This spring, JLC is following a renovation job in Brooklyn, New York. The project includes a deep energy retrofit done to the demanding Passive House EnerPHit standard, but it also involves some extensive structural reinforcement. This week we talked with contractor Jose Maldonado and structural engineer Kathleen Dunne about the structural part of the job (for a closer look, see "Slideshow: Beefing Up a Brooklyn Brownstone's Floors and Roof").
Kathy Dunne, an RPI-educated engineer and architect who is a faculty member at the Pratt Institute, has been practicing in New York for decades. For her, this project is routine. "This is just a brownstone renovation. It's our bread and butter," she told JLC. "We're not doing anything particularly miraculous here." It's also not the first project she has done with James Wagman, the architect on the job: "We have done probably ten or fifteen townhouses of various kinds together over the years," Dunne says.
New York City brownstones are the legacy of a building boom in the late 1800s and early 1900s. "These buildings were not designed, as such," explains Dunne. "It was all empirical: They built them that way and they stood up, so they kept building them that way." The townhouses share basic characteristics in common, she says: "They always did side-bearing masonry walls, with nominal 3x wood joists going across the span."
The width of the homes varies: "They're either 18 feet, 20 feet, or maybe 25 feet wide, depending on how the lots were subdivided," says Dunne. "And the joists are either 3x8, 3x10, or 3x12. The typical floor layout is that there's a staircase on one side, along the length of the party wall, and the joists go all the way across. So say it's a 19-foot-wide townhouse and they used 3x8 joists: well, 3x8s won't span 19 feet without some help. So there is a wall that runs down the length of the unit at about the third point of the width, which we call a load-relieving wall, because it's really there to stiffen the joists against deflection."
Credit: Cramer Silkworth
As is typical in townhomes of its vintage, the Brooklyn project house has a staircase that hugs one of the long party walls between units, and a "relieving wall" that helps support the floor joists, which run across the whole width of the unit. This wall was removed during renovations, requiring the floor system to be reinforced.
Even with the load-relieving wall breaking up the span, joists are typically under-sized. And the load-relieving walls don't usually run the whole length of the building: "Typically," says Dunne, "about three quarters of the floor would have the load-relieving wall. Usually the front and rear rooms will not, and they will usually sag the worst over time."
An inch or two of sag in a floor is not uncommon, Dunne says. "Wood deflects one and a half to two times its initial deflection, over time," she says. "Whatever you got in the first place, it's likely to double over the hundred years. My own house has 17 foot spans, I have 3x10s, and it's 80 years old, and I am out a good inch and a half in the kitchen."
Sagging floors are not dangerous, Dunne says. "You have to understand the difference between deflection and stress. Failure modes have to do with stress. Shear stress, bending stress, tension stress, compression stress--whatever stress. But deflection is simply a measure of how much it deforms due to the load. All materials that are loaded deform."
Despite an inch or two of sag, the floor framing is as strong as it was when it was new, says Dunne: "In fact, it's actually stronger, because as wood dries over time it gets stronger. If it sags so much it makes you nervous, then you might want to level it. But it's not unsafe in any way, shape, or form."
However, floor plan changes often call for the removal of the load-relieving wall in some locations. And even with some mid-span support, the floor joists may be undersized. In this job, some sections of floor have been re-framed with steel C joists, and other sections have been reinforced with C joists between the existing wood joists.
Stairs often present another structural issue, says Dunne. "Stairs, more than anything, tend to sag," she says. Frequently, she says, stair headers and trimmers are joined with mortise and tenon joinery that has slipped or broken, so connections need to be reinforced. "In this house," she says, "we moved the staircases around. So we used engineered lumber to create new stair openings."
But it was the roof structure that required the most beefing up. Says Dunne: "At the rear, they put a walkable roof deck on. The New York City snow load for these old roofs is 30 pounds per square foot (psf). But the minute you go to occupy them, you are up to a 60 pound per square foot live load, so you have doubled the live load. And these days, people are putting metal gratings, concrete pavers, trees, plants, cast iron furniture -- they're putting everything they can think of up on a roof that was never sized to take that kind of load. So we're always reinforcing the roofs significantly, because we have more than tripled the load on them."
Credit: Cramer Silkworth
A worker sets a heavy PSL timber to help reinforce the townhome's roof structure. Loads on the roof have more than doubled because of a new walkable rooftop patio.
Dunne added parallel strand lumber (PSL) beams to the roof frame to help with the extra roof loading. In a few locations, she also specified steel flitch plates. "One constraint that we have in townhouses is the height of the floor," she explains. "Nobody wants to see the beams hanging down in a room where we only have an 8-foot ceiling. But a lot of times we make openings, and we gather loads, and I am going to tell you that to stay in the wood vocabulary, you need an 18-inch-deep beam, so you can't occupy the room below. That's a problem. So then we have to come up with a creative way to do a shallow beam. The flitch beams put the stiffness of steel into the wood, but still provide two side plates of wood so it's still a nailable situation. It's an interesting engineering problem because you have two materials with two very different properties, and you have to figure out what their capacities are. But there's a company now called Better Header, who makes them as a standardized product, so they are very easy to get."
Even with those added roof loads, however, and the point loading introduced by heavy composite beams, the existing masonry walls are up to the task—within reason. "The walls have capacity," says Dunne. "But you couldn't put, say, two floors and a roof on one of these buildings without doing a steel frame inside the whole thing to bolster what the walls can carry."