Steel and Wood beams are used throughout homes and commercial buildings to provide proper distribution of floor and wall loads. There are a few different types of beam designs and various materials that can be used but the most important factor is matching the structural rating of the beam to the loads being supported. For this reason under most conditions Beams will need to be specified by an Architect or Structural Engineer.
Mid-span Beams are the most common type used in home construction and an example can be viewed in most basements or crawlspace. Most homes are designed around a rectangular floor plan shape. When Architects design layouts for the floor Joist System they want to span the shortest distance between the foundation walls. This is because the farther a joist has to reach between end supports the larger the board must be. Beyond a span of about 16 feet the size a floor joist must be reaches past traditional lumber sizes. To get around this limit a mid-span beam can be placed in the center of the basement and 2 boards can be used with their outer ends resting on the foundation sill plate and the inner ends supported by the beam.
They are placed at a midway point that will allow the carpenters to use the longest lengths of joists possible. Commonly a 2″ x 12″ x 16 foot Joist will be used in floor construction. This means if the house’s shortest span is longer then 16 foot a support beam must be installed. A 30 foot basement width would have a mid-span beam installed at the center point or 15′ dimension. The use of 16′ joists will allow for overhang on the outside foundation wall and overlap at the beam.
To the right you can see a conventional first floor that was built over a 4 foot crawl space. A concrete footing was poured on the outside of the foundation and a block wall was built. If you look closely you can see that there is a bump-out of block on the left wall to accept the steel beam that runs mid span along the center of the building. Block walls that support a beam can be made of either solid core block or block that has had its core filled with concrete.
Above the steel beam there is a nailing strip made of 2x lumber and the joists are nailed to this strip. To connect the nailing strip to the steel beam nails have been toe-nailed into the bottom edge and then bent to wrap around the top portion of the beam.
Because the joists are staggered but still laid out at 16 inches on center you will have to cut your first pieces of plywood decking material to fit the offset.
Note that the joists are overlapped and face nailed above the beam. Wood or steel strap blocking may also be necessary depending on the span distance of the joists from the beam to the outside foundation wall.
Blocking is made of the same size lumber as the joist and is placed between the joists to prevent twisting.
Some new products like Wood I-beams and floor trusses have reduced the need for steel beams by providing a structural joist that can span the whole distance of a foundation. The cost reductions in using these new products can be seen through reduced labor and equipment. The weight of Steel Beams will almost always requires some type of crane or lifting equipment to place it in position additionally altering a steel beam at the jobsite can become next to impossible even with portable band saws and drill presses.
On our Duplex we are using I-Beams to frame the first floor. Standard 2×10″ size I-Beams allow us to span the full distance between the foundation walls of the first floor without the need for a mid-span beam.
Steel hangers can be used to attach joists to wood beams. Here you see two hangers on top of a piece of glue laminated wood that is being used for a rim joist. Glue laminated wood beams can also be made out of this material.
There are areas where a header beam must be used If you look closely you should be able to see the Double I-Beams used to offset the load around the basement stairway. Steel hangers will allow a glue laminated wood beam to be attached between the I-Beams to support the rest of the floor.
You can see the structure of the I-Beam a little better on this small cut piece. The materials used are structurally rated OSB board that is placed between two 2x nailers.
When you size a beam or joist you must account for many loading factors that may not be for seen or predictable.
Beam Span Charts charts are available from manufacturers and lumber industry associations to help you pick the size you will need but this is one of the few things that is better left to a professional.
You may also find that the company that sells you the trusses for your roof structure will have a structural engineer on staff that will review your home building plans for free and provide the specs for the materials you should use.
If you are a owner builder you may find that a combination of I-Beam joists, Glue Laminated and conventional built up wood beams will meet your needs and budget better then steel. However if you are building with contemplation of whirlpool spas and other heavy items a steel beam may be your only choice.