Metal connectors for timber frame buildings

Started by kyounge1956, August 18, 2009, 03:37:56 PM

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Don_P

No those are not constants they are variables. Each species and grade has a different allowable bending strength (Fb), stiffness(modulus of elasticity, E), and allowable horizontal shear (Fv)... amongst other variables (these plus Fc perp, side grain crushing strength, are the most commonly needed variables).

The variables, called "design values", are available in a download here;
http://www.awc.org/pdf/2005-NDS-Supplement.pdf

Be careful as you wander through the use of those tables. Heavy timbers are in table 4D, these are members 5x5" and larger, table 4A is for dimensional sizes (lumber) 2-4" thick. In the heavy timber there is another spot to pay attention to, timbers that are within 2" of being square are classed as "posts and timbers" where those of rectangular shape are "Beams and stringers".

I was sizing around some pretty nice materials based on your area #1 Dougfir is good stuff and readily available to you. To give you an idea I use alot of eastern white pine here Fb 575,E .9,Fv 125. That kicks us up to some pretty hefty sized timbers to compensate for the comparative weakness of the material. To look at another material, Fb for steel is around 28,000 psi  :o

The purlins can tie the building together but do not support the rafters. A ridgebeam, properly sized, supports the rafters. In the case of the socket systems the bents support the purlins which support the roof, they do not strengthen the bents, they load them. Nothing wrong with that as long as its all designed correctly.

kyounge1956

Quote from: Don_P on August 28, 2009, 11:41:01 AM
(snip)The variables, called "design values", are available in a download here;
http://www.awc.org/pdf/2005-NDS-Supplement.pdf

Be careful as you wander through the use of those tables. Heavy timbers are in table 4D, these are members 5x5" and larger, table 4A is for dimensional sizes (lumber) 2-4" thick. In the heavy timber there is another spot to pay attention to, timbers that are within 2" of being square are classed as "posts and timbers" where those of rectangular shape are "Beams and stringers".(snip)

I have a nasty cold this weekend and my brains are only working at about half-power. To see if I understood how to use the tables, I tried to find the design values you used in your previous example in the linked Design Values document, but I didn't see anything that matched exactly. There are three places in Table 4D with variations of Douglas Fir in the title: Douglas Fir-Larch, Douglas Fir-Larch (north), and Douglas Fir (south). These are at the bottom of page 45 and top of page 46.  Of these, only Douglas Fir-Larch shows a value of 1350 in the first column (bending, Fb) opposite #1 grade, but it's in the Beams & Stringers category. The calculation was for a 6x8—that's within 2" of being square, so wouldn't it be a Post & Timber? The Fv values are in the third column. The table says 170 here for all grades of Douglas Fir-Larch and Douglas Fir-Larch (north). The modulus of elasticity values are in the 6th column, and several of these say 1,700,000 (i.e. 1.7 million) as used in the calculation, but I don't see anywhere that says Fb=1350 and E=1,700,000.

What am I missing? Am I completely in the wrong table, or neglecting to apply an adjustment factor, or what?  ???

To test my idea, I need to plug the correct values for a 4x6 into the Windy Hill Logworks calculator. In table 4A, about halfway down page 33 of the Design Values, I see for Douglas Fir-Larch, 2" and wider, #1 Grade: Fb=1000, Fv=180, and E=1,700,000. Then on pg 31, I see a size factor adjustment table. Looking in the section of the table labeled "Select Structural, #1 & better, #1, #2, #3" reading across from 6" depth and down from 4" thickness, I see a size adjustment for Fb of 1.3, so do I multiply the table value by 1.3 to get the correct Fb for a Douglas Fir #1 grade 4x6, and then use that value in the beam-size calculator?

Another wood that's available here is Western Red Cedar. For that I would use the next-to-last section of table 4A, at the bottom of page 34, right? But for cedar, the values even for Select Structural Grade are lower than those for #1 Grade Douglas Fir, so with my weight constraints, cedar is probably not an option, because I would have to use a larger timber for the same span.


Don_P

QuoteI don't see anywhere that says Fb=1350 and E=1,700,000.
What am I missing? Am I completely in the wrong table, or neglecting to apply an adjustment factor, or what?
That would be, or what...I should have used Fb=1350 and E=1.6  :-[
Sorry about that, I must have hopped a line while reading too fast, hope you didn't spend hours trying to figure it out.

I used beams and stringers, we were right on the break line of being 2" deeper than wide. It would be more conservative to use posts and timbers' design values, nothing wrong with being conservative. When you order the materials you can specify which grade i.e. "#1 doug fir, beams and stringers" Doug fir has the widest selection of heavy timber available.

You may multiply Fb and Fv x the duration of load adjustment factor (full snow load is only assumed to be on the roof for 2 months). I usually don't for heavy timbers, my concession to conservative, we cannot visually know as much about what lies within a large timber. Notice E does not get modified, it is not an allowable design value, it is an average stiffness value, a serviceability issue rather than a safety issue so we want a "real" number there not a safety factor adjusted value.

Table 4A I take the adjustments, I feel the grading is more accurate on a smaller timber.
Fb is the only value we need to adjust here, I'll walk through the adjustments and how I would take them.
Fb=1000psi x Cd (snow)1.15=1150xCf(size factor)1.3=1495
Fb=1495psi
You can multiply Fv x Cd but there is no need on these beams, you should pass in shear by a mile.
You may use judgement on whether to apply the adjustment factors, if it sits under a full head of snow for 4 months of the year I'd think about whether to apply that adjustment.

Cedar would not be my choice for structural timbers if at all possible. Also with the exception of Dougfir when I'm sizing heavy timber I typically use #2 values. As you shop you'll find that its harder to find very high grade in other species.

Also do be aware that this is preliminary design and should all be checked by an engineer. I wrote that calculator, it is correct, but neither of us is an engineer. We could be scrutinizing a tree and miss the forest   d*.

kyounge1956

Quote from: Don_P on August 29, 2009, 10:06:45 PM
QuoteI don't see anywhere that says Fb=1350 and E=1,700,000.
What am I missing? Am I completely in the wrong table, or neglecting to apply an adjustment factor, or what?
That would be, or what...I should have used Fb=1350 and E=1.6  :-[
Sorry about that, I must have hopped a line while reading too fast, hope you didn't spend hours trying to figure it out.

Nope, I took a nap. That's about all I'm capable of doing for hours at a stretch just now.

Quote(snip) You may multiply Fb and Fv x the duration of load adjustment factor (full snow load is only assumed to be on the roof for 2 months). I usually don't for heavy timbers, my concession to conservative, we cannot visually know as much about what lies within a large timber. Notice E does not get modified, it is not an allowable design value, it is an average stiffness value, a serviceability issue rather than a safety issue so we want a "real" number there not a safety factor adjusted value.

Two months! Snow usually doesn't even stick around for two weeks here—not at lower elevations anyway. Last winter, when it snowed repeatedly over a period of three weeks or so, and also stayed cold enough that the snow didn't melt in between the storms, was just unheard-of. I didn't see the duration factors in the table. What page are they on?

QuoteTable 4A I take the adjustments, I feel the grading is more accurate on a smaller timber.
Fb is the only value we need to adjust here, I'll walk through the adjustments and how I would take them.
Fb=1000psi x Cd (snow)1.15=1150xCf(size factor)1.3=1495
Fb=1495psi
You can multiply Fv x Cd but there is no need on these beams, you should pass in shear by a mile.
You may use judgement on whether to apply the adjustment factors, if it sits under a full head of snow for 4 months of the year I'd think about whether to apply that adjustment.

Well, it could work. My idea is to put a gable over the living/sleeping room, and then a lower-sloped roof over the bath & kitchen. It's the same sort of roof profile as the Screen House over in the Owner Builder Projects Forum http://countryplans.com/smf/index.php?topic=6670.40;topicseen. That reduces the span to 8'9". Dividing the house in thirds the other way gives a bent spacing of 7'4". 8.75' x 7.3333' x 35 lb/sq ft=2246 lbs (rounded up to next whole pound). Inputs to the calculator Fb=1300 (table value x size factor), Fv=180, E=1.7 (million).  It wasn't clear to me whether I get to use actual size for a rough-sawn 4x6 or have to use 3.5" x 5.5"—if I can use the full dimensions, a 4x6 will work; if I have to use the smaller dimensions it won't. To pass using 3.5 x 5.5 dimensions, I have to get the roof load down to 1747 lbs, which means reducing the bent spacing to about 5.7 feet (1747/35 psi/8.75' span=5.704). In practical terms this means dividing the width of the house in quarters instead of thirds, and I'm guessing it would be a real hassle working the bales around bents that close together. I would have to come up with another idea in that case.
QuoteCedar would not be my choice for structural timbers if at all possible. Also with the exception of Dougfir when I'm sizing heavy timber I typically use #2 values. As you shop you'll find that its harder to find very high grade in other species.
I mostly looked up cedar out of curiosity,because it's another species that's available here and to see if I understood how to use the table. Cedar is gorgeous stuff, but with a structure like I'm planning where the frame isn't going to be exposed anywhere, its good looks are irrelevant. And knowing that I might not be able to find the necessary grade (and/or would have to pay through the nose for it if I did) takes it off the list of possibilities.

QuoteAlso do be aware that this is preliminary design and should all be checked by an engineer. I wrote that calculator, it is correct, but neither of us is an engineer. We could be scrutinizing a tree and miss the forest   d*.
Right, but it makes a useful screening tool. I'd rather find out an idea of mine won't work by plugging some numbers into an online calculator than by paying an engineer umpteen dollars an hour to check "what if" scenarios. When I come up with a design I like that passes the preliminary tests that I can do myself or find online, then it's time to have an engineer give it a thorough going-over.

Don_P

#29
Quote
Quote
Also do be aware that this is preliminary design and should all be checked by an engineer. I wrote that calculator, it is correct, but neither of us is an engineer. We could be scrutinizing a tree and miss the forest   .

Right, but it makes a useful screening tool. I'd rather find out an idea of mine won't work by plugging some numbers into an online calculator than by paying an engineer umpteen dollars an hour to check "what if" scenarios. When I come up with a design I like that passes the preliminary tests that I can do myself or find online, then it's time to have an engineer give it a thorough going-over.
Excellent, that's the reason I put those calcs up. I just feel I should remind folks  :).

QuoteI didn't see the duration factors in the table. What page are they on?
If you look at the note at the top of each table it refers you to section 4.3 in the NDS. The full description of the adjustment factors is in the NDS, unfortunately it is not online, they do sell it online. Out of the 8 possible adjustment factors these were the 2 that were applicable to this situation. For duration of load adjustment 4.3 refers to table NDS table 2.3.2. Basically, wood is capable of handling "overload" for short duration where if that same load was applied for long duration it would lead to problems. The duration of load adjustments allow us to bump up the allowable bending strength to reflect that characteristic of wood. For instance if this were a floor beam I would not apply that adjustment. For construction loads (7 day) I'm allowed to increase x 1.25, for wind or earthquake (10 minutes) I'm allowed to increase x 1.6. If you want to see this in use in another calc open the AWC joist and rafter span calc and run some scenarios clicking snow and non snow options, wet service vs dry service,or incised, these are all adjustments they are asking about. Then look at the bottom of the page after running the scenario and you will see the adjusted design values.
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
I wrote my beam calc to fill the void where their calc stops with dimensional lumber. I have requested more calcs from their head engineer and hopefully one day they will be able to write and post those.

My calc uses actual dimensions, plug in the real dimensions. There are some commercially available calcs that allow you to specify timbers from a drop down list of timber dimensions. Another void I was trying to fill was that timberframers and sawyers can make a timber of custom size, so this adressed that and allowed us to plug in whatever size we needed.

We've been exploring bent type construction. There could be a series of posts with a heavy beam for a top plate at the top, then trusses of either light or heavy timber can sit on top of the plate, not necessarily aligned with the post spacing. 


Don_P

Since we've been talking about design values I thought this might be interesting. This stamp showed up on some treated 2x8's on the job yesterday. This is some awesome stuff.

The MSR means this wood was machine stress rated rather than visually graded. typically wood like this ends up in engineered products like trusses.
Fb=2400 psi
E=2.0 million psi
It is southern yellow pine graded by SPIB, southern pine inspection burea
Kiln dried to 19% or below
Heat Treated to above 150 degrees to kill pathogens (exportable)
It came out of mill number 205, if I have a complaint I can call spib and report that mill for scrutiny or recourse

Compare those design values to LVL's, this is some very strong lumber, it could save some bucks in a beam if an alert shopper paid attention at the right time. Anyway, just more trivia  :)