Girder construction

[jtom]

When building a 3 ply girder for a house,do the ends of the lumber need to break on a pier?I can not seem to find where it addresses it in the ICC residential code.Typically we see the ends break on piers and staggered.Thanks for any information.

 

[Mark K]

I assume that you are talking about taking 3 joists and nailing them together so they share the load and that the laminated girder is longer than the length of one piece of wood.

I would terminate each piece of wood over a support. Locating the splice of an individual lamination away from a support complicates things.

 

[TheCommish]

The prescriptive code assumes your splice ends over support points, you would not build a header for a window with a splice in the middle. if you go outside of this assumption you need engineering.

 

[jeffc]

Functionally, a cantilevered member is much stronger than a simple span member. Think about what it would take for the center of cantilevered member to sag, the end of the cantilevered portion would need to move. If the cantilevered end is anchored, it provides additional capacity to the main span. The problem is how do you use the prescriptive path in the codes and justify staggered joints? I use the Wood Structural Design Data book to help with beam sizing on non engineered projects. Yes, non engineer verifying beam sizes. Let the beatings about the head and shoulders begin. The formulas give you a good idea on what configuration provides the best structural efficiency.

 

[TheCommish]

For 1 & 2 family work, if you can show me in write that the proposal works using a credible source, I will accept most time, no RDP required. if you are making it up because pick one; 1) we have done it before, 2) the other towns let me do it, 3) you know it will work, 4) we always done it that way= you can hire an engineer

 

[Mark K]

Un qualified statements like "Functionally, a cantilevered member is much stronger than a simple span member." are misleading. In some situations a beam that cantilevers beyond the support can allow the use of a smaller beam cross section but in other situations can result in the need for a much larger member. The safety of such members depends on a number of variables that can result in either improved or inadequate performance.

The situation contemplated by the original question has to do with a multiple span beam which would be considered an indeterminate beam that normally cannot be properly sized by rules of thumb. In addition ending one or more of the laminations away from the supports would be similar to introducing a big notch in the beam. In my experience only engineers have an appreciation when such construction methods can be accommodated.

From a code perspective when such laminated beams with discontinuous elements are not clearly addressed in the IRC they can only be used if somebody can show compliance using the IBC.

 

[Paul Sweet]

Multispan girders usually have an inflection point (moment = 0) near the 1/4 spans, and the maximum moment (usually the negative moment at a pier) is less than the simple span moment. If you size the girder for single span, and only one member breaks at any 1/4 point the girder will be conservatively sized and more rigid than single spans. However, constructing a long multi-span girder is more complicated than nailing three single span 2-bys together on the ground then lifting them into place.

 

[Glenn]

502.6 is the section. It may not read that each "ply" must be supported, but that's what it means. If you were to choose to interpret it otherwise, you could also interpret it that NONE of the plies of a multi-ply beam, double joist or double trimmer rafters have to be continuous from support to support, and I really hope you wouldn't do that.

502.6 is old code, and few other than AWC or NAHB care much about these sections in code development. Deck codes are getting recent attention, and notice when this same section is written new, you get the details about plies. See. 507.5.1

Some opinion and history.

Do not try to learn framing from the IRC. It is absolutely not meant for that. The framing provisions in the IRC are somewhat stagnant from the days when the code was a professional standard for professionals, not an attempt of a "how-to" book for step-by-step construction.

Honestly... if you want to learn wood framing concepts and fundamentals, search Ebay for "Light Frame House Construction" from the Federal Board for Vocational Education. From what I can tell, this was from a time when society didn't tell children they were losers and failures if they didn't go to college. A time when carpenters were taught engineering without engineers being threatened or offended. Both professions were respected, from what I can gather from records. A time when a carpenter did math instead of only read tables, and those tables had words like bending, fiber stress, and modulus of elasticity. A time when you learned the profession of framing from other framing professionals and those professionals were respected as professionals.... so they treated themselves like professionals... They sought excellence and pride in their framing.

Clearly my bias as a former framer is shining through.

Here is a quote from my 1932 edition of the above mentioned book, page 43. "Splicing - No member of a built-up girder should be cut or joined in any way except over a bearing post."

You decide, but it looks to me like an upcoming 2024 IRC code change proposals is going to be citing 1932.

 

[Glenn]

Follow up with three more old documents.

1) US Department of Agriculture, Forest Service, Agricultural Handbook no. 73 "Wood-Frame House Construction" 1955, page 23
"The build-up girder is usually made up of two or more pieces of 2-inch-dimensional lumber spiked together, the ends of the pieces joining over a supporting post."

2) Forest Products Laboratory, US Department of Agriculture, in collaboration with the technical staff of the housing and home finance agency, "Technique of House Nailing", circa 1930's. Page 7. Figure of three-piece girder.
"Girder joints shall occur only at supports"

The Army had some more sophisticated ideas, and perhaps some great carpenters.

3) Department of the Army Technical Manual TM 5-460 "Carpentry and Building Construction", 1960, pages 16-18.
This document provides a few pages of text and a figure of details for splices. However, it describes compression, tension, and bending splices separately and uniquely. Bending splices are shown in a figure as a beam and joist type of horizontal spanning member. There is one splice described and it's not a butt splice end-to-end. Its a splice like nothing you would ever see or have ever seen in modern framing practices.

So... I found one Army manual that does suggest a specific type of splice is acceptable for a bending member. Of course, there is little specific information about loads or spans. This document is for army housing, so I'm guessing we are talking pretty modest size and design.

Fun stuff... now back to paying work. I appreciated the distraction. Thanks.

 

[ADAguy]

For 1 & 2 family work, if you can show me in write that the proposal works using a credible source, I will accept most time, no RDP required. if you are making it up because pick one; 1) we have done it before, 2) the other towns let me do it, 3) you know it will work, 4) we always done it that way= you can hire an engineer

not can, "must" hire.

 

[TheCommish]

not can, "must" hire.

no, they do not have to hire an engineer. They can choose to take the work out and complete in accordance with the prescriptive code