Built up vs. Dimensioned Headers ~ IRC

[codewonk]

Do the span tables in chapter 5 of the IRC for girders and headers (built up headers) correlate directly to dimensioned headers of the same size? I assume they do but I can't find the exact reference which states so.

 

[GHRoberts]

Pretty close. Large solid girders have lower strength values.

 

[brudgers]

Do the span tables in chapter 5 of the IRC for girders and headers (built up headers) correlate directly to dimensioned headers of the same size? I assume they do but I can't find the exact reference which states so.

Your assumption is incorrect.

The strength of larger members is often greatly reduced.

I recommend referring to the specific design criteria for the sizes and species involved.

Or hiring a design professional.

 

[codewonk]

Thank you for the feed back. The IRC does not include 4x and 6x members which are commonly used for headers and girders on the west coast. FYI: The following URL is a presentation by John Henry, PE, with ICC. He includes a table showing comparisons between built up headers and dimensioned headers and asserts that the values are close enough that in most cases a direct substitution may be valid.

http://www.nsicc.org/Links/IBCSpanTables_NapaSolanChapterFinal.pdf

 

[brudgers]

He's a PE.

Are you?

 

[RickAstoria]

Generally built-up and solid timber has relatively equal values and treated as such. This is because built up members are usually not engineered and tested in a control environment facility. However, there is some fundamental factors to consider. Provided the built-up beam is glue-bonded with high-strength industrial resins/glues (not cheap wood construction glue) AND nailed with a fastening schedule so that all plys will act as a unit - built-ups will often be stronger then solid heavy timber of same exact cross-section. This is due to the ability to ensure that all layers are of the same grade lumber. Being all SS grade for example. Solid hewn timber is really not graded of able to be consistantly one grade throughout its entire mass.

Engineered Glu-Lam attains a portion of its increased strength from that very fact. There are other factors as well in the designing and engineering of glu-lam beams but a field-Built-Up beam can be carefully designed with backed structural calcs with competent calcs. The importance is the built-up would need to be treated much like joists which are in fact gets its span figures through the same math procedure as beams. Joists are beams but just caring a smaller load. Can be calculated and spec'ed out through same methods as calculating beam loads but just smaller tributary loads as "BEAMS".

For general sake, a beam is calculated through a series of math equations for calculating a number of load types and a cross section is determined as well as minimum breadth and depth of beam member with inclusion of composites of load and how that would effect final cross section dimensions of beam. Beams also must be able to span a certain distance without deflecting unacceptably. Beams shall not deflect more then joists do in many circumstances.

If this is for an exempt building and you are in a state like Oregon with exemption of certain buildings from the Engineer's law, a competent designer (whether licensed or not) may perform the math. Unlicensed persons would most likely need to show the math and how you reach your results. Much like math class where the teacher tells you to SHOW the Math. That means not just writing down the math equation and the final answer but show the math process. Registered Engineers can just use a computer program. Where I am at, I have to show the math in the old-fashion way. A computer can then be used to verify. But you have to do it right. I do agree that the B.O. shall reserve the authority to request that you get an RDP if you simply can not adequately perform the task after a reasonable chance to do it right. Allowed to be submitted does not mean it shall be approved if isn't done right.

On non-exempt buildings, there is no choice. It must be performed by an RDP.

For what it is worth, the overall cross-section of the built-up beam and the unsupported span of the built-up beam means more then the individual ply thickness.

 

[brudgers]

That's why I recommended a design professional.

 

[RickAstoria]

That's why I recommended a design professional.

Well, sure. You already know my stance on the wordage of 'design professional' vs. 'registered design professional'.

The important question for the OP is, does the OP know how to do the task without failure beyond that expected by a prudent, competent architect/designer/engineer that knows how to do the work.

Codewonk, you better know how to do it the first time and perform to the same standard of reasonable care as for be expected from an RDP in performing structural calcs and what not - as you will be held to that same standard. Otherwise, hire a competent designer/engineer.

 

[GHRoberts]

Unlicensed persons would most likely need to ...Registered Engineers can just ...

It is best for either class of person to simply write on the plans that the girder meets the code requirements. If the AHJ wants to do the math, he/she can.

But ...

 

[brudgers]

Well, sure. You already know my stance on the wordage of 'design professional' vs. 'registered design professional'..

And you know mine.

It's based on the nonsense you post.

 

[Paul Sweet]

As GHRoberts and Brudgers pointed out, you can't substitute directly because timbers have lower strengths than 2-bys. For instance, #2 Southern Yellow Pine has a bending strength of 1200 PSI for a 2x8, 975 PSI for a 2x12, and only 850 PSi for a timber ( http://www.southernpine.com/designvalues.shtml )

I like to use the Western Wood Products Association Span Calculator to size joists & beams. http://www.wwpa.org/store/product1.asp?SID=3&Product_ID=13

 

[RickAstoria]

As GHRoberts and Brudgers pointed out, you can't substitute directly because timbers have lower strengths than 2-bys. For instance, #2 Southern Yellow Pine has a bending strength of 1200 PSI for a 2x8, 975 PSI for a 2x12, and only 850 PSi for a timber ( http://www.southernpine.com/designvalues.shtml )I like to use the Western Wood Products Association Span Calculator to size joists & beams. http://www.wwpa.org/store/product1.asp?SID=3&Product_ID=13

Codewonk:

Solid hewn beams has essentially the strength of a #3 grade and is as good as the weakest link. This is because of the internal make up of a piece 10" x 12" has grade variability ranging from #1/SS to #3 all within the same beam. When you slice beams down into thinner pieces such as 2x members, you can easily learn that the beam that the grade can change dramatically from the exterior face to the most inner wood.

A built-up is all controlled grade. Giving better design value option. Solid hewn can not be given as high a design value and you have to go with the lower grade even if the particular piece might happen to be completely clear/straight grain with no knots. You have to be able to determine precisely what the grain make-up is and determine the lumber grade.

At 2x dimensions, it is much easier to determine but in a 10x or 12x, it is alot harder. There isn't much visual difference between SS grade and #1 grade. #2 just has a few knots and little bit more wavy grains. #3 has more knots and more crappy quality.

Design values does have some factor of safety built-in for the relatively statistical variability of one grade lumber from another of same grade.

When designing, you need to have that understanding to properly size the member. Certainly, you can base your built-up based on the same figure for same dimensioned timber frame but you may end up using a larger built-up member then you need. For example: A 7-ply 2x12 Built-up beam may be stronger and serve the same load as a 12x12.

This is because of the controlled-lumber grading of each ply. However, Ply thickness doesn't really matter.

Brudgers: you misunderstood my post earlier. The bottom line: it needs to be properly calculated and cross-section of built-up beam means more then ply thickness. Depending on state laws, an unlicensed person may perform the calcs but must be done correctly and the math must be shown to show how you arrived to the specifications such as beam dimensions and spans. So an unlicensed person would need to do the math the old-fashion way.

 

[codewonk]

Thanks all. Good information.

 

[brudgers]

Design values does have some factor of safety built-in for the relatively statistical variability of one grade lumber from another of same grade.

Brudgers: you misunderstood my post earlier. The bottom line: it needs to be properly calculated and cross-section of built-up beam means more then ply thickness. Depending on state laws, an unlicensed person may perform the calcs but must be done correctly and the math must be shown to show how you arrived to the specifications such as beam dimensions and spans. So an unlicensed person would need to do the math the old-fashion way.

I didn't misunderstand your post.

It was perfectly clear that you were offering the sort of bad advice that requires the very licensing laws you dismiss.

And continue doing so, as I highlighted.

 

[RickAstoria]

I didn't misunderstand your post.It was perfectly clear that you were offering the sort of bad advice that requires the very licensing laws you dismiss.

And continue doing so, as I highlighted.

Design values does have some factor of safety built-in for the relatively statistical variability of one grade lumber from another of same grade.

Since you quoted that line. Lets start with - we are talking "Design Values" not "Ultimate Strength" design. By the vary fact, design value takes into consideration statistical variability of wood within each grade and there is a built-in factor of safety to it from the statistical means.

 

[Phil]

"Beams and stringers", "post and timbers" and "dimensional lumber" have different grading rules and have different reference design values. Wood members with the least dimension less than 5" are graded as dimensional lumber. Members with the least dimension greater than or equal to 5" are timbers. Timbers with with the depth 2" greater than width are considered beams and stringers. If the depth is not 2" greater than the width, the timbers are considered to be a post and timbers. Also, the NDS has size factor that are applied to the reference design values. This can cause a 2x12 to have a differnt allowable bending stress than a 4x12 of the same species and grade.

The dimensions used for design are nominal. A 2x12 is 1-1/2x11-1/4; a 4x12 is 3-1/2x11-1/4; and, a 6x12 is 5-1/2x11-1/2. So, a 4x12 is larger than (2) 2x12's. Built-up members should not be confused with glulam beams. Built up members have the same design properties as the pieces that are put together to form them. Glulams are fabricated in a factory and will typically have greater capacity than built-up members.

The building codes prescriptive span tables don't come close to covering the gamut. The NDS Supplement has 30 or 40 pages of tables with only the reference design values (bending stress, shear stress parallel to grain, etcetera).

 

[peach]

don't see many solid hewn lumber girders in these parts. It's really a factor of the strength of the type of lumber being used... it still needs to be graded lumber. (If it's dimensioned whitewood, the values will be significantly lower than Doug Fir)

Which means.. a grading agency needs to evaluate and give you the strength of the girder.. it's not addressed per se in the code.. except in R502.1

Engineered beams (Glu-lams and such) come with their own engineering.

 

[RickAstoria]

Solid-hewn timber grading never exceeds what would in lumber grading #1 / #2 grade because grains can not be consistant throughout entire cross-section. Built-Up beams can be built stronger then a solid-hewn lumber even if actual dimensions are slightly smaller because you can be more controlled on the lumber. Glu-lam is even stronger yet. (Glu-Lam is the term for any pre-manufactured and engineered beam. Field-built glu-laminated beams are called "Built-Up beams" as well as ones nailed together. Including combination nailed and glued.

Another factor is Built-Up beams plys are stacked/connected together horizontally (while some field-built might be stacked vertically) while Glu-Lam is stacked one on top of the other in a controlled pattern. The grading and species is also handled. Hence an even smaller dimensioned beam then a Built-up that is a Glu-Lam beam- can be used.

Beam size is determined by specific formulas which in case of wood beams takes into account modulus of elasticity / cross-section of members.

You can determined what that is for a 2x12 (11.5 x 11.5) and adjust mathematically for a 10.5 x 11.5. When it comes down to engineering (the actual dimensions matters more because a true 12x12 would be stronger then a 11.5 x 11.5 (nom. 12x12). More mass. May not be much but it is when you calc it down to the letter.

If absolute minimum cross section dimensions needed is 12" x 14" (a nom. 12x14 is insufficient.) You must go up a size or two.

 

[peach]

That's my point.. sewn lumber needs to be field graded.

 

[Phil]

Solid-hewn timber grading never exceeds what would in lumber grading #1 / #2 grade because grains can not be consistant throughout entire cross-section.

I suggest you look at the reference design values for DF-L timbers in the NDS supplement http://www.awc.org/pdf/2005-NDS-Supplement.pdf . While dense select structrural timbers may be impractical (cost and availablitlity), the grading rules and design properties are published.

Built-Up beams can be built stronger then a solid-hewn lumber even if actual dimensions are slightly smaller because you can be more controlled on the lumber.

Price and availability rather than strength make built-up beams attractive.

Glu-lam is even stronger yet. (Glu-Lam is the term for any pre-manufactured and engineered beam. Field-built glu-laminated beams are called "Built-Up beams" as well as ones nailed together. Including combination nailed and glued.

Glulams are a good alternative to timbers when the appearance of the timber is not as important. Glulam, Glu-Lam, GLB are all short for Glue-Laminated Beams (also used for posts). Glulams should not be confused with PSL, LVL or other engineered lumber. People will be confused if the term glulam is used for site constructed built-up members. Glulams have stress rating and combinations with published design values similar to dimensional lumber.

 

[RickAstoria]

Phil, I understand that. That is why a glue laminated built-up, I call a "Glue-Laminated Built-Up". Normal Glu-Lam is engineered AND factory produced. Field-built Built-Up maybe engineered (ie. structurally calculated and spec'ed out when a designer like myself calls it out in the plans. Then it is field built to specifications.

When I spec out Built-Up beams, I spec the size, number of ply and span. This is why it is called DESIGNED versus aimlessly slapping it together with no thought.

Strength is a prime factor of all beams. Price and availability is always a factor of any project/product.

 

[RickAstoria]

Phil, I understand that. That is why a glue laminated built-up, I call a "Glue-Laminated Built-Up". Normal Glu-Lam is engineered AND factory produced. Field-built Built-Up maybe engineered (ie. structurally calculated and spec'ed out when a designer like myself calls it out in the plans. Then it is field built to specifications.

When I spec out Built-Up beams, I spec the size, number of ply and span. This is why it is called DESIGNED versus aimlessly slapping it together with no thought.

Strength is a prime factor of all beams. Price and availability is always a factor of any project/product.

BTW: Dense-Select practically doesn't exist in fresh-cut lumber as Dense grade can only come from trees that live long enough for the growth-ring density. With Douglas-Fir, the tree has to be over 300 years old to get to Dense level. At least the way it was in the NDS. I'm aware of NDS. Remember, lot of the chart info goes back to the 1980s. The only time you'll see Dense-Select in like Douglas-Fir is if it was re-cycled timber from old buildings that were torn down. Growth-ring density is what gives the grain density.

Grading of timber is not available in all the grade that lumber is available in because it is too difficult to achieve in the twigs we have today. If the tree was 10-ft. diameter to 15-ft. diameter and was 300-600 years old then yeah, dense grade beams of nom. 12x12 or 12x14 or 12x16 or larger would be possible. We won't find that from a tree that is only 6-8-ft. diameter and 30-80 years old. That is why the new wood is crap.

Built-up beams are often somewhere between solid hewn timber found in today's trees and Glu-Lam because of the ability of a controlled grading and not the variability of grade. Grade is based on number of knots, size of notes and straightness of grains. A surface inspection of a solid timber beam leaves a big potential for grade difference from the portion of wood deep in the beam from the surface. Grading of 2x is much more consistant and built-up beams can be more consistant.

 

[peach]

So, then, why does the code specify lumber grades? If it's all crap (which I don't recall seeing a crap grade)... Grading individual lumber pieces is available (expensive.. but available).. if someone wants so dearly to have the look of hewn lumber over an engineered product.. they can pay for the grader to come to the site.

Built-up headers usually strikes something like 2-2x10.. not an engineered product.. and the typical built up header is what the code tables are based on.

Once you're out of the cookbook (prescriptive) provisions of the code.. you are into an engineered structural design. Not that hard.

 

[brudgers]

Once you're out of the cookbook (prescriptive) provisions of the code.. you are into an engineered structural design. Not that hard.

Until it falls on your head because a building designer decided to reduce the factor of safety.