Bearing location

[Sifu]

I have been told a couple of different things about bearing measurements. Some say bearing is measured from inside dimension, some say outside, some say center. What say you folks?

 

[fatboy]

Inside to inside.........if we are talking about joists, rafters beams & such. At least here anyway.

 

[north star]

$ $

Here also, "inside to inside dimension", ...per Section R502.6, `06 IRC.

$ $

 

[dhengr]

Since your question lacks much in the way of specifics; I would say it depends on whether you are talking about ball bearings, spherical roller bearings or journal bearings. It might also be that this doesn’t have much bearing on your real problem, you just didn’t splain it well.

 

[Sifu]

Joists, rafters, beams etc. Sorry, I thought being in a framing forum it would have splained itself. Sometimes I have trouble figuring out what I'm talking about, I shouldn't expect others to know if I have trouble myself. I have always gone inside to inside but was told center of bearing is a better measure so I wanted to ax the experts.

 

[TheCommish]

i would say inside to inside, of course there could be an argument made by those more learned that it might depend on what, waht as bearing on, would the jost or beam fail before or after the concrete shelf broke off the face of the wall so the embdment depth (bearing) need to be futher back than the inside face.

 

[brudgers]

Length of span is measured inside to inside. Bearing length is measured from where it starts bearing to where it finishes bearing.

 

[DRP]

I think the code is silent but clear span is generally used.

Nitpicking-

NDS 3.2.1;

For simple, continuous and cantilevered bending members, the span shall be taken as the distance from face to face of supports, plus 1/2 the required bearing length at each end. (required in this sense is the compression perp to grain requirement, not the 1-1/2" code requirement).

That is for the bending calc.

For shear 3.4.3.1(a) for beams supported by full bearing on one surface and loads applied to the opposite surface, uniformly distributed loads within a distance from supports equal to the depth of the member, d, shall be permitted to be ignored.

 

[dhengr]

Sifu:

So far, I think we are talking about several different things. I think your OP was asking ‘what is the span length of a beam or jst., a spanning member?’ Do I measure this from inside to inside of bearing points/surfaces (the clear span) , or from center to center of the bearings? But, I’m not sure, because you won’t ask a complete question; don’t make us play 20 questions. In your case, for your purposes, I would be inclined to measure from center to center, that is what I usually use for my span lengths. It is not very often that a couple inches in a 8' or 10' span length makes or breaks the beam design, all else being properly considered. Building and designing in wood, and the material itself is just not that precise a process/science. If I were looking at jsts. bearing on 2x4 wall top pls., I would use the 10' clr. span, plus 3 or 3.5", probably 10'-3" which is 10.25', a nice clean number for my calcs. If this same jst. was bearing on 2x6 or 2x8 top pls. or sill pls., I would probably still use the same span length, not the increased length for the wider pls. My reasoning is that the real bearing point is something more than the clear distance but not as much as the 2x6 or 2x8 might imply. When the beam starts to deflect it starts crushing the sill pl. and the bearing point moves toward the center of the top pl., maybe the top pl. or sill pl. rotates a bit under this reaction too. At times clr. span might be slightly un-conservative so that’s not good for your purposes. The NDS sections DRP cites (NDS 3.2.1 and 3.4.3.1) are pretty much what defines my thinking above. This is something of an engineering judgement call depending on many different things. And, in some instances I might finesse a calculation a bit and in others the mode of failure and the variability of the material makes this kind of risky.

IRC Sec. R502.6 Bearing. Refers primarily to the min. bearing length, which leads to a min. bearing area; i.e. (min. length)(member width) = min. bearing area. The bearing stresses must still be checked, so they do not exceed allowable bearing stresses or compression perpendicular to the grain, the bearing length might have to be extended, since member width is a constant.

To really understand and use the IRC it is helpful if the Bldg. Inspector can learn to think a bit like a Structural Engineer, even if he/she has little engineering training. It will help you understand how some of the tables are developed, and why some of the code sections are the way they are. You should probably not be doing much design or engineering if you are not a Structural Engineer, but you should work on thinking like one. There are several good text books on elementary Structural Engineering and the basic design concepts in the various bldg. materials. They won’t make you an engineer but they will help you understand what you are doing as an inspector, and why parts of the codes are written the way they are.

 

[Sifu]

http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0077.jpg

I will be more specific in my questions. The reason I ask is to be precise when I am dogged by those that I turn down for over-spanning a member. In this case it refers to a rafter that appears to be over-spanned. The plan roof span is from outside wall to outside wall, making the rafter span half of that number, which is 20'2". The actual span measures about the same but is difficult to be precise when by yourself and climbing across the CJ. The rafter is a 2x8 finger-joint hem-fir which according to the table in IRC (802.5.1(1)) is permited to span 17'11". The rafter pictured here also appears to be over-notched though on another thread that has been called into question. This one might get nasty....school teacher pulling permit as owner/builder, purposely covered work without framing corrections or insulation being done, continuing to work after stop work order.... so I am trying to determine the exact extent of the deficiencies. There is a footnote in the WFCM that permits an increase in span (x 1.07 for 8/12) which allows a span of 19'2. So I am trying to figure out if his rafter is over-spanned by 2'3 (without using the WFCM and using outside dimensions) all the way down to 8" (using the increase in the WFCM and inside dimensions). I realize they would be over-spanned either way but I would like to have a definitive number when everybody starts screaming. I guess if the question included that it was a rafter it would have made a difference but sometimes you don't know what you don't know. I do try to study and understand some of the engineering principles behind the code, as the question (however poorly or incompletely phrased) in the OP might demonstrate. The owner has told me he talked with an engineer and was told everything was fine. I told hime to give me a signed, sealed letter addressing each point specifically but nothing yet so I am doing my homework.

 

[rogerpa]

Per the American Wood Council span tables tutorial. http://www.awc.org/technical/spantables/tutorial.html

When sizing joists, use the clear span - the length from support to support - not the full length of the joist.

Use the horizontal projection of a rafter, not its actual length, when figuring rafter span.

 

[dhengr]

Sifu:

You’ve just made my point about a complete question, with enough info. so the real problem can be discussed, with your last post. Look at how you have expanded the info. and scope of the problem from your OP. The trick always is knowing how much detail and info. is enough. A brief OP or insufficient problem definition can lead to wrong answers or directions and you certainly don’t want that, nor do we. And, pictures and sketches truly are worth a thousand words. It seem to me you are doing the right thing..., he brought up the fact that his engineer said it was all right, so just list the details you want the engineer to address, and have him provide calcs. and explain how they work and are o.k. in light of such and such code section. And, let them prove it! You should not have to do their design and member sizing for them, but you are obviously smart to be prepared and ready for some dust-up when you say something doesn’t meet code. You should be able to explain why it doesn’t meet the code, and why that’s important. But, you don’t have to provide the guy with a Ph.D. diploma in Structural Engineering.

There should be blocking btwn. the clg. jsts. (or a rim jst., rim board) and under the 2x4 sill pl., so the jsts. can’t roll over and the sill pl. is better supported. The way the guy has notched the rafter effectively reduces it to a 2x6 at the inside of the bearing on the 2x4 sill pl., the tail canti. should be o.k. if it’s not too long. Thus, you are probably looking at inferior 2x6 rafters rather than 2x8's. When that rafter splits, due to the reaction loading, at the inside reentrant notch, it might as likely split up into the rafter making it a bad 2x4. Tell the guy to have his engineer check the horizontal shear stress in the rafter at that point, and explain how he is going to prevent splitting up the rafter. The guy has made a lot of incorrect notching work for himself, for lack of understanding how to frame a roof. If he had raised the roof plane a few inches he would have more std. birds mouths, etc. NorthStar pretty much said this in your other thread, in his post #13.

Without studying Rogerpa’s AWC tutorial on span tables a little closer, I’m hard pressed to comment with certainty. Those tables could certainly be set up so that the defining span length would be the clear span. And, the fact that the actual span is another 3" or 4" longer is buried in the calcs. and development of those tables. The statement that the span length does not have to be the full length of the joists, is certainly correct. And, the way we typically treat and design common rafters is to use the horiz. projected span length for their design. Roof loads (snow, etc.) are based on the horiz. projection, and the dead load of the roof system should be adjusted from in-plane lbs./s.f. to a horizontally projected load. This is really a mathematical or geometric manipulation. Suffice-it-to-say, that the common rafter can be designed based on its sloped length or its horiz. projected length, when all other considerations are adjusted accordingly.

 

[rogerpa]

So you don't have to tax your brain, here is the AWC's on-line span calculator.http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

 

[Sifu]

In the other post I posed this question: Does the notch get measured from the inside of the seat cut (1 1/4") or the outside (3 1/2")? I am pretty clear on span lengths, just not where they should be measured from. This kind of ties into where the notch would be measured from doesn't it? In any case the difinitive question is "are his rafters over-notched?" Some have said yes, some are not so sure. I think they are according to the code language and I try not to over-ride that language without some better education...which I truly get on this forum (some of it I even understand). I have yet to hear back from this guy and he keeps right on working, right through the stop work order so he is making his own bed right now. At this point I am inclined to ley his DP address it. I have way to much on my plate to chase him around.

 

[dhengr]

Sifu:

In your photo, you are measuring the critical depth of the notch correctly, parallel to the member depth, square to the t&b edges; from 7.5" at bot. edge, to 6.25" at the reentrant corner, or 1.25" notch depth. I can about sketch on that photo how that member will split up its length, from the reentrant corner. The approx. 2x4 canti. out to the fascia is not as critical because its bending action tends to close a longitudinal crack or not cause one.

To understand the problem with that notched bearing, try this experiment: take a 2x4 - 3' or 4' long, at one end of the member cut a notch, thus; measure down from the top edge 1" and draw a line parallel to the top, in 2.5" from the end into the member, then draw a square line down to the bottom edge of the member. Cut the 2.5"x2.5" notch piece out of the member, but don’t over cut on either line or you just make the problem worse. Support this member on two sawhorses, one end bearing on the bottom edge and the other end bearing on the 1" remaining part of the member; this is a fairly good representation of your notched rafter. Load this member at the middle and it will split lengthwise, starting right at the reentrant corner, at a fairly low load, and very much dependant upon the grain orientation down the member length. On a new member cut the notch so 2" of material remains on the top (cut out is now 1.5"x2.5") the failure load and mechanism will be about the same. That’s the nature of this beast. If you now cut the notch so 3" of material remains on the top (cut out is now .5"x2.5") and if you put a generous radius at the reentrant corner this last member will start to act almost like a full depth 2x4. But, it will still likely split at less than full cap’y. because of the notch affect.

Your valley rafter has the same problem. Take a 2x4 and cut a 12" x 2.5" sloped seat cut on the end of it, like your valley rafter. Support this 1" from the end and load the member, it will split lengthwise, from the edge of the bearing surface. Support this right back at the heel, near the bottom edge and it will act like a full depth member.

When we write a prescriptive code, whatever it says exactly about notch depth, we have to be fairly conservative, because some users won’t have spent two hours in a college Structural Engineering class on wood design, on this exact topic. Furthermore, this problem is so sensitive to the wood grain and its orientation, that any notch might be a problem, and no notch is best.

You might want to send the guy a letter telling him he is going to be ripping out a bunch of his work, so you can do your framing inspection; and until you have the calcs. and details and explanations, signed and stamped by his engineer, he might be replacing some of the framing too. So, at this point he is working at his own risk, and may want to expedite his engineer’s certification of the existing conditions.

 

[Big Mac]

In previous code editions and in most older structural engineering textbooks, bearing points were often considered to be from the center of bearing support to the center of another bearing support.

I belive it wa sonly with the I-codes that spans were based on the clear span dimension, and then perhaps only for very specific applications.

 

[Big Mac]

In previous code editions and in most older structural engineering textbooks, bearing points were often considered to be from the center of one bearing support to the center of another bearing support.

I belive it was only with the I-codes that spans were based on the clear span dimension, and then perhaps only for very specific applications.

The second post is an attempt to correct grammer.