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.

Curious Cracks

Beth is buying a 100-year-old house in Barre, and hired PERCH to investigate some cracks in a concrete foundation wall. The cracks were discovered by the current owners 10 years ago when they removed a wall covering in the basement. Beth needed a structural engineer to assess the cracks’ severity before proceeding with the sale.

The terrain outside the wall slopes at a steep angle, up to 45 degrees, with a window above the high point of the grade. Several roof lines direct water into the corner; gutters were added at some point to divert the water. The outside face of the wall jogs inward at two locations: a stair-step jog near the southwest corner, and a vertical jog near the bottom corner of the window.

Exterior view of the foundation wall.

On the interior side, there are several vertical and diagonal cracks in the wall between the floor slab and the window. The widest cracks extend from the top east to the bottom west, and vary from ¾ inch to 2 inches wide. (Other cracks are smaller.) There’s also a crack in the floor slab itself, which has a slight downward slope toward the corner. All the cracks were filled with spray foam by the current owners and have remained stable over the last 10 years.

Wall crack – filled with spray foam. Floor slab crack – slopes down into corner.

PERCH determined that the culprit for the cracks was a familiar one: differential settlement. Soils under a heavy load may compress at different rates, causing one location to sink lower than another. This is especially true of old houses built on whatever soil happened to be on site, with no fill or compaction. Here it’s clear that the corner of the floor slab has settled lower than the middle of the slab. The stress concentration in that corner, along with the lack of damage elsewhere, makes differential settlement a better explanation than frost heave.

As for the wall, concrete is most likely to crack at stress points, where the forces in the concrete change rapidly. The bottom corner of a window, in which the wall next to the window supports two floors and a roof while the wall under the window supports only the window itself, is a textbook example of a stress point. Here one can imagine the wall sliding along the diagonal crack, down to the lower point as a result of differential settlement.

So should Beth be concerned? In this case, no. The cracks have not widened in the last 10 years, and gutters divert water to prevent further impact on the soil. Also, the wall is plumb and the cracks don’t extend all the way through. (There’s a simple explanation for the oddities on the outside of the wall: imperfect formwork when the concrete was originally placed.) PERCH delivered a report to Beth with some long-term suggestions for keeping the cracks insulated and the soil dry. The sale proceeded on schedule.