[rydrienne]

What I have learned here is that I still don't know the answer.

[jsquared]

The illustration at this link really cuts through the fog..

How To Drill Through Floor Joists | thefamilyhandyman.com

Holes that are no more than 1/3 of the joist depth can occur anywhere along the joist provided that larger holes are centered along the long axis and all holes are away from the edges. Vertical shear forces are strongest at the bearing points. This condition is generally handled with additional material, so think about what's going on above before drilling a large hole here, better not to. Where horizontal tension and compression forces are greatest i.e in the center third of the span NO NOTCHING should occur.

[onedayold]

I'm no expert ( I did sell building materials a long time ago) however this question did jog my memory. As I recall, I believe the load bearing in a joist is done mostly toward the ends and diminishes as you approach the center. To picture this imagine the joist with a shape of a long football drawn on it from end to end. The area outside the football is the load bearing area of the joist, the area inside the football carries less if any load depending upon the construction. For example engineered I-beam joists have perforated knock-outs for running wire as well as duct within (in the web) but not but not outside of this football shaped area.

[jsquared]

I'm not an expert either.. but as far as my understanding goes you are on the right track "onedayold".. be careful when using the term "load". There are lots of kinds of "loads". If you are referring to "shear load".. that increases as you approach the bearing points.. imagine the difference between having someone step on your arm on a flat surface (shear above is localized at the foot but gets a little more spread out below)... or with your arm resting on a brick (shear is localized above and below).. the pain with the brick would be different and the more the muscle the less pain.. therefore shear tends to be handled with additional material. "shear" can crush or punch through... other "loads" that spans can havel.. the "smile" that a simply loaded beam (supported at either end) assumes is from deflection.. pretty much handled by compression above and tension below.. this is more a question of vectors and geometry.. disrupt the geometry at your peril... too much "deflection" can snap a beam like a broken twig.. it can also raise heck with windows and doors... "bending", that other "load" is a little hard to explain but engineers seem to like to compare it to "loaded taffy".. not a smile, more like a stretched out upside down omega shape in a beam under simple loading.. this can be a big problem... other loads include "impact" like when you suddenly drop a piano on the beam.. "thermal" causing expansion and contraction etc.. etc..

Anyway.. the football shape sounds pretty good as an illustration of where the work is being done as far as the material is concerned.

[ScarboroughME]

OneDayOld and Jsquared, You both have Just enough knowledge to be dangerous. The shear stress in the joist is maximum at the ends and goes to zero at the center. The bending stress is maximum at the center and goes to zero at the ends. The bending stress is resisted by the top and bottom of the joist whereas the shear stress is resisted by the whole depth of the joist. A 3/4" hole centered in a 10 inch floor joist is not going to get you in trouble but if you hack away at your floor support structure without knowing what you are doing... Go find a structural engineer.

[jsquared]

ScarboroughME.. a structural engineer distinguishes between bending stress and deflection. bending stress is dependent upon the strength of the material and the location and degree of load points above and below... not necessarily at mid point. The danger is assuming that it is always at midpoint. What happens when a piano gets moved? Or a lot of people start dancing at a party?

The handyman website that I sited on Jul 21, 2008, 03:23 PM does a pretty good job of explaining how to keep out of danger.

What's with the condescending tone? Who was talking about 3/4" holes? And what was inaccurate?

[ScarboroughME]

Jsquared, I apologize if I sounded condescending. I meant no disrespect. I only mentioned the structural engineer as a solution to a complicated problem. You are absolutely correct that points of maximum shear and bending can vary depending on the application of load. What I described was the stress distribution for a joist loaded uniformly over its entire length. I mentioned a ¾” hole because the string started with the task of routing “some 12/2 Romex” and it would be hard to get into trouble with a small hole.
Again, sorry to have offended you.

[jsquared]

ScarboroughME

It says a lot about this issue that the thread is so old.. you are to be commended for actually going back to the original question. I had forgotten the 12/2 romex.

Having enjoyed the framing modifications made by certain plumbers through the years (not all plumbers don't get mad out there some of you are friends of mine), I believe that this question should be more widely understood by one and all. The long term state of things found beneath bathrooms during demolition can be shocking.. and a tribute to the resilience of light frame construction. As engineered products capable of very long spans become more commonplace someone should come up with a rhyme or something to help us all get it right.

Thanks for the gracious reply

[jafoman]

From an engineers perspective, the bottom center of the joist is the area of the joist seeing the greatest tensile (pulling apart) force. Drilling a hole in this location should be avoided because it could significantly reduce the overall structural capacity of the joist (and if drilled through several spans, the overall structural capacity of the floor itself). The bottom half of the outer 1/3 of the joist over the span is primarily undergoing compressive (pushing together) forces. The floor joist would be better suited to have holes drilled in the portion experiencing compressive forces rather than in areas where tensile strength in the joist is required. The buckling capacity of the beam is affected less by the holes drilled in the compressive portion than the structural capacity of the joist would be affected by the holes in the area under tension. If you would draw a sine type wave from the top area of one end of the beam, dipping down along the bottom center of the beam and then back up toward the top portion of the beam at the other end... drilling above all areas above that line should be avoided. That leaves the bottom outer third on each end for use to drill for placement of wiring.

[mrcastrovinci]

I Think oldmanspoon answered the question?. ITS CODE right?;)

oldmanspoon

Jun 28, 2006, 07:23 PM
The 2003 International Residential Code covers this:
Not exceeding 8 feet (2438 mm).
R502.8Drilling and notching. Structural floor members shall
Not be cut, bored or notched in excess of the limitations speci-
Fied in this section. See Figure R502.8.
R502.8.1 Sawn lumber. Notches in solid lumber joists, raf-
Ters and beams shall not exceed one-sixth of the depth of the
Member, shall not be longer than one-third of the depth of the
Member and shall not be located in the middle one-third of
the span.