LaPlatte Bridge Survives Historic Flooding

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.

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.

2019-11-01_Lewis Creek Bridge
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.

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

Supporting a Staircase

Zach is a carpenter who partially gutted a house he bought. Before outfitting the house with a new hardwood floor and an oak staircase, he hired PERCH to analyze his framing and see if it was up to the task.

Top of the new stairs.

Zach was smart to hire a structural engineer in this situation, because as it turned out the framing needed several improvements. The additional weight from the staircase was too great for the existing first floor joists to carry, so PERCH specified an additional 2×10 joist to pick up each of the stair stringers. The next step was to follow the load path and note that the joists are all supported by a triple-wide beam running across the middle of the basement ceiling. With increased weight on the triple-wide beam, it needed an extra support post too.

What about the rest of the first floor, where Zach would install new flooring? A finish floor is typically ¾” thick and doesn’t add a ton of weight. But some of the existing joists were notched to accommodate utilities like air ducts, and this weakened them beyond the minimum code requirement. PERCH added sister joists across the notches to bring these joists back to full strength.

Now it gets weird. The house has a partial second floor with a balcony overhanging a central loadbearing wall. (That loadbearing wall is directly above, you guessed it, the triple-wide beam in the basement.) The staircase ends in two winder stairs supported by the overhang. Anticipating the increased cantilver load, Zach had already added an extra joist where the staircase was supported. Analysis confirmed this extra joist was adequate for the load, provided he fastened it to the original joist over the entire length.

But again we need to follow the load path, and in this case it leads past the central loadbearing wall to the back wall. If there’s a big weight on the end of the cantilever, and no weight anywhere else, then the joist actually pushes UP on the back wall. (If you sit on one end of a seesaw, the other end goes up.) That means there needs to be a stud ABOVE the joist to carry the load up through the back wall, where it gets balanced out by downward forces from the roof. In reality the weights on the joist will be evenly distributed most of the time, and uplift will rarely occur. Still, a situation that causes uplift is plausible (like a crowded party on the balcony with nobody in the other rooms), so PERCH designed for it.

Original second floor renovation plan in PERCH Engineering Report.

When Zach received the Engineering Report, he asked if there was any way to support the stairs without the last stud. Opening up the back wall would require a lot of extra labor on his part, refinishing a wall he had already completed. We pitched ideas together and settled on supporting the top half of the staircase with a post leading back down to the first floor. PERCH located and sized the post to leave room for a piano behind the stairs, and sent along the revised design. This exploration of options is called value engineering, and it always proves that good communication makes good projects.

Mockup of the staircase support post location.

LaPlatte River Bridge

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.

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.

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.

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.

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.

Donors and volunteers carved their names in the deck boards.