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4 x 4 #2 rafters, pine max span

C

codeworks

Gold Member
Joined
Jun 12, 2011
Messages
579
Location
South Texas
got a building, put togehter with 4 x4 # 2 pine rafters, 24 " o.c. don't have a chart or beam table. is 13'-6" too much span for these. does it need collar ties ? it's about an 8 on 12 pitch, standing seam metal roof over solid 5/8 decking
 
I would use the 2 x 4 rafter chart. the strong dimension is the 4" and it doesn't matter too much that the other dim is 4" instead of 2". . . . was the question collar ties, or rafter ties?
 
A single 2x4 doesn't get very close to the 13'6 you have and without a table I would be looking for some other source of verification if you were to approve it. Either way it would typically still need both collar and rafter ties.
 
Codeworks,

Not sure if we have enough info,13-6" span @ 8/12p makes the building about 22ft +/- ?

Collar ties would only be used every other rafter in the upper 1/3 of the roof

Rafter ties (ceiling joist) would help in preventing wall spread

Is there a center beam or center support wall? (2x4 #2 YP @ 24" o/c with no ceilings attached will span approx. 8'7")

Is there a (beam rafter handbook) or calculator?

pc1
 
center beam is 6 x 14, bolted with angle iron braces at both ends. i dont have a beam /rafter handbook, or calculator, otherwise i could find the answer that way. your width is right, +-. i guessed on pitch based on field measurements.
 
A 4x4 has a section modulus of 7.146"3

As compared to a 2x4 at 3.063 , a little more than double

As compared to a 2x8 at 13.14, just making sure you don't go there, a 4x4 isn't a 2x8 it is more like a double 2x4. One simple way to use a table is if the 4x4's are on 24" centers the 12" 2x4 column would be fine, slightly conservative. At 6x the design values change and this won't work.

sounds like a simple beam;

Beam and Column Calculators

Index
 
I just checked it with Strucalc and it came up inadequate by 277%, controlling factor: Deflection/Depth required 5.45". I assumed no snow load as your profile says South Texas and used a 12" eave length.
 
I'm curious, Texas I'm assuming #2 SYP, which design values are you using in Strucalc? I tried it with new post 6/1/12 allowable design values and had an adjusted Fb of 1509 psi, which required a rafter section modulus of 10.89"3 remember a 4x4 is 7.146"3.

But the controlling factor is deflection, the new E value is 1.4 million psi giving a deflection of 2.56". Plugging in Rio's 5.45" depth deflection drops to .678", l/240 for this span is .675". certainly passes l/180, 4x6's would have worked. Putting 4x4's on 12" ctrs still has 1.28" deflection.

This was probably pulled before 6/1/12 and who knows what is being enforced, so in trying to make the math fit the existing.. Using the old design values I get an adjusted Fb of 2156psi. With the higher bending strength material the section modulus required drops to 7.607"3. Overspanned based on strength alone but not horribly. MOE was 1.6 x 106psi so the deflection even using old values is 2.24". Allowable span for this situation dropped 8" due to the recent derating.

Sistering a 2x6 to the existing scrapes by on paper.

They are planning on voting this upcoming January on whether those design values that they put into play in June are valid (they are). The SYP cliff :D
 
Sounds like a lot of engineering calcs going on. It isn't in the table, use the table value for a 2x4 OR ask for engineering/stamp.
 
Mainly hoping to flesh this out, it is incorrect,

I would use the 2 x 4 rafter chart. the strong dimension is the 4" and it doesn't matter too much that the other dim is 4" instead of 2".
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If so the 12" oc table would be appropriate for 4x4's@24" oc, it is more than equivalent to 2-2x4's.
 
DRP said:
Mainly hoping to flesh this out, it is incorrect,If so the 12" oc table would be appropriate for 4x4's@24" oc, it is more than equivalent to 2-2x4's.
Click to expand...
I don't understand your post?
 
In the op the 4x4 rafters are on 2' centers. From my read of your original post (#2) it sounds like you are proposing using the 24" oc 2x4 table. In that post you roughly equated a 4x4 to a 2x4 and suggested ignoring the the additional thickness. The section modulus and the moment of inertia for the 4x4 are more than twice that of a 2x4. That extra half inch of thickness in the 4x4 accounts for the increase in section properties, nothing mysterious. If we are engineering just from tables it would be fine to use the 12" oc 2x4 table for the allowable span.

The new inspector came on a log and timber job I was doing a few years back, looked up at the roof framed of 4x10 DF @4'spacing, and remarked that it had to be one of the strongest looking roofs he had seen. It looked impressive but was structurally about the same as a stick framed roof, we were simply twice as thick and half as often. I pointed out that we had a potential sheathing issue, flipped to the lumber sheathing table and I took him up to show that we were using T&G sheathing that was appropriate for the 4' spacing.
 
DRP said:
In the op the 4x4 rafters are on 2' centers. From my read of your original post (#2) it sounds like you are proposing using the 24" oc 2x4 table. In that post you roughly equated a 4x4 to a 2x4 and suggested ignoring the the additional thickness. The section modulus and the moment of inertia for the 4x4 are more than twice that of a 2x4. That extra half inch of thickness in the 4x4 accounts for the increase in section properties, nothing mysterious. If we are engineering just from tables it would be fine to use the 12" oc 2x4 table for the allowable span. The new inspector came on a log and timber job I was doing a few years back, looked up at the roof framed of 4x10 DF @4'spacing, and remarked that it had to be one of the strongest looking roofs he had seen. It looked impressive but was structurally about the same as a stick framed roof, we were simply twice as thick and half as often. I pointed out that we had a potential sheathing issue, flipped to the lumber sheathing table and I took him up to show that we were using T&G sheathing that was appropriate for the 4' spacing.
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Right. . . . as Building Officials, we don't (or shouldn't be) engineering anything. That is why I suggested using the table for the closest section property available.
 
I'm going to relate to Prescriptive Residential Wood Deck Construction Guide pg 7 Post Requirements: All deck post sizes shall be 6x6...

Based on that (if I'm reading it correctly), then regardless of how close to the ground or small the deck is a 4x4 post is never adequate. So, then if it can't support be used as a vertical support then it shouldn't be allowed to be used as a horizontal support.
 
When you picked which table to use you were engineering.

I hope we are now agreed that the 12" table does work a bit closer to the situation at hand.

That will not get the op out of the jam his client is in, it was just an opportunity to hopefully turn up the light. I think we've had 3 threads this week with understanding problems on simple beams. We might not ought to do engineering but we obviously need a better understanding.

I've been working on a little beam tutorial for a construction forum, but where to begin.

galilcanti.jpg
 
DRP,

there are two things here.

1. Understanding the solution proposed. (this sort of engineering)

2. Asking for engineering to affirm your plan review concerns.

Only ones who are going to be annoyed with a plan review comments asking to affirm the use of a materials or construction system is the one that avoids design and engineering.
 
Continuing with #1... I do see a need on both sides of the counter for more understanding

This is where I was stuck last night, its early on in writing the first section but I think it might be good to see my intentions. I would appreciate knowing if something along these lines would be helpful, feel free to pm suggestions/ corrections. I think it might be better to start with moments.

beam_design

Cat,

Do you have R407.3 in your version allowing a 4x4 as a minimum house support column? (R317 gives the ground contact rating required of the embeded posts from another thread, a U1 classification on the tag.)

You are confusing a beam and a column in your analogy. This is a calc I put together awhile ago to kind of walk through how a column is designed. It is set up for 5x5 and larger timbers ;

figuring a simple column

(personally I start columns at 6x and think a 4x4 makes a fine mailbox post)

I've carried us off topic, sorry codeworks, feel free to redirect.
 
no issue, man , i appreciate the conversation. this thing is already built, we're a "no approved palnsd on site set up (grgrgrgrrg) sooooo, i get out there, and it's up. owner hired it built, he and i both have converns on the span for rafters, come back to office, can't find appication, plan review or file for job. go figure
 
97catintenn said:
I'm going to relate to Prescriptive Residential Wood Deck Construction Guide pg 7 Post Requirements: All deck post sizes shall be 6x6...Based on that (if I'm reading it correctly), then regardless of how close to the ground or small the deck is a 4x4 post is never adequate. So, then if it can't support be used as a vertical support then it shouldn't be allowed to be used as a horizontal support.
Click to expand...
I think this publication is enormously useful, but it isn't CODE, and although I agree and would always like to see 6x6 posts, it is not required prescriptively in the code. what is required is that the post be sufficient to carry the loading and in order to know for certain, one would need the loading and spacing for each project and then do the ENGINEERING (which is not what we do). In reality, hopefully many of us have a sense of where we MUST see a 6x6 instead of a 4x4 without seeing the actual engineering. My "boss" calls this "common sense". . . I have come to despise that phrase. Common sense is an accumulation of all of one's knowledge applied to a situation, therefore every single person will have a different solution based on their "common sense".
 
DRP said:
When you picked which table to use you were engineering.
Click to expand...
I don't think that would pass the common understanding of the word engineering.Engineering is using the formulas you have given for beam moments, which have no place in a typical building permit review.
 
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