Anyone know what weight 4x4s can take?

RalphVa · Jan 1, 2004

I've a little 8' bridge over a spring-fed stream between my property and the next neighbors to the left. Its 3 parallel 8' long 4x4s with 1x4s crossways to them. Reckon the 3 4x4s will take the weight of a new BX1800 with FEL and 54" hog?

I'll have to swap out the 1x4s for 5/4x6s or maybe 2x6s; I think. Pretty sure the 1x4s won't take the weight.

My Gravely with hog and me on sulky goes across it fine. Even if it collapsed, the BX would probably be able to go across.

Ralph

JJT · Jan 1, 2004

No way will 4x4's support this weight if the span is 8'. Post the weight of the tractor and the total free span of the bridge and I'll do the stress calc to get you to the right size timbers.

Bob_Skurka · Jan 1, 2004

How far is the drop if the 4x4's don't hold up?

What is the actual span you are crossing? If the main supports are 8' long, I am guessing that the span would be only about 6' across, maybe only 5'.

You need to know the span and the tractor weight to determine the lumber needed to carry the load over any given distance. I'd guess a 3 spans of double 2x8 lumber to carry any tractor would be a lot better than a 4x4 across a span of about 6'. 4x4 lumber is usually used for vertical compression loads like deck supports, not for spanning loads. I believe a 4x4 has less spanning capacity than 2 2x4's nailed together.

Knowing the actual distance of the unsupported span and the weight of the load (tractor, implement & operator) is really critical to determine the actual lumber needed.

RalphVa · Jan 1, 2004

The total span is about 7' out of the 8 total length of the 4x4s.

The drop, if a 4x4 broke, is only about 6". The BX would be able to still go across, but it would be ruined for the Gravely.

The BX1800 is around 1400#. Not sure what the 54" hogs run. Maybe 3 or 400 #? Probably best to remove the FEL?

Ralph

wroughtn_harv · Jan 1, 2004

Instead of removing the FEL I think I'd consider shimming the existing beams. Consider using some flagstone pieces stacked to give you a larger footprint while keeping it decorative. /forums/images/graemlins/grin.gif

A stacked support in the middle of each span would not only make the bridge much stronger but maybe visually more appealing.

Something like this underneath makes the whole thing appear not as an error in judgement during original construction but a clever decorative alternative to massive beams. /forums/images/graemlins/grin.gif

RalphVa · Jan 1, 2004

Yeah, I've been thinking about putting a support right in the middle. Those and changing to 5/4x6 for the cross pieces will probably carry the BX weight. Wonder if I need to go to 2x6 cross pieces for the approx 6" overhang on the sides? Could widen the 4x4s to be even with the BX tire tread.

Ralph

TOMLESCOEQUIP · Jan 1, 2004

What about using RR ties in place of your 8' 4x4's ?? Most ties are 8'6" long and are at least 6x8........a couple nice used ties are $10- $12 each around here and will still be in use by your grandkids when they inherit the antique kubota.......Another plus.......They won't break under trains if you want to add a rail siding later !!!

Egon · Jan 2, 2004

The RR tie solution sounds terrific. Maybe just a few more ties and you can do away with the planking which will rot long before the ties.

Egon

JJT · Jan 3, 2004

Ralph, Good idea to widen the support beams to the width of your tractor. Then you will not need to redeck the bridge.

Quick calc's - you need to scab on 2 - 3 2x4's on each of those outside 4x4's to keep this safe. A pair of 2x6's would be better, but I'm guessing you want to keep this a low profile bridge, and the 2x6's would require some excavation and you'd loose some clearance under the bridge.

Try to get clear 2x4's, (no knots). The quality and the strength of pressure treated wood is pretty poor.

RalphVa · Jan 3, 2004

Thanks for the calcs.

The clearance underneath the bridge is more like a foot. In the stream, there's probably gonna be 6" or so of mud.

Another problem I have is this bridge gets shifted a bit by heavy rains that cause the pond up above to flow a lot of water down through the stream bed. I've it tied to trees on both sides so it doesn't float away, but very heavy rains (only about once/year) will shift it 4-6" on either end. This makes putting a support in the middle difficult.

Sounds like I need to scab on a couple of 2x4s or 2x6s and widen to the width of the new tractor's tread. The 2x6s would be easy to dig into the mud on either side.

I often thought about building a side piece on this bridge and putting a large rock on the side piece. This would hold it down in the floods but would add to the stress on the lumber.

Moved this bridge once already, further upstream. Where I had it the first time was an impossible situation, as it flooded in both directions, e.g. parallel and perpendicular to the stream. It's actually just upstream of the corner to my property.

Ralph