Wood beam load calculations

[BFBOB]

This should be easy to calculate - but none of the online load calculators will tell me what I need to know. Figure a 2x4 (actually two 1x4's) supported at the ends, long way up - just like a normal floor joist. Figure the max load. Simple, right? Well, no. All the load calculators I can find assume an evenly distributed weight- expressed as pounds per foot (or kg per meter). What I need to know is the allowable point load. It's to be a foot bridge for one person, so for worst case figure a 200 lb. load at the center of the span. That's where the calculators I've found fall short.
Surely some of you gurus can do this in their sleep (or better yet, point me at a calculator I can get to do it for me -- teach a man to fish, y'know)
Thanks!

 

[gsmith22]

for a uniformly distributed load (your pounds per foot), the max bending moment in the middle of the beam = uniform load * span^2 / 8
for a point load positioned at the middle of the beam, the max moment in the middle of the beam = point load * span / 4
you can figure out the equivalent point load from your online load calculator by setting those two expressions equal and solve for the point load

equivalent point load = uniform load * span / 2

this is for any material. the structural behavior of a simply supported beam is the same in wood as in concrete, masonry, brick, steel, plastic, etc.

 

[mike93lx]

You are building a bridge out of 1x4? That needs to support people?

I can't imagine doing that for even 1'

 

[Shiftless]

You are building a bridge out of 1x4? That needs to support people?

I can't imagine doing that for even 1'

I’d walk on it if the span was only one foot but I bet the OP wants a longer bridge.
I built a foot bridge over a shallow stream for a friend‘s garden. I didn’t consult any engineers. For a 6 foot span I used a 4 x 6 on each side and 2 by material for treads. Handrails included. It turned out quite solid.

If it had been high above the ground, I would have had some engineering input. My bridge was only 2 feet above ground level.

 

[Sumboodie]

I'd be figuring more than 200lbs unless you mean 200lbs per beam? X2, 3, etc?

 

[gearhead1]

What kind of wood?

 

[alwaysFlOoReD]

I've made walking planks up to 20' long, 16" wide from (3)x2x6 with 1/4" plywood glued and screwed to the top. I doubt there is more than 1/4" deflection. I would have no problem building shorter from 1x4 in shorter lengths. I weigh 245#.
In fact I was planning on using (3)x1x6 with plywood both sides to make another 20' x 16".
These are used up 2.5 stories up as staging to apply siding.

 

[Innovate1]

Western Wood Products Association (WWPA) had a nice calculator called WWPA Lumber Design Suite that was free and could do what you want. I see that that it is no longer available from WWPA. They suggest span tables now - I didn't look at them but you can find them here:
https://www.wwpa.org/resources?text=design suite
You might be able to find them at other sites. Just spreadsheets that run under Excel.

Unless your bridge is very short it could have more than one person on it at a time so would be good to design for more load.

No way to tell what you need without the span. Type of wood makes a big difference.

 

[toplessHO]

go with engineered wood products
trussjoist,paralam etc.

 

[Cleave]

Like said above, you can get the bending moment for a simply supported span with center point load from the following equation,
Mmax = P*l/4
where Mmax is the maximum bending moment within the beam(s)
P is a center point load
l is the span

That's the easy part. Then you have to figure out what the allowable moment is on your particular material and beam shape.

The reason wood analysis is more complicated than steel is because of the variability with knots, runout grain, etc.

It is common in structures for deflection to govern, not bending stress. This implies that if you do no engineering, but the bridge has minimal deflection, you're good to go.

I'm assuming here that the span is short and the consequence of failure is modest.

As an engineer by day and DIYer on the weekend, I wouldn't do any math on this one unless the structure was substantial.

Edit to add: If this is "a long way up" do whatever necessary to make sure it is adequate including the following,
1. Proper end terminations to the footings/structures that support it
2. Code worthy guardrails
3. Proper construction best practices throughout
4. Depending what structures support this do you need to think about having an intentional slip joint at one end to account for sway in the support structures and wood movement?

 

[Innovate1]

I threw some numbers in WWPA program. Greatly simplified but maybe helps as a point of reference. I had to use a 2x4 - that was the smallest member allowed and equivalent to two 1 x 4. With no dead load and 6' span it is just over 100% load. This was #2 lumber and could go to #1 - that reduces that number to 90%. You should add more margin and also allow for dead load (weight of structure itself.

 

[tominboise]

You need some info about the wood you intend to use, the beam orientation and the layout. The Moment of Inertia of the beam, and the Modulus of Elasticity of the wood used. You should be able to get the deflection information from these.

Beam deflection table

M of I calculator

Wood database

 

[coldh2o]

Still need more information - length of span? And how do you walk on a bridge consisting of 2 laminated 1x4 on edge?

 

[Cleave]

Also if you're doing calculations, you can't just use 200 lb.
What if somebody bigger walks across it?
What if more than one person uses it?
What about dynamic loading?
What about the wood degrading with time?

When analyzing bridges we typically use distributed loads, in pounds per square foot (psf).
For a low usage structure 60 psf is a typical value.
Say you have a 6 foot span, that's 360 lb.
A dynamic loading factor of 1.6 is common on the live load, so that's 1.6*360 = 576 lb May as well call it 600 lb.

2x4s cost about $4 each right now - no sense trying to engineer this thing like a rocket ship. For the cost of hiring an engineer to look at this thing you could just build it out of SOLID laminated 2x12...

 

[gte718p]

Also 2 1x4s are not necessarily equal to a 2x4. Depending on how they are affixed might be stronger or a good bit weaker.

 

[laser3kw]

does that calculate a dead load? How does it change for a live load? like someone jumping up and down?