Holes in Joists, Beams, and ... Diaphragms!

"Where do the stairs go?  In the middle, silly!"


I love it when things come together and make sense on a big scale. Here is such a case.



As a consultant I sometimes have to deal with holes drilled in glulam beams that are `not okay’, or at least feared not okay. Generally I turn to the American Institute of Timber Construction (AITC) Technical Note (TN) 19, “Guidelines for the Evaluation of Drilled Holes and Notches in Glued Laminated Timber Beams.” Figure 1 of that TN is shown below (with permission).

Manufacturers of engineered wood trusses publish similar ... showing where, and where not to put holes, or notches.



Sec. 2308.8.2 of the International Building Code intimates similar for sawn wood joists.



In summary:



... you can have notches as long as they are not too large, and are not along the edges mid-span (or ends mid-depth) ...



... they may be in the corners, if small, except ...



... they may not be at load or bearing points (bearing critical zone of a beam) ...



... holes may not be too large, and they must not be near bottom or top edges, especially midspan, or ends, mid-depth.



Holes may-should be (if required) in the MIDDLE!



Sweet!



So why does the title include `diaphragms’???



Well, a diaphragm is just like a (big, flat) beam, isn't it!???



And so here is the situation: my clients, and students (architects, and architect majors) like to put holes in floor (diaphragms) ALONG THE EDGES! ... and not just small holes, BIG ones ... STAIRS!!!



Some may not even know clearly what I diaphragm is, or does ... so I’ll just say `FLOOR’!!!



The building codes, and engineers, know what a diaphragm is, and generally require floors (and roofs) to function as such.



They (my architects, practicing and aspiring) argue that stairs mess up `space’ when they are in the middle of a floor.



Well they mess up STRUCTURE if they are at the edges!!!



Okay – you can put in a small hole – shoot for a corner.



And it needs to be small.



No long stair runs parallel-alongside a wall (boundary to the diaphragm)!



The boundaries of the diaphragms – at the walls – are where load is transferred in/out.

AND, ... if the floor (diaphragm) is intended to provide lateral support for a basement retaining wall – serious load comes in along the edge! And then goes back out at the sides!



In (re-)summary, then, keep holes and notches small. Keep them away from the edges mid-span, and ends mid-depth. If they must be along edges – put them at the unloaded corners.



MY PREFERENCE IS TO PUT THEM IN THE MIDDLE !!! (of the joist, beam, diaphragm) ... and then I might even let you make them be non-small!



JRF ...