built up beam shear flow- calc check 2

[jgeng]

I am making a composite wood beam out of a double 2x5 with a 2x4 flat on top. The beam is spanning 16 ft and is supporting 40 plf. Using shear flow to calc the fastened spacing for the flat 2x4 I am coming up with 50 lb/in! This seems very excessive to me. Anyone want to double check for me?

 

[ash060]

How close to the end is the 50 lb/in? Near the middle of the span the shear should be about 0. Maybe just use a few more nails at the ends and a typical spacing in the middle.

 

[jgeng]

That's at the ends (max shear). So my number sounds reasonable? What do u think about averaging the shear flow between zero and max and then use ftp spacing assuming all are engaged d to meet the requirement?

 

[ishvaaag]

Attached, integral way

 

[ishvaaag]

And now, closed form

 

[jgeng]

I meant a standard spacing that is good for the average shear flow

 

[ishvaaag]

Since the assurances of fasteners are not as big as those in steel I'd better follow the shear flow law (I wouldn't rely in some plastic distribution of the shearflow requirement, that could result in bigger deflection and worse behaviour than wanted). Hence divide the span in zones, and dimension each for the max shear flow.

 

[ToadJones]

I'd do as Ishvaag suggests....calculate the shear in several zones and use the max shear from each zone in order to calculate the flow.
From there, you can make a nailing schedule (that the contractor will probably dismiss entirely)

 

[ash060]

That is about a 16d nail every 2" c/c. That is tight, but you will probably be able to increase the spacing to 4 or 6" pretty quickly from the end.

 

[PMR06]

Are these 2"x4" & 2"x5" or 1.5"x3.5" & 1.5"x5" (ripped from a 2x6?)

Either way, wouldn't that shear flow be divided by two since you have two web members?

 

[woodman88]

The problem, as I see it, is that nails and wood screws require critical slip to achieve their full shear values. So that the center half (of a simple span) beam will basically not be available for shear transfer. I would take the full shear for each side and divide it by the nail value then determine the nail spacing required within the 1/4 span distance from the ends. If less than 4" (but I would not go less than 2.5") oc per 2x5 I would require pre-drilling the nails/wood-screws and structural glue. I would also give an alternate option of self-drilling screws like Simpson SDS's. The center section I would go with 12" oc per 2x5. The 2x5's I would nail/wood-screw at 12" oc staggered.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[a2mfk]

I have to know WHY are you doing this? What's a 2x5? What's preventing you from using a traditional lumber size or LVL? Just seems overly complicated. Piece of steel not an option? An HSS3x3 can easily be trimmed out with wood to look like wood...

 

[woodman88]

a2mfk, The 2x5 is a standard size (it is in the NDS) mostly used in Canada to my knowledge.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[hokie66]

Good to see engineers learning that it is not very practical to make wood members composite in bending by nailing.

 

[ToadJones]

I love building 2x10 "box" headers with a two 2x10's with a 2x4 flat on the bottom and a piece of 3/4" plywood on top. When building 8 foot stud walls with "precut" 92 5/8" studs, with a double top plate, it puts the top of the windows and doors all at the same height while maintaining the proper rough opening for most doors (usually around 82.5") ....header right up against the double plate.

 

[aeeglrock]

This is actually more like a built-up member (rather than "composite") and seems very uncommon for use as a wood beam (at least here in S. California). The wood members also seem too small for the 16-ft span and would allow deflections that might be unacceptable (1/2" or greater at mid-span). Also, why not use (2) deeper members such as (2)2x8's with similar combined moment of inertia as the other proposed 2x5's and 2x4 combined. This way, there will be no need to calc shear stresses or provide nailing between members except at the sides. Then again, I still think there might be too much deflection.

 

[msquared48]

Seems like it would be easier and cheaper to use a PSL and rip it to fit the situation. 3.25 X 9.25. Even a Microlam... E is greater too, so deflection will be less of a problem.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Mike is right.