Load bearing capacity of 4×6
stephenr
| Posted in General Questions on
Hello, I have built a ramp out of 6 18 foot 4×6 beams (spruce, pine) to move a skid steer in and out of my foundation hole to deliver sand fill and stone for the radon system beneath a concrete free slab. I have posted it solidly and at no point does the ramp have more than a five foot span. There is a space of 6 inches between beams and it will be capped by 3/4 inch plywood. The skid steer has a continuous track and when loaded will weigh about 4,000 lbs. I can’t quite negotiate the online span calculators confirm my design. Help would be appreciated.
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Seems low for a skid steer
lookup 'calculate a simple beam'. Math is not overly complex
I doubt all of the beams will share the load evenly
HOw far to fall when it fails...?
I am surprised how hard it is to get an answer to the question, "what is the maximum load of a 4x6 with a 5 foot span" using online resources.
No one ever loads beams to the maximum capacity. In construction, you care about deflection, which is quite a bit less than the failure point.
Thanks DC. Much to learn. IYHO, will my ramp work?
I'm not a structural engineer so I can't say. But...
There is an app called the "Sagulator" that allows you to calculate the sag in a piece of wood at a given load. It's at: https://woodbin.com/calcs/sagulator/
I put in Eastern White Pine (they didn't have Southern Yellow Pine, which is what you'd get around here if you bought a 4x6); span of 60" secured at ends, load of 2000 lbs, width of 3.5", depth of 5.5".
The calculated sag is 0.02". Usually in residential construction you don't want horizontal members to sag more than 1/180 of their length, so for a 60" piece the acceptable sag would be 0.33". That's far from the breaking point, that's the point where you start having problems with floors being visibly out of level and tile and drywall cracking.
So I would say in your application the beam will be well within its breaking strength. The problem -- and this is where I can't offer much expertise -- is that there are other failure modes. The beam could roll over on its side. The plywood could punch through. Whatever the beam is resting on could give.
I would be concerned with the 3/4" plywood between beams. Normally 3/4" plywood is pretty robust in terms of punch through, but you're talking about some pretty extreme localized loads here, especially with the track drive. I'd use at least 2 sheets, maybe 3 here, since it's likely to get pretty chewed up in the upper layers as you drive over it.
Bill
Let see what Chat GTP says:
To estimate the maximum load-bearing capacity of a 4x6 SPF (spruce-pine-fir) beam supported at a 5-foot span, we can use some typical values for bending strength and other properties.
### Properties of 4x6 SPF Beam:
- **Bending Strength (Fb)**: Approximately 1,200 psi
- **Modulus of Elasticity (E)**: About 1.6 million psi
- **Moment of Inertia (I)** for a 4x6 beam:
\[
I = \frac{1}{12} \times b \times h^3 = \frac{1}{12} \times 4 \, \text{in} \times (6 \, \text{in})^3 = 48 \, \text{in}^4
\]
### Load Calculation
1. **Maximum Bending Moment (M)** for a uniformly distributed load:
\[
M = \frac{wL^2}{8}
\]
where \(w\) is the total load in pounds and \(L\) is the span in inches (5 feet = 60 inches).
2. **Maximum Allowable Moment (Ma)**:
\[
Ma = Fb \times I \div c
\]
where \(c\) is the distance from the neutral axis to the outermost fiber, which for a 4x6 is 3 inches (half the height).
3. **Setting the Moments Equal**:
\[
\frac{w \cdot 60^2}{8} = \frac{1,200 \cdot 48}{3}
\]
4. **Solving for w**:
\[
\frac{w \cdot 3,600}{8} = 19,200
\]
\[
w \cdot 450 = 19,200
\]
\[
w = \frac{19,200}{450} \approx 42.67 \, \text{lbs/in}
\]
### Total Load
To find the total load:
\[
\text{Total Load} = w \cdot L = 42.67 \, \text{lbs/in} \times 60 \, \text{in} \approx 2,560 \, \text{lbs}
\]
### Conclusion
Under these calculations, a 4x6 SPF beam supported at a 5-foot span can typically bear approximately **2,560 lbs** uniformly distributed.
### Safety Factor
It's important to consider a safety factor in real applications, typically around 1.5 to 2. This means the safe working load could be lower than the calculated maximum. Always verify with local building codes and consider other factors such as lateral stability and live loads.
Didn't check the mat, but kind of close especially if you are carrying a bucket of gravel which can be an extra 1000-1500lb.
I don't think that's right. So with a safety factor of 1.5 the safe working load is 1706. That's for a true 4x6. For a beam a solid piece has the same strength as equivalent sizes of wood laid side by side. So that means a 2x6 would have 1.5/4 of the strength, or 639 lbs. So 2x6's spaced 24" OC would have a safe working load of 63.9 pounds per square foot. Just looking at a joist span table a 5' 2x6 can do over 110 PSF at 24" OC. And that's max deflection, not even getting close to working load.
I know it's a shock that ChatGPT might be wrong...
I used BCCalc for a lot of the loads in my house design. You could probably plug in the weights you listed and go from there. It seems like a pretty safe bet, 4,000 lbs is not too heavy of a skid steer, are you sure on that part too?
I'm with Bill at least one sacrificial layer of plywood might be good.
We typically size beams based on an acceptable amount of deflection, but that's mostly an aesthetic concern, not a structural one.
There are three ways that beams can fail:
1. The load on each end can be so heavy that the wood crushes. That is calculated using the value for allowable load perpendicular to the grain, notated as Fc. (F for force, c for compression.) Even a small amount of crushing is not acceptable for permanent structural members, but would probably be ok in a case like yours.
2. The load can be so heavy that it shears the beam along the grain, typically near the supports where the shear stresses are greatest. That's usually limited to very heavy loads on very short beams. (Fv)
3. The load can bend the beam so much that the grain on the bottom pulls apart. The safe limit is the "extreme fiber in bending." This is the failure mode most likely to be a problem for you. (Fb)
To calculate the allowable load on a beam, we also need to know the exact wood species and the lumber grade--knots make a huge difference in strength. There are adjustment factors we also need to know: is the wood saturated with water? Water makes wood softer. How long will the load be on the beam? Long-term loads "creep" over time. Are the members repetitive? With several beams relatively close together, such as floor joists, the load is partially carried by adjacent members.
One usually trusts 2-2x6 more than one 4x6. Grading and whatnot
10 second google says a skid steer weighs 7500 pounds
This is not a floor with evenly distributed load.
It is a pair of beams really, with a potential point load of [?] with a full bucket downhill when you stab the brakes.