100lb point load rule per SJI

[SeizeTheMoment]

Per SJI, a 100lb point load applied between panel points does not need additional reinforcements.

Have you interpreted this to be MULTIPLE 100lb's applied across the entire joist? (i.e. Across the entire top and bottom chord, can there be a 100lb concentrated load between all the panel points)? Or is it only one 100lb across the entire joist?


Don't bend to the stress, seize the moment!

 

[sbisteel]

Draw a shear/moment diagram for the uniform load the joist is capable of supporting. As long as the induced shear/bending from the 100# loads does not create a larger bending moment or shear load than the envelope, it would be acceptable.

 

[SeizeTheMoment]

I was originally under the impression that the 100lb pt load is in addition to the uniform loads from the load tables. I.e. if a 28LH05 has a uniform load of 626plf per the table, it has a capacity of 626plf + 100lb between the panel points.

But now you bring up a good point, the additional 100lb's must be checked to ensure that the shear/bending is still within capacity? Maybe the 28LH05 now only has a 600plf capacity if it has a 100# point load between all panel points?

Thanks.

Don't bend to the stress, seize the moment!

 

[sbisteel]

The intent is that the top chord has local bending capacity as if you treated it like a beam spanning from panel point to panel point, to carry the 100# load. Anything in excess should have a field installed diagonal web member bringing the load back to a panel point (i.e. carried only by axial loads in webs & chords).

 

[Celt83]

@SeizeTheMoment:
Get yourself a copy of SJI Technical Digest 12, it works through several examples of manually evaluating joists, Link to SJI TD12

 

[cliff234]

Is the 100# value published in the SJI specification? (I’ve seen it vary from 100# to 150#.) That’s the maximum permitted point load between panel points to prevent flexural overstresses in the joist chord (presumably from miscellaneous loads from suspended pipes, suspended ductwork, and small loads on the roof). I always wondered about this load, especially when I stand on a joist-framed roof - because I weigh 200 pounds!

To my knowledge (I may be wrong) the global design of standard joists are based on the published uniform loads in the SJI load tables – and do not consider additional secondary moments due to 100# point loads between panel points (unless the joists are special (“SP”) joists designed for loads (uniform, point loads, etc) specified by the EOR. But the joist design DOES consider secondary moments due to the uniform design loads between the panel points. (example: If the SJI table lists the joist uniform load capacity as 400 PLF, then the global moment and shear are computed using the 400 PLF value, and additional secondary moments are computed based on the 400 PLF loading between the panel points. (Perhaps this is what prevented me from causing top chord local buckling failures when walking on roof joists all these years!))

 

[dold]

You can specify a "bend check" load on your drawings too that will supposedly eliminate the need for field installed web members.

 

[SeizeTheMoment]

@cliff234 thanks for the insight. The SJI 45th edition does have the 100# listed on page 15. It states "For nominal concentrated loads between panel points, which have been accounted for in the specified uniform design loads, a "strut" to transfer the load to a panel point on the opposite chord shall not be required, provided the sum of the concentrated loads within a chord panel does not exceed 100 pounds and the attachments are concentric to the chord."

Furthermore... "Although standard K-Series, including KCS joists, and standard LH-series and DLH-Series joists are designed specifically to support uniformly distributed loads applied to the top chord, research conducted by SJI, using 2nd order inelastic analysis, has demonstrated that the localized accumulation of uniform design loads of up to 100 pounds within any top or bottom chord panel has a negligible effect on the overall performance of the joist..."

SJI responded that provided the overall capacity of the joist is not exceeded, 100# can be applied to as many locations between panel points as possible.

I guess from your example, a 400plf capacity joist may be reduced to only allow something like 360plf with 100# point loads in all the locations between panel points...



Don't bend to the stress, seize the moment!

 

[lexpatrie]

FYI this is fairly recent, circa 2005 as I recall.