Existing Open Web Steel Joists
Existing Open Web Steel Joists
(OP)
I found an interesting web site about the analysis of existing open web steel joists.
http://www.pdhonline.org/courses/s117/s117.htm
Mr. Stuart recommends against analyzing existing joists if you don't know the manufacturer because of the variability of manufacturer's products from the SJI catalog. What do you do if you have to add a load to an existing joist, such as a new HVAC unit, and all you have to go by is 16H5 joist designation (circa 1973 construction)?
http://www.pdhonline.org/courses/s117/s117.htm
Mr. Stuart recommends against analyzing existing joists if you don't know the manufacturer because of the variability of manufacturer's products from the SJI catalog. What do you do if you have to add a load to an existing joist, such as a new HVAC unit, and all you have to go by is 16H5 joist designation (circa 1973 construction)?






RE: Existing Open Web Steel Joists
Alternatively, you can use the SJI standard load tables to reconstruct a joist capacity diagram (for both shear and moment). Then compare that to your new shear/moment diagrams to verify whether you are OK or not. This usually suffices for small concentrated loads.
You can also add steel beams between joists to support new loads - connecting them to the supporting girders.
RE: Existing Open Web Steel Joists
JAE's approach is one that I have used over the years to evaluate new loads on old joists. This, obviously, gets tempered with greater safety factors as less information is known.
RE: Existing Open Web Steel Joists
JAE's other comment: "Alternatively, you can use the SJI standard load tables . . ." is not a bad idea to estimate the average capacity of a given size / span of owsj, but there are hundreds of fabricators, sometimes including Ma & Pa operations. Subtle differences in the connection details can result in substantial differences in joist capacity so I doubt the SJI tables alone can offer the requisite precision you need for your analysis.
RON's comment that: "Early joist design was largely empirical." is true, but for joists fabricated in 1973 the SJI standards or the CISC Standards in Canada should apply. The rest of RON's comments are right on the mark also.
I'd add a few more thoughts that may be relevant, namely:
Have you considered full scale in-situ load tests of a few joists? Obviously the size of the project dictates whether this is justified, but testing is a definitive solution to the assessment of load capacity.
Regards,