Structural engineer helps keep the walls from tumbling down

HOMEWORK

February 13, 1993|By Karol V. Menzie and Randy Johnson

When you're working on an old house, structural problems often come with the territory. Sometimes it's a reflection of the age of the house -- wood wears out, and even brick and stone wear down.

Sometimes it's a reflection of when the house was built -- at a time of war, building supplies may not have been plentiful. Sometimes the materials just didn't exist; mass-produced structural steel wasn't around until the latter part of the 19th century.

Sometimes early do-it-yourself builders simply didn't know what they were doing; and sometimes structures were put up quickly and cheaply, with no real thought for the distant future.

The house we're working on seems to fall into the latter category. It's a good lesson in the vicissitudes of city life: The house, built before the middle of the 19th century as "worker" housing, now sits in a prime real-estate market, one of the more desirable in the city. If this little house is going to last another 150 years, at least, the structural problems have to be addressed.

Assessing such problems and devising a remedy is not really a do-it-yourself activity unless you're a structural engineer. If you're not, you need to hire one. The service is not usually expensive, and it's cheaper than having to tear the house apart later to satisfy a building inspector.

Our house has two basic problems: A second-floor support beam wasn't large enough to support the load on it -- a third of the weight of the roof and a third of the weight of the second floor. Once that beam began to sag, it put extra weight on a first-floor wall that was designed to be decorative, not load-bearing. The first-floor wall is supported by a single side-to-side header joist; even if that joist had to carry only the weight of the first-floor wall, it should have been double, not single.

The result is the whole front section of the house sags in the middle.

We hired Tim Sibol, of Skarda and Associates Inc., a Baltimore structural engineering firm, to design a solution for the problems.

He didn't have to go much farther than the front door to figure out what was wrong. His remedy: On the second floor, replace an inadequate load-bearing wall with a better one that will support the roof; install a new laminated beam beside the existing second-floor beam, braced in a pocket in the masonry wall on one side and on the other loaded on a 4-by-4 wood post that goes all the way into the basement and rests on an existing brick corbel, part of the foundation wall. For the first-floor sag, Mr. Sibol specified a 4-by-4 support post in the basement, on a new concrete footing, under the existing single header joist.

Mr. Sibol also said we could take out the old second-floor wall while we leveling the second-story floor, as long as we temporarily shore up the roof beam, "in case it snows."

The result will be to distribute the weight from the middle of the house to the brick side walls; with the load off, we can level the floors.

There was one other structural detail for Mr. Sibol to resolve: the whirlpool tub for the second floor. Since we already knew the framing was inadequate, we were pretty sure the tub was going to need extra support. Mr. Sibol recommended building a "mini-floor" under the tub, to be made of 2-by-6s anchored into the masonry wall on one side and supported on the other by two new 2-by-8 joists installed next to existing joists. The joists on the second floor run front to back; the new ones will go into an existing header at the back and will rest on the new laminated beam in the front.

"We get a dozen calls a year from people asking, 'Can I put a whirlpool bath on the second floor?' " Mr. Sibol said. "We get the technical data on the tub and check the size of the existing framing. Sometimes you don't have to reinforce, but you should always check."

Among some general rules for support structures, Mr. Sibol said, are these:

* A wood beam can be supported by a post made of wood or steel, but a steel beam needs a steel column. Steel beams can be smaller than wood ones that support the same weight, if space is a problem. But steel is more expensive than wood.

* Laminated beams, used primarily in residential construction, will support more weight than traditional wood beams.

* When working in a rowhouse, make sure you stay on your side of the party wall. (And let your neighbors know what you're doing, in case there are problems. Pounding on your side could cause plaster cracks on theirs, or knock pictures off the wall.)

* Don't set anything directly on a concrete slab; columns and load-bearing walls will need new concrete footers.

* All beams, columns and footings need to be professionally sized. Just because something worked for a neighbor, don't assume it will work for you.

Above all, try to think further ahead than the original builders did about what the house needs. Or you could be cited in a newspaper column 150 years from now as the one who caused a structural problem.

Next: Answers to readers' questions.

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