Credit: Ted Cushman
Engineer Kathleen Dunne
’s prescription for supporting a shared rubblestone foundation wall called for a stepped reinforced buttress, sloped at the bottom to leave existing bearing soil undisturbed, and braced at the toe by the new basement slab at the lowered floor level.
The New York City borough of Brooklyn is a hot market these days (see New York Daily News, "Brooklyn real estate keeps soaring as both sales and rental markets end 2013 on a high note," by Phyllis Furman; and Park Slope Patch, "Brooklyn Real Estate Median Sales Price Sets 11-Year Record," by Rich Arleo). Unlike next-door Queens, Brooklyn is feeling the powerful economic spillover from upscale Manhattan and its rebounding financial industry. And the borough is also boosted by an influx of young people attracted by New York's identity as the nation's cultural capital; for that crowd, Brooklyn is a practical alternative to higher-priced Manhattan. (For an interactive heat map showing property value changes in all five boroughs, see this Vizynary graphic: "Where in New York City has property become more expensive?")
Where there's property appreciation, there's remodeling. So in recent years, Brooklyn has become a hotbed for major renovations — and, increasingly, for deep energy retrofits. That's a good thing for RPI-educated engineer Cramer Silkworth: as a Passive House consultant, Silkworth is making his living guiding Passive House makeovers of New York City row houses. "And I'm not just one person in the middle of nowhere," says Silkworth. "It's a burgeoning community down here. There are a lot of people digging it."
This winter and spring, JLC's Coastal Connection will follow along on the gut-rehab and energy retrofit of a historic brick home in Brooklyn's Prospect Heights neighborhood. The project brings together a strong team of professionals: along with Silkworth, the lineup includes architect James Wagman, whose portfolio boasts a long list of upscale residential makeovers in the city; structural engineer Kathleen Dunne (who also is a registered architect, and a professor at New York' Pratt Institute); and contractor Jose Maldonado, whose company has been renovating Brooklyn buildings for more than twenty years.
Maldonado is carving out his own niche in the Brooklyn Passive House community. Several years ago, he decided to fly to Ireland for a training course to earn certification as a Passive House builder, before a similar course became available in the U.S. "Taking the course was one of the best things I've ever done for my business," Maldonado says. "It is helping me a lot."
But it also doesn't hurt that Maldonado and his crew have the skills to handle the other parts of a complete gut-rehab — including aspects of the job that have nothing to do with energy performance. Maldonado is typical of JLC's readership: a hands-on tradesman who started out swinging a hammer, but who long ago graduated to the role of business owner and manager, executing one or two projects at a time with a crew of about 15 employees.
"I don't swing the hammer like I used to," Maldonado explains. "My Midas touch is the quality control, and the organizing: making sure the materials are there, and scheduling the tradesmen. I make sure that everybody has everything that they need, and I make sure that the work is done. So every day I'll go onto the job site twice a day. I make a stop around in the morning, and then in the afternoon I'll come in and make sure that what was expected when we walked through in the morning, that it's done in the afternoon."
In the case of the current Passive House job, the design includes major layout changes that require a complete re-working of the building's floor structure. Architect Wagman's program also called for a garden addition at the rear of the building, with a step-down into a new lower level in the main house. And there was no access for heavy equipment. So the job started with excavating behind the house, as well as beneath the house — using shovels (see Slideshow: Stepped Buttresses for a Row House Basement).
To accomplish the lowering of the basement floor beneath the house, Maldonado's crew had to dig out beside the existing basement wall and buttress the structure in stages, in order to avoid undermining the foundations of the adjacent dwellings. "First we dug out a three-foot wide section at each of the corners, and at the mid-point in each wall," explains Maldonado. "We formed up those points and poured the concrete there. Then we dug out the sections in between and set the steel and formed those, and poured those sections."
In a phone call to Coastal Connection, project engineer Kathy Dunne explained the structural considerations. It starts with an understanding of the soil. "I'm a structural engineer, not a geotech," Dunne makes sure to point out, "so my knowledge of soils is limited to what I need to know, and not extensive knowledge." But with that disclaimer out of the way, Dunne gets to the point: "Brooklyn is part of Long Island. And Long Island is mostly something left over from old glaciers that came across Connecticut. So from our point of view, Brooklyn is sand. And because these old houses usually were the first thing built, they are in effect resting on undisturbed soil. So we operate on the assumption that any bearing capacity we are going to get is the bearing capacity of sand – about two tons a square foot. We usually don't assume anything better, because it's unlikely that we're going to get anything any better."
Sand is plenty good enough bearing for a three-story brick house. But there's a twist: a typical New York City row house shares its brick or stone bearing side walls with its neighbors. They're party walls built on the lot line. "So what we do is make sure that the load of both buildings coming down on the common wall does not exceed that two ton per square foot allowable soil pressure," Kathy Dunne explains.
"All of those brownstones have party walls," Dunne goes on. "You're talking about very narrow sites — sometimes they are only 17 or 19 feet wide. If both of them had their own wall, you would end up with a width of building that is almost unbuildable, because you couldn't meet all the width requirements for habitable rooms and hallways and doorways and all that stuff. So the way they got around it, a hundred years ago, is they just built one wall and everybody used it. And actually, in a lot of these buildings, you will find evidence of an old doorway in the brick party wall that has been bricked up. Because what they would do is they would frame three or four or five of these buildings simultaneously, and keep an opening between them so the workmen could go back and forth for their trade, and then brick them up before they finished them."
Those shared party walls rest on shared rubblestone foundation walls between adjoining basements — and Wagman's plan called for deepening the basement. So how do you deepen a basement inside a shared rubblestone foundation supporting a shared brick bearing wall? Answer: very, very carefully.
Dunne says, "What most people end up doing is full underpinning of the side walls. If you are on a lot line, New York City says you have every right to underpin everything next to you. And in fact, you are obligated to underpin anything next to you, if you go below it. But underpinning is fussy, and it's tedious, and understandably, a lot of clients and architects are afraid of it. So, since they weren't interested in this case in obtaining the full width of the basement, we did stepping buttresses instead."
"When you mess with soil," Dunne explains, "the other thing that you get besides the vertical load of the structure, is you get a certain amount of horizontal pressure as well. So that's actually what the reinforcing in the buttresses is for: to resist the lateral pressure, not the vertical."
When a building rests on soil, the load of the building's weight spreads out from the footing as the load goes down into the soil. To prevent undermining of the existing party wall foundation, Dunne's solution called for a sloped underside to the stepped buttress sections. "You can't go straight down," she explains. "You have to do it at the slope that we show on the section. The reason is to bypass the influence of the load of the adjacent building. Because the last thing in the world you want to do on a structure that has a party wall or an existing building that has a structure right up against the lot line, is cause any damage to the house next door."