Author Topic: A Bridge to...the Backyard  (Read 3150 times)

frugal_engineer

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A Bridge to...the Backyard
« on: September 21, 2017, 06:55:31 PM »
The newlywed DW and I purchased a house this year.  It has a little less than a 2 acre lot, but only ~2/3 acre was accessible without an exciting balance beam adventure crossing a fallen tree over a 10-12ft deep ditch eroded by a small stream that cut the lot in two.

Hence, it was necessary to build a bridge to reach the rest of the lot.

First, a bridge design was necessary.  26ft unsupported span was desired to avoid a center support, so the main beams are sistered 2x12s with a Howe truss to maximize bending stiffness.


Next, a quick FE to verify the feeling that the bridge is over designed can be backed up with math.  I used a 5,000 lbf load at the center joist which is a little conservative vs the local deck loading code of 60 psf (26ft span X 42" width X 60 pdf = 5,460 lbf spread across the whole structure). vM stress is below some southern pine guidance of 2,900 psi I found and deflection with the 5,000 lbf center load is ~5/8" at the deck. Looks good!


To begin construction the abutments were set.  These were dug 2.5-3ft deep and set in compacted pea sized gravel.  Didn't use concrete footers since the soil there had a very high clay content and I wanted to give the 4x4s the best possible chance against rotting from being set in wet ground.


DW and I constructed the initial sections of the main beams from 3 2x12x16s.


These were surprisingly flexible at 32 ft long.  And VERY heavy.


Used a dog run cable, shackle, and some rope to guide the beams across the ditch.  No way could I lift them back out of the ditch without machine help if they got down to the bottom; they probably weighed 250+ pounds each.


Once secured to the abutment posts, it starts to look like a bridge could go there!


Started with  the joists between the abutments for stability, then started again at the center.



As might be expected, but I didn't plan for, lateral stability was on the low side with just the beam/joist ladder shape.  Addition of a lateral truss beneath the 2x8 joists provided extreme lateral stability.


Next it was time for railing vertical members and the truss. Even though the bridge was what I;d consider pretty sturdy with just the beams and joists, its so solid with the truss railings that my highest jump and stomp on the deck barely moves it.



Completion of the truss, addition of the deck, and few extra safety railings finish off the main structure.



Dog enjoys the new laying area.


After a few months for the pressure treatment to dry out of the boards, its time for some paint!  Used Behr Deckover with texture for the deck to add some non-skid properties.  Not sure how that'll hold up over time but its fantastic when new.


Poundwise

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Re: A Bridge to...the Backyard
« Reply #1 on: September 21, 2017, 08:27:48 PM »
Wow.  Just Wow!!

You've built a really handsome asset to your house. I'm in awe. Do you do any of this professionally, or are you just an extremely gifted amateur?

Fishindude

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Re: A Bridge to...the Backyard
« Reply #2 on: September 22, 2017, 06:20:37 AM »
Neat project !
Good workmanship and sounds like it has been well thought through.

frugal_engineer

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Re: A Bridge to...the Backyard
« Reply #3 on: September 22, 2017, 03:21:23 PM »
Thanks!  Definitely not a professional carpenter. More of a spreadsheet and powerpoint person these days at work but engineering school paired with a little creativity goes a long way (or can get you into trouble by tackling projects like this depending on who you ask).

lthenderson

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Re: A Bridge to...the Backyard
« Reply #4 on: September 22, 2017, 03:46:26 PM »
Looks really nice. I probably would have made it a bit wider though in case I ever wanted to get some sort of portable power equipment (thinking wood splitter, stump grinder, atv) into the back half of my lawn. Also looking at the pictures, I might be tempted to install a zip line from one of your back decks to your back lawns to skip the walking over the bridge element altogether on the way out there.

frugal_engineer

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Re: A Bridge to...the Backyard
« Reply #5 on: September 22, 2017, 03:58:15 PM »
Its about 42" wide min, I figured I can get a reasonably large piece of gear across there.  Even a 40" riding mower, nominally. 

You are the first to suggest a zipline! Thats fantastic and is now on the list for next summer.  Much appreciated.

paddedhat

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Re: A Bridge to...the Backyard
« Reply #6 on: September 23, 2017, 09:05:46 AM »
Nice!  I've seen a number of these on backwoods trails, that ended up with pretty flexible, and wavy top rails on the truss. On others there was "flying buttress" style bracing, where a beam crosses under the structure, perpendicular to the truss, under each vertical truss member. The beam projects roughly 2' past the structure and supports an angled brace that extends from the end of the beam to the top of the post. It really stiffens and straightens the rail.

Dave1442397

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Re: A Bridge to...the Backyard
« Reply #7 on: September 23, 2017, 01:31:56 PM »
That's exactly what I've been saying we should build since we moved here in 2001 :) Nice job!

Most of our back yard is on the other side of the creek shown below (with daughter for scale), but because the side we can't get to is all woods, I've never really bothered doing anything about getting over there.

The other problem is that it can flood, and I'd hate to see a bridge float away, or get destroyed by debris.




Mgmny

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Re: A Bridge to...the Backyard
« Reply #8 on: September 28, 2017, 09:57:40 AM »
that bridge is badass. Nice work.

Northern gal

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Re: A Bridge to...the Backyard
« Reply #9 on: October 27, 2017, 05:50:11 AM »
Wow. Just wow

bacchi

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Re: A Bridge to...the Backyard
« Reply #10 on: October 27, 2017, 09:06:02 AM »
Nice engineering. I love building solid structures.

Are you worried about creek erosion at all? Do you get a lot of heavy rains?
« Last Edit: October 27, 2017, 11:03:38 AM by bacchi »

acroy

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Re: A Bridge to...the Backyard
« Reply #11 on: October 27, 2017, 09:17:56 AM »
Badass!
Looks like a friggin mansion there. when are you going to right-size? ;)
I like the Gorilla Cart. Great investment in a great tool.

trollwithamustache

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Re: A Bridge to...the Backyard
« Reply #12 on: October 27, 2017, 02:05:00 PM »
Does the grade of lumber purchased match the one used in the FEA? The HomeDespots of the world often do not stock dense select structural or select structural wood.

one piece at a time

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Re: A Bridge to...the Backyard
« Reply #13 on: November 07, 2017, 02:01:26 PM »
Great Job! It certainly suits the house.

For others considering this type of project a gentle upward arch removes the need for the truss. Potentially allowing a heavy load to get dragged over the bridge with the handrails off. BUT an arch would look a bit naff in this setting.

FallenTimber

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Re: A Bridge to...the Backyard
« Reply #14 on: November 07, 2017, 03:37:52 PM »
Wow, it looks like this project was right up your alley. Or you're just extremely talented in everything you take on. Incredible job.

We have a 30ft deep ravine on our property. My thought process was more along the lines of telephone poles and some 2x8 decking. Your bridge is on an entirely different level.