WMRR BRIDGE DESIGN OVERVIEW
November 25, 2009

By Larry Gomes
Assistant Trail Master
White Mt. Ridge Runners

After receiving some questions from interested members, we have prepared the following document on how we build our bridges.  Feel free to copy any of our ideas for your bridge projects.
(Pictures provided are courtesy of Roger Richard, 2009)

Wood Beam Bridge Design

The club started using 6" x 6" beams for building bridges in 2007.   These beams are available in 12' or 16' lengths and are light enough that they can be handled by 2 or 3 people yet strong enough to take the weight of a groomer or an excavator.  Another advantage to building with these type of beams is their low profile.  When the deck and runners are installed, the top of the bridge is less than 9" off the ground.  To make a ramp up to the bridge, cut two 6" diameter, straight, dead trees and place them on either side of the bridge.  These trees keep sled skiis from going under the bridge and form a nice rounded ramp onto the bridge deck.

The key to building any wood bridge is beam spacing and the location of the runners.   For WMRR wood bridges, a total of 7 beams are used, with one beam in the center and 3 beams on either side.  All of the decking is 2" x 8" x 14'.   For snowmobile bridges, a single runner is placed over each beam, except for the center beam which gets 2 runners side by side.  The reason for the double runner is that sleds usually run up the center of the bridge and in low snow conditions, a snowmobile track rides perfectly on this center runner.

The beams on either side are spread out in a deliberate pattern to take the weight of the groomer, even if it is not centered on the bridge.  For multi-use bridges that must accommodate both snowmobile and wheeled vehicle traffic, a slightly different design is used that incorporates additional runners and dirt ramps instead of logs at either end of the bridge:


Roger Rulers

When building one of these wooden bridges, the beams are placed starting with the center beam first and then the beams on either side are installed.  The first step is to determine the center of the trail and make sure the center beam is aligned to the direction of travel on the trail.  Once the center beam is in place, then the side beams are placed using measurements off the center beam.

To make lining the beams easier, Roger Richard marked off his tape measure using a permanent marker with the beam locations.  Instead of measuring center to center, he could simply hook the end of his ruler on the edge of the center beam and it would show the proper location for each of the 3 side beams.  It proved to be a real time saver and we all ended up making our own "Roger Rulers" during the project.


Beam Alignment

Once all of the beams are spaced correctly, they need to be aligned on each end.  This process usually involves using a sledge hammer to bang down the ends of the beams that are too high and shoveling dirt or placing flat rocks under the beams that are too low.  We simply use one of the 2" x 8" decking planks standing on its edge across all of the beams to perform the alignment process.

16' Bridge with decking almost complete.  Note the spacing and alignment of the beams.


Decking and Runner Installation

Once the beams are spaced and aligned, one end plank is centered and nailed to act as a guide for the other deck planks.  The remaining planks are placed on the bridge but are not nailed.  Once the planks reach the other end of the bridge, we determine the location of the other end plank, center and nail it down.  If there is some open space between the planks that needs to be taken up, we will move the planks so they are 1/8" apart to spread the open space along the entire length of the bridge. 

A string is run between the outer corners of the two end planks and this string is used to line up all of the planks in-between so the ends of all the deck planks are even.  Then we snap guide lines for nailing along each of the beams.  The two end boards get 4 nails per beam and all of the other boards get two nails per beam.

The runners are then installed and nailed down with 4 nails on each end and two nails per deck plank.  Finally the ends of the runners are cut an an angle to insure that the drag does not catch on them.

The completed bridge with runner ends cut at an angle and end logs in place. 


Steel Beam Bridge Design

The club uses steel bridges for any bridges that must span 13' or greater.   Steel bridges can be made up of 4 or 5 carrying beams, depending on the length of the span and how heavy the steel carrying beams are. 

Regardless of the number of carrying beams, there is always a steel base beam installed on either end of the bridge.  This base beam is 15' long and lays on its side lengthwise along each the bank of the river.  The top inside edge of the base beam has cutouts for the carrying beams.  The base beam is pounded into the bank using the bucket of the excavator until it is level in both directions. 

When the base beams are installed, they are carefully measured to make sure they are the right distance apart and square to each other.  If the carrying beams are 20 feet long, then the back edge of the base beams will be placed 20' 4" apart to allow some extra room for the carrying beams to be aligned with each other.

The carrying beams are swung into place using a high-strength strap wrapped around the center of the beam.  (Chain is not used to swing the beam into place because it can slip).  The carrying beams simply rest on top of the base beams until they are welded into place.

 

Welding the Beams

The club has an arc welder and cutting torches mounted on a heavy duty trailer that is pulled into the bridge site.  The carrying beams are first welded to the base beam and then round bar supports are welded at each end of the beam in to keep the beam from tipping over.  The final step involves cutting and welding angle iron cross bracing between the beams.

The club welder on its way out of the woods.

Prior to any steel bridge project, club members prepare the bracing materials in advance.  End braces are built using 3/4" round bar cut up into 18" long sections and then bent with a 22.5 degree bend at each end. 

 2" wide angle iron is cut up into 3" long pieces to be used as tabs for the cross bracing.  These tabs are welded onto the carrying beams and then angle iron cross braces are cut and welded to the tabs.

In the picture below, all of the round bar end braces and angle iron cross bracing has been welded into place.  Also the 2" x 8" stringers have been bolted to the beam just prior to the decking being installed.

Once the bridge has been welded, the final step is to burn holes in the tops of the carrying beams so stringers can be bolted to the beams.   After the stringers are in place, the decking is carried onto the bridge, lined up and nailed down.

The completed steel bridge with runner ends cut an an angle.  Logs were later dragged in and placed at each end of the bridge to guide snowmobile skiis up on the bridge deck.

Railings

At one time, the club used to install railings on almost every bridge.  But over the years we realized that most bridges were actually safer without railings.  As snow piles up on a bridge, the railing becomes much less effective at preventing a machine from going over the edge.  Also railings tend to break down faster than the bridge deck, exposing riders to nails and broken railing pieces. 

Therefore we will only install railings on bridges that have heavy pedestrian traffic or where the bridge decks are located at least 10' off the ground.

The other problem with railings is they can take up a lot of deck space if they are not constructed properly.  After examining several types of railings, we chose the following design because It takes up very little deck space and posts can be pre-built at the clubhouse saving installation time in the field.

Railings on a 35' bridge using the above design .  The railing on the left was hit by the club drag.  It has since been winched back into place and refastened.

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