Sizing bridge beams

[Sawyer Rob]

Someone posted a pic of a steel bridge farther up this thread with the beams laying on their sides. do not do that. I'm not sure if that pic was just while the bridge was being built but...

You mean, don't do it IF you expect the bridge to carry heavy loads!

The bridge posted above with the beams lying down is only used as a walk, ATV or lawn mower bridge, it's not going to see a heavy tractor.

SR

 

[Slackdaddy]

I found this US Forest Service guide covering in detail the following bridge types (including CAD templates) very useful. No reason to reinvent the wheel.


We have a small stream in a valley we want to span and originally I was thinking a bridge, but realized everywhere close by the spanning of streams was done with causeways and culverts due to the silty/sandy soil. There are three ways to solve the issue, create a ford, add a causeway and culvert or add a bridge.

A resource I've also found useful for planning this sort of project is the USGS National Map Viewer that allows you to overlay layers, adjust order and transparency to see pretty detailed Lidar images of your project. This shows the use of a measurement tool showing the area off a potential causeway overlaid on lidar layers, "hillside stretched" and contour lines which for our area are very accurate in our area, though the wetlands data seems offset from the lidar data, though accurate in the aggregate AFAICT, just annoyingly mis-aligned.

View attachment 778979

The county, state (and probably the feds) would have a heart attack if I dropped a culvert in the stream and backfilled.
This creek is a tributary of the Patuxant river (part of the tidal Chesapeake Bay),, The creek on my property is not tidal, but about 1/2 mile down it is.

 

[2manyrocks]

I think he was just saying that design information was available from the Forest Service and that those topo maps were available.

I detest culverts because most of them I've seen are woefully undersized causing the water flow to either backup in a storm or cause stream water to overflow the banks into the highway. Or they aren't maintained, fill up with debris and backup the flow of water.

 

[jonsstihl]

about the bridge with the beams sideways. unless there is some specific dimensional reason to do that you really are loosing a lot of the beam capacity. the beams I used were 6" wide and 6" high so I had no difference. It obviously worked for you and with light traffic its fine. but why compromise the capacity of the bridge in that way when a simple 90 degree flip of the beam would give a huge increase in capacity.

some taller beams and larger spans would not even support themselves on their sides.

 

[KDubU]

Following as I have a similar project for this summer.

 

[Sawyer Rob]

about the bridge with the beams sideways. unless there is some specific dimensional reason to do that you really are loosing a lot of the beam capacity.

Yes there's a specific reason, that's why there's 3 instead of two beams.

SR

 

[LD1]

Yes there's a specific reason, that's why there's 3 instead of two beams.

SR

Got me curious as to the reason. Definitely arent as strong that way....but obviously strong enough for what you need them to do.

 

[jonsstihl]

i'm curious as well as to the reason. If it worked for you than that's great. Sometimes challenges come up and how we adapt to them within the constraints of the project make all the difference.

 

[beeforty]

A steel beam with the web vertically that is long can be curved sideways just like a long plank can be curved while not flexing downward any appreciable amount when in the vertical position. The top flange I believe is in compression and the bottom is in tension working to keep it straight with the webbing keeping them apart, just like in a floor joist. That short of a span on those beams it might not matter, but when they are placed vertically you have more support for your decking. Usually with steel beams you'll have a nailer board attached to the top flange for your decking to be secured to, not sure how he's planning to secure his deck boards.

 

[Slackdaddy]

Did some final measuring today.
Looks like the best spot the top of the bank width is about 12',, so adding 2' on each side to get away from the bank,, I will need 16' beams.
I'll be dropping by the scrap yard later this week.
With the 16' beams, my clear span would be about 14-15'
Now how to keep them from twisting.