, rather than load capacity .... but at the end of the day ... and this seems to be a never ending problem here .... how do you determine the structural capacity? SJI aside, how do resolve shear requirements when you analizing a pin jointed truss? ... panel points being < 24 inches.

 

[chicopee]

Thanks to the responders for their inputs. I finally got to find out the type of joist by snaking my way thru the suspended ceiling and all the electrical wiring. The tag on one joist indicated the following" Ganon Steelworks;made in Canada;J3- Cumber". So obviously we have a 24J3 joist but I coul not figure out the word "Cumber", so if any one knows what it means let me know. Also I would appreciate a copy of a load table instead of using my stress calculations.

 

[ntpe]

chicopee,

The older AISC steel manuals contain joist information including load tables and Fy's. Check the light blue manual - I think thats the Seventh Addition.

 

[JedClampett]

Could "Cumber" actually be "Camber"?

 

[rholder98]

Be careful--'J3' may be the fabricator's piece mark, and not necessarily the size. That is, a whole row of 24J#, all the same size, could be numbered J1, J2,. . ., for manufacturing and erection purposes. The tag said "J3", not "24J3". If you checked other joists, then you've probably already verified. It may well be the size, but just keep that in mind. All the joist shop drawings I've reviewed number the joists that way.

"Cumber": maybe the manufacturer's client; i.e., the general contractor who built the building? I like JedClampett's thought better.

 

[JedClampett]

I got out my old Blue Steel manual and looked at the joist table. They don't list a 24J3. The smallest 24 they list is a 24J6. So either they didn't make the 24 inch joists with a J3 configuration or AISC didn't list it. Either way, I'm no help.
Maybe if you call Vulcraft or another large joist supplier, they can tell you if they made 24J3's and what the capacity would be.

 

[chicopee]

To Jed clampett: What does your manual list for uniformely distributed load and moment of inertia for a 24J6 joist?

To rholder98: I am incline to believe that you are correct when indicating J3 does not indicate either an actual or nominal value for width of joist eventho this had been my impression. Publishers appear caggey about the design data of joist cross sections.

To apetr26542: Ca not answer your question because of lack of data on J series; however these are my values:
uniform load : 81 lbs/lf
point loads : 379 lbs at x=26'
190 lbs at x=33'
reactions : 2185 lbs at x=4'
1950 lbs at x=44'
Overhang : -324 lbs resisting shear at x=4'
-648 ft-lbs resit. moment at x=4'
average shear stresses: 1537 psi at x=4'(A=1.42 in^2)
1248 psi at x=7.83'(XA=1.24)
Truss Analysis @x=26' where bending moment is max
at +20,637 ft-lbs
Sc=15,400psi thru centroid of top chord
St=20,324psi thru centroid of bottom chord
Rc=341 lbs thru 3/4 dia diagonal
Based on yield strength of 22,000psi I am about 5% lower in my resultant stress for the lower chord which is too close for comfort--What do you think?

 

[JedClampett]

Don't forget, this is the old Blue Book, AISC 7th Edition. It's on page 5-249. Also, it doesn't give Moment of Inertia, but has moment capacity and shear capacity.

 

[apetr26542]

To Connect2, if you compare the actual shear diagram of your loading to that of the trusses shear envelope, this can be used to determine if you are overstressing the webs. The shear envelope is based on the middle portion of the joist having a minimum 25% shear capacity of the ends.


Chicopee, if you are under the allowable stresses, would think you are ok, for a 24J6 which is the smallest published for a 24" joist the allowable moment is 30.5k-ft, this is based on load tables published in 1971. However, since you are adding point loads, you may be overstressing certain web members, can you construct the allowable shear diagram, also these point loads should be added at a panel point or a reinforcing web angle added.

 

[connect2]

to apetr26542, Yes understand the SJI requirement. Re. as a udl across owsj, the central webs increase from zero as you move left and right from center symetrically, therefore minimum 25% of reaction needs to be applied to all webs since 'truss analysis' gives zero load to center webs given symetry. However the 25% reaction was removed in canada in 94 or so, dont have it here to be sure on date. we recently analysed an existing joist under SJI 25% reaction requirement and canadian requirement and found the canadian requirement more stringent ... ie larger axial loads, plus sign. We assume .. there's that word again, that the 25% of reaction (SJI) are compression loads? Is there a parallel in strut and tie modeling with RC and OWSJ webs?