Designing A Replacement Structure For A Wall


January 19, 1991|By Karol V. Menzieand Randy Johnson

There are lots of reasons to remove a load-bearing wall: to open up a house; to gain access to adjoining space; to connect an addition.

It may not be easy, and it may not be cheap, but it can almost always be done.

The hard part of the job is determining what sort of structure -- some kind of beam and columns -- will replace the wall. The new structure has to be capable of bearing the same (or greater) "load" as the old wall.

"Load" is a complex calculation of exactly how much weight each adjoining structural member must carry. The total will determine how strong the beam and columns must be.

The process of designing the replacement structure is complicated by the fact that it has to be supported underneath -- usually, but not always, by the foundation.

In addition, the load also has to be supported, or shored up, during construction. The eventual strength of your beam and columns won't matter much if the house falls down before you can install them.

The shoring has to be in place before the old structure is removed. It also has to be strong enough to carry the load during construction -- and it has to leave room to work. If the load can't be supported during construction -- in a very tight space, for instance, or a very large one -- you may not be able to replace the wall.

Because this is all so complicated, and so important to get right, it's not a project ordinary homeowners should attack unassisted. Minimum loads are specified in building codes, and in many locales, structural work can not be done without the approval of a structural engineer.

If you're contemplating removing a wall, start by hiring a structural engineer to inspect the wall and determine if it's a good candidate for replacement. If it is, have the engineer design the columns, the beam, the foundation support and the shoring.

The engineer will begin the new design by determining the load. Bob Elliot, a structural engineer with Skardi and Associates of Baltimore, explains that there are two basic types of loads to be considered:

*Dead load, which is the actual total weight of all the building materials.

*Live load, which is the weight of people and furnishings (including waterbeds or whirlpool baths), plus any natural factors that increase stress on the roof, like snow, or earthquakes.

According to Mr. Elliot, local building codes govern live loads, which are based on the intended use of the space or building. In Baltimore, for instance, Mr. Elliott said, the code mandates live loads of 40 pounds per square foot for living room and dining rooms and 30 pounds per square foot for upper level sleeping rooms.

Because the code-mandated loads are only minimums,Mr. Elliott suggests that you tell the engineer if you are installing large heavy items -- "like large aquariums, walls of books, and waterbeds, because the beam may need to be designed for heavier live loads."

Michael Walkley, of Michael Walkley Associates of Baltimore, explained how the process works in a hypothetical situation of removing a load-bearing rear wall between a rear addition and the main house, so that a kitchen in the rear part could be expanded into an adjoining room.

He begins by calculating the live loads and dead loads for each level of the house -- the high roof (of the house), the third floor, the second floor, and the low roof (of the addition) -- to be supported by the new beam and multiplying each by the square feet supported (called the "tributary area"). The result is a list of loads per square foot.

He also calculates the dead loads of the new building materials. Then he adds all the load totals, which determine the load that must be carried by the new structure.

But that's not all he needs to design the new beam. There's a lot more math, concerning such esoteric considerations as "moment," "shear capacity" and "deflection." And there are other factors, such as lateral stability of the building, to be included.

Once Mr. Walkley has determined how strong the beam must to be, he is able to designate construction materials.

Beams can be made of a variety of materials. If there's brick or block above it, codes usually require that the beam be steel. If the house is made of wood, you may, depending on the loads, the length of the beam, the deflection, the size requirements, be able to use a wooden built-up beam.

Some wood beams can be as simple as a couple of 2-by-10s glued and bolted together. For extra strength, plywood can be glued and screwed between a sandwich of 2 by 10s. (The plywood adds extra deflection capacity). There are also pre-made lumber beams on the market which are very strong and may start to approach the load-carrying capacity of steel beams. Once the type of beam is determined, the next step is to design the supports or columns, which may be wood, masonry or built-up wood studs. The columns are as important to the structure as the beam itself.

The load the new beam carries must be transferred through the columns to the foundation. Sometimes the columns can sit directly on the foundation, or on another beam. In some cases there must be another column underneath to carry the load from the first floor down to new poured-concrete footings beneath the basement floor.

Once the structure is in place, it can disappear under any wall finish you want to apply. You may forget that wall was ever there.

Next: Dealing with linoleum.

* Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.

*If you have questions, comments, tips or experiences to share about working on houses, write to us c/o HOME WORK, The

Sun, 501 N. Calvert St., Baltimore, Md. 21278.

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