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The design of the walls and partitions* (even if they are load-bearing) will not take us into mathematical tables since the structural criteria aren't as critical as those for floors. Walls and partitions are subjected to gravitational loads, which are compressive, and most structural materials are very strong in compression. Be that as it may, it's important to under stand how these vertical supports work if you are thinking of adding, removing, or modifying a wall, partition, or, especially, a column. The load-bearing vertical supports of a building transfer the roof and floor loads down to the foundations. For the most part, the stresses on a wall or column are mostly compressive. In some situations, vertical supports, especially columns, are subjected to bending stresses. Different structural materials respond in different ways to the various stresses. Unreinforced concrete is very strong in compression but extremely weak in tension, as in bending. We must reinforce these walls and columns with steel bars if we expect any bending stresses. Masonry, as in brick, concrete block, or stone, is a very strong material when the stresses are compressive. You are probably most familiar with load-bearing walls that are constructed of double-thick brick or brick and concrete block. These walls are very good in compression, but will crack if subjected to bending stresses, such as the uneven settling of the foundation. Wood, on the other hand, although not as strong as concrete in compression, can assume a great deal of bending stress. A variation of the load-bearing wall is the wood-frame stud wall. WOOD STUD WALLS and PARTITIONS Essentially, there is little difference in appearance between the load-bearing and non-load-bearing partitions and walls. They are both constructed of 2 X 4’s or 2 X 6 (which are actually 1 1/2” X 3 1/2” or 5½” in cross section) spaced 16” o.c. Partitions that aren't load-bearing use studs that are at least 3 1/2” deep for stability. In some circumstances, as in the stub wall between closets, shallower studs (2 X 3’s) or 2 X 4’s set sideways are used to save space. But these thin partitions, even if non-load-bearing, are never used in trafficked areas where they may be leaned against. Exterior stud walls used in platform-frame construction are built after the floor below has been completed. The structural elements of the wall are the studs and the sheathing. The non-structural elements consist of the insulation and condensation-controlling elements, the interior wall finish, and the exterior siding. *The 2 x 6 stud allows for more insulation in the exterior walls. *Walls separate the inside of the house from the outside. Partitions separate one interior space from the other. The outside walls consist of 2 X 4 or 2 X 6 studs which are approximately 5’ long positioned vertically between the sole plate on the bottom and the double plates on the top. Plywood sheathing (generally ½”, 5/8”, or ¾” thick) is an important structural element since it's needed as bracing. Many of the old, pre-plywood houses have diagonal bracing that has been “let in” to notches made in the studs. The sheathing also acts as a skin which, to some extent, allows for the spreading of the load from one stud to the adjacent studs. Studs are traditionally spaced 16” on center, but can be spaced 12” or 24” apart if conditions warrant. (The 24” spacing is often used in houses with 2 X 6 studs.) The stud spaces in the walls of houses in mild climates are filled with 3½” of insulating materials. Houses built in northern climates have 2 x 6 studs and 5 1/2” insulation. The most commonly used insulation is the blanket variety available in 15” -wide rolls, which comes with and without a vapor barrier and is designed to fit snugly in between the studs. Many old houses have 2” of insulation or no insulation at all. If your house lacks insulation and you live in a climate that requires the house to be either heated or cooled, you should add insulation (see Section 19). Interior partitions, whether load-bearing or not, are either 4 1/2” thick (for walls with 1/2” of gypsum board on each side of the studs), 4 ¾” thick (for 5/8” gypsum board), or 5½” thick (for walls with 1/2” of backer board and ½” of gyp sum board on each side). Exterior walls are about 6” to 5” thick (counting the stud, 5/8” interior gypsum board, ½” sheathing, and about 1” of exterior siding). Since the exterior siding and sheathing extend over the edge of the foundation wall (, the thickness of the exterior wall, as measured from the edge of the foundation to the inside of the room, is only 6 1/8” (or 4 1/8”). REMOVING PARTITIONS Many of you may be considering the removal of a partition to combine two or more rooms. If the partition you remove is nonstructural, you will probably not have to reinforce the structure. On the other hand, if the partition is structural, it will probably have to be replaced with a beam and , perhaps, one or more columns. The removal of any wall or partition is so very critical to your safety and to the integrity of the structure that we strongly advise you to consult with an architect or structural engineer before taking any action. The following discussion on how to determine whether or not a partition is structural, or how to design the replacement girder and columns, shouldn't be considered a substitute for this professional’s advice. It is sometimes difficult even for a professional to be absolutely sure if a partition is load-bearing. First, you may have to make a hole in the ceiling under the upstairs floor to determine the direction of the span of the joists. If the joists above the partition are running parallel to it, it may suggest that the partition is non-load-bearing (unless, of course, it's propping up the extra load of a bath tub). If the joists are running perpendicular to the partition, and their ends are resting on it, it's very likely that the partition is load-bearing. When in doubt, trace the partition down to the basement or the cellar. If there is a partition or line of columns under the partition in question (especially if the basement supports aren't needed to separate rooms), you may suspect that the partition is load-bearing. Sometimes a partition that's perpendicular to the joists is non-load-bearing and sometimes it's partially load-bearing. If the span is short, under 20’, and the joists go over the top of the partition but don't end there, it's difficult to tell if the partition is completely extraneous to the structure or if it serves as a “prop wall”. The prop wall is commonly seen in a brownstone row house. It is the partition between the parlor and the front foyer. However, not all of the partitions between parlor and foyer are prop walls. Some are completely load-bearing, and some are non-load- bearing and can be removed. In the case of a brownstone, you learn a lot about its structure by tracing it from cellar to roof. If you are considering removing any old townhouse partition that runs parallel to the long lines of the building, consult an architect or engineer. He or she is likely to go down into the cellar and see if there is a masonry wall or a line of columns directly under that partition. If there is, and the line of support can be traced up to the partition in question, the partition shouldn't be removed without providing some other support for the floor system above it. In an apartment building taller than eight stories (with some few exceptions) you can be relatively sure that the building’s structure is a steel or reinforced-concrete frame and that all of the partitions are non-load-bearing. Old apartment buildings up to seven or eight stories are often constructed of masonry load-bearing walls and wood-joist floor systems. In this type of building about half of the partitions are load-bearing. If you are living on the second floor, it's likely that the partition between the bedroom and the living room is supporting the floor systems of all of the bedrooms and living rooms above you. It is best not to remove that structural wall even if you would add a steel beam in its place. If the building is old, there may be a lot of cracking and resettling above the new beam that may cause many of your neighbors a great deal of distress. Evaluating the structure of old buildings can be tricky. There are a number of apartment buildings that have both load-bearing walls and some lines of columns. In addition to structural considerations, there should be no chimney flues or any major plumbing, electrical, intercom, or cable TV lines in the partition to be removed. GIRDERS and COLUMNS If you want to open one space into another and remove a load-bearing partition, have an architect or engineer evaluate the situation. He or she is likely to suggest that you use a girder* spanning between two columns to support the floor joists above. The columns might be designed to be free standing or they might be enclosed as part of an adjacent partition. Columns are generally constructed out of wood, steel, or reinforced concrete, or are Lally columns, which are hollow steel shafts filled with concrete. In private residential construction, wood is by far the most popular choice. When designing columns, two sets of criteria must be met: first, the column must be strong enough to hold up its load (that is, it must resist the compressive stresses applied to it), and second, it must also have adequate thickness to withstand possible buckling (which subjects it to bending stresses). *The procedure for selecting the girder is exactly the same as the one described in Section 21. TABLE A: PRELIMINARY GUIDE TO MAXIMUM ALLOWABLE AXIAL LOADS FOR COLUMNS P = Maximum allowable axial load in pounds P/A = Allowable axial stress for buckling in psi (pounds per square inch) In light-frame wood construction, it sometimes isn’t even necessary to calculate the cross-sectional dimensions of a column needed to support a relatively light compressive load, if the column isn't much taller than 8’ and is enclosed in a wall. As a rule, an enclosed wood column with a 4 x 8 cross section will support a portion of one story and the roof of a house above it if the column is short and , therefore, not subjected to bending. But have your individual situation evaluated by an expert. As in designing girders, the span of the column, which is its unsupported length, is critical. If the column is long and thin, and eccentrically loaded, it will tend to buckle along its slimmest axis. If the load on the column is applied centrally (along its neutral axis) and evenly, the stresses on that column are purely compressive. If the loads are applied off center or unevenly (even as little as a quarter of an inch), the column may bend or “buckle” slightly. In wood-frame construction it's very difficult to control the loading and subsequent bending on the column. Even if you are very careful to center the load on the column, the lumber itself might contain an invisible defect which unbalances its load-carrying capacity. To make sure that the column selected (using the above rule of thumb) will not buckle, subject it to a simple mathematical test. The length in inches of the column is divided by the depth in inches (its smallest dimension). The result of this equation must be less than 50. If it's not, the depth of the cross section will have to be in creased. Table A integrates both sets of criteria. Procedure for Column Design Remember when designing columns that the loads are cumulative. That is, if a column is to be installed on the lowest floor, loads from the top of the building downward must be calculated. 1. Establish all loads on each section (floor to floor) of the column separately. Multiply the live load (L.L.) as established in the code by the floor area sup ported by the column. For the dead load (D.L.) include the weight of the materials used in the flooring system (joists, subfloor, finished floor) plus the weight of the partitions. 2. Working from the top down, determine the loads accumulated on the lowest column section. (Add all of the floor loads.) This gives you F, the axial load in pounds. In Table A, look under the heading corresponding to the height of the column for that section of column only—not for the whole length from basement to roof; you want the unattached length only. Read down to a number greater than load P previously calculated. Read left to determine the nominal dimensions and the cross section. Make sure that the P/A for the chosen section and column height is less than that recorded in the table. Example: The axial load P for the lowest section of a column on a two-and-a-half-story house is 15,000 lbs.; the column height is 14’. Check Table A. Read down under P in the 14’ column. Stop at a number greater than 15,000 lbs. Look to the left. Nominal size of the section is 6 X 6. The area of the 6 X 6 is 30.25 square inches. Check: P/A: 15,000/30.25 = 496 Check against the maximum P/A allowed (just to the left of the maximum allowable load for that height column), which in this case is 568. 496 < 568. Use a 6 X 6. Have an architect or engineer review your calculations before proceeding further. Next: Designing or Modifying the Roof |
