Bridges

LaPlatte Bridge Survives Historic Flooding

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The LaPlatte River Bridge, designed by PERCH and opened last fall, still stands after a severe Halloween rainstorm brought the highest flood stage in at least 25 years. The pedestrian suspension bridge in Shelburne’s LaPlatte Nature Park was designed for a high-water level of 7.63 feet, achieved in 1996 and 2017. Early on November 1, USGS recorded a water level of 8.07 feet, high enough to put the entire bridge deck underwater. The lower west tower withstood water at least 4 feet deep.

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Richmond, Vermont at peak of flooding on November 1.

The storm washed out roads and flooded homes in northern and central Vermont; over 100,000 customers lost power according to the Burlington Free Press. Vermont Emergency Management is seeking FEMA assistance to pay for repairs. Among the storm’s casualties was the nearby Lewis Creek Bridge, which served as a model for the LaPlatte Bridge. The Lewis Creek Bridge towers had a rubble foundation that apparently succumbed to the boulder-moving power of high water. The LaPlatte Bridge’s ground anchors, giant screws buried 7 feet below grade, remain intact.

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Lewis Creek Bridge after flooding.

PERCH feels great respect for the laws of physics, as well as all the building codes, design guides, and outside engineers consulted in the design of this bridge. The LaPlatte’s amazing construction crew often wondered if the bridge was overbuilt with a design live load of 90 psf (equivalent to about 70 people standing on the bridge at once) and a 3.0 safety factor applied to the bearing strength of the clay soil. But extreme events like this flood demonstrate why structural engineers demand high standards.

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Aerial photo of LaPlatte River Bridge last summer. (Credit: Josh Halberg.)

LaPlatte River Bridge

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PERCH was designer for the LaPlatte River Bridge, a footbridge located in LaPlatte Nature Park in Shelburne. This suspension bridge crosses the LaPlatte River with a clear span of 68 feet. It opened to the public last weekend in a festive ceremony with refreshments and a band.

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Swedish band Kolonien leads a parade across the newly completed LaPlatte Crossing.

The bridge owes its existence to a Shelburne resident who would walk to work every day through LaPlatte Nature Park. Joplin built about a dozen bridges over a seven-year period to cross the LaPlatte River, some as simple as a single log with a handrail. Most of these primitive bridges washed away; one was condemned because it was built on town land without a permit. So Joplin set about to build a permanent bridge, with full permission from the town.

Joplin also wanted a landmark, something that would be fun to cross and look at home in an Indiana Jones movie. A girder bridge seemed far too pedestrian (no pun intended), and he decided early on that a suspension bridge was the way to go. But designing a suspension bridge is complicated: the load path goes from the deck to the hangers to the main cables to the towers, and the load distribution changes as people walk across and deform the deck. That’s where PERCH came in.

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Using a handheld driver to install the ground anchors.

The foundations for the towers (compression) and the main cables (tension) went through many iterations based on constructability and environmental impact. With no vehicle access to the site, all materials were designed to be transported and installed by human power. And the permits prohibited excavation, eliminating the possibility of metal piles or concrete anchorages. An initial plan to wrap the cable ends around sturdy trees was scrapped when the strength and longevity of the trees could not be confirmed. We ended up renting a generator-powered handheld driver to install screw-like ground anchors for both the towers and the cables.

Transporting the 8×8 timbers.
Erecting a tower.

Almost every detail of the bridge was revised or refined as construction challenges cropped up. The girders supporting the deck kept falling off the needle beams as the bridge moved, so a splice was devised to enable full bearing on the needle beams without changing the deck’s overall flexibility. The 3/16” diameter hangers, spaced 4 feet apart, connect to the 5/8” diameter main cables via wire rope clips that can’t slide along the main cables. Turnbuckles were added to the main cable ends so builders could easily adjust the cable tension post-installation.

The towers are 20 feet tall and made from 8×8 timbers; fabricated steel base plates that connect to the ground anchors and fabricated steel saddles that hold the cables in place were designed late in the game. Rubber thresholds were added at both towers to provide an unbroken surface between the moving bridge deck and the stationary access ramps. Even the tower locations were moved 5 feet east from the initial plan due to erosion concerns, requiring a revision to the site survey.

Tower base plate.
Cable anchorage with turnbuckle.

Attending the chilly opening ceremony, I didn’t need to tell the crowd they should test the bridge to its limits. They recorded the first piggyback crossing, the first to skip across, the first parade (led by the band) and many other variations of their own volition. There may never be another day when 50 people try to cross this bridge all at once, so I’m confident it will last many years. Endless thanks to Joplin, the Town of Shelburne, and the hundreds of volunteers who made this bridge possible – it has been a wonderful opportunity.

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Donors and volunteers carved their names in the deck boards.

Tucker Hill Bridge

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The Mad River Path Association (MRP) manages a series of trails in the towns of Warren, Waitsfield, and Fayston. The trails, nearly 10 miles in all, range from the flat, pastoral Mad River Greenway along the bank of the eponymous river to the steep Vista Trail that gains an overlook behind Warren School.

Last year MRP removed a failing bridge on the Millbrook Trail, which runs 2.1 miles from the Mad River Barn to Tucker Hill Road. The missing bridge breaks the trail in two for everybody except those hardy hikers and mountain bikers willing to ford a stream. Before proceeding with construction of a replacement bridge, MRP needed a Professional Engineer to approve a new design.

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Proposed footbridge location marked with stakes.

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Proposed footbridge design by architect.

In this case PERCH did not design the bridge – an architect did. But PERCH did what an architect is not qualified to do, which is to verify that the design is structurally sound. The starting point was a set of sketches by the architect, Charlie Hosford, which show the bridge from several perspectives and label the materials and dimensions. Conversations with Charlie helped PERCH determine what was needed: an analysis of the structural components of the bridge, and a design for key connections.

The primary loads on the bridge are its own weight (dead load), pedestrians (live load), snow, and wind. According to NCHRP Guide Specifications for the Design of Pedestrian Bridges, the standard live load on a footbridge is 90 pounds per square foot, which equates to adults standing shoulder-to-shoulder across the entire span. This load case might occasionally happen in a city park, where pedestrians might crowd onto a bridge to watch fireworks, but it’s ludicrous for a rural trail. MRP proposed that they could post the bridge for a load limit and bypass the NCHRP provision, which in any case is a guide rather than a legal requirement. This allowance makes a slimmer (and cheaper) design possible.

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Part of girder splice design by PERCH.

PERCH confirmed that the decking and girders are adequate as designed for a live load of 30 pounds per square foot, or up to 25 persons spaced equally across the bridge. PERCH also issued a connection design for the girder splices, using Simpson metal plates and a specific nailing pattern. An Engineering Report was submitted to the Town of Fayston, and I’m proud to report that they issued a building permit right away. Construction will proceed this summer.

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The permit!