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Author Topic: Can someone help me understand the results from a load calculator on here?  (Read 253 times)

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Offline williaty

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Well, reading this forum has helped me mightily with logging and firewood harvesting around the property and a google search tonight surprised me by bringing me back to this website for a different reason. My real job is as a photographer. Today, a client who was in for a portrait saw the exposed beam running through my studio and asked if it would support her weight. She's some kind of aerial yoga instructor which looks like some kind of acrobatic stuff involving ribbons looped to the ceiling. In trying to find out if it was safe or not, I stumbled onto the "Midspan Point Loaded Simple Beam" calculator here on ForestryForum. I plugged in the following:

Span of beam: 205"
Width of beam: 3.5"
Depth of beam: 7.1"
Max fiberstress in bending: 575 psi
Modulus of elasticity: 10 million psi
Max allowable shear: 125 psi

Notes: I have no idea what species the beam is. It's a pair of 2x8 boards sistered together with a zig-zag nail pattern. Because of this, I chose the values for No.2 Spruce-Pine-Fir from the chart on this website at Maximum Allowable Fiberstress in Bending Values The "spruce-pine-fir" sounded like it translates to "I don't know but it's gotta be some kind of softwood" and I went with No.2 because I'm sure this was just cheap lumber from a construction yard in 1979 (year the house was built).

I increased the load on the beam until the "Section Modulus Required" matched the "Section Modulus Input" and that point was 325lbs.

1) Did I pick the right set of values to put into the calculator for the mechanical properties of the beam?
2) Does the calculator include a safety factor? In other words, is 325lbs the failure point of the beam or the maximum safe load on the beam?
3) Reducing the span helps things immensely. If I cut a pair of 4x4 posts just strong of the actual height of the floor to beam gap and then knock it into plumb with a hammer (common way of lifting/supporting a beam to work on it) at, say, an 8' gap between the posts, does that actually count as reducing the span to 8' as far as the load in the center is concerned?
4) Anyone out there have any experience with designing something like a wooden swingset have any experience with safety in this case?

Thanks guys.

Offline Don P

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Re: Can someone help me understand the results from a load calculator on here?
« Reply #1 on: November 01, 2018, 07:20:27 PM »
 Let me give you some better numbers to plug in;
Span 205
Width 3
Depth 7.25
Fb 1069.5
E 1.1
Fv 135

Assuming this beam is not supporting anything else! If that beam at that span is structural it is already in severe distress.

Deflection controls at about 450 lbs
Bending strength would be safe up to about 545 lbs

Deflection is a serviceability issue. Bending is a strength issue.

Bending strength is an allowable load not an ultimate load, factor of safety should not be less than 2... I have seen lumber break at less than allowable, rare, but your eyes do matter.

You were looking at a table for 5x5 and larger timbers, I used the correct table for dimensional lumber 2-4" thick and applied an increase of 15% for multiple member use and 20% size adjustment factor for depth to the base design bending strength of 775 psi which gave the adjusted design strength of 1069.5 psi.  

Reducing the span as you describe does work as long as whatever is under those posts is capable of supporting the load without deflecting and throwing the load out to the longer span supports, the old adage "load goes to stiffness".

If you can find a grade stamp we can do more but yes you chose a conservative species group.
If you move the load off midspan the bending moment improves as well. That calc is the next one down the list, "point load any location".

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