Most problems with floor/ceiling structural assemblies are related to excessive sagging/deflection and can be attributed to a number of deficiencies, including:
- Beam strength that has been reduced by extensive notching at joist-framing connections and joists that are excessively cut at ends (below) where they frame into girders (typical in pre-1900s houses);
- Joists that have been excessively cut, notched, or bored to accommodate material changes, pipes, wiring, or ducts (code agencies and manufacturers of engineered wood beams and joists stipulate limits to such modification);
- Insufficiently sized supporting beams; inadequately sized or spaced floor joists and fasteners; excessive spacing of posts supporting the beams; rotting of posts at bearing points; and insufficient or settled footings under the posts.
Problems with partitions are usually related to insufficient floor support or shrinkage of the studs, which are dealt with below.
Most current codes limit deflection for floors to L/360 (length of joist/360) which is derived from long-standing standards based on the deflection at which a plaster ceiling of the space below the floor would crack. While this is generally considered adequate to control deflection, some architects, engineers, and designers believe that stiffer floors are necessary for a user's sense of well-being, and design to higher deflection limits (such as L/480) or increase the floor load requirements from 40 psf on the non-bedroom floors to as much as 100 psf in the more public spaces such as kitchens, entries, family rooms, and living rooms.
Another approach is to limit deflection to a maximum, for example 1/2". Many older houses built before 1920 have floor joists sized considerably below current requirements. It is not uncommon to find 3x6 and 3x8 joists in pre-1900s housing that, in some cases, have been notched where they frame into girders by as much as one-half their depth. These members may well be split, specially if they have been affected by rot or insect damage. Because of the large safety factor used in the design of newer floor systems (up to factor of four), floors will rarely fail structurally, but they may have excessive bounce and feel unsafe.
Techniques for solving problems
1. REINFORCE EXISTING MAIN BEAM BY ADDING SUPPORT.
A house's main beam that is over-stressed, has deflected excessively over time, or has been affected by termites can be reinforced by adding a steel or wood column or a masonry pier to reduce the beam span (below).
It may also be possible to jack up the sagging beam to reduce or eliminate a slope in the finish floor above, although long-term settling is often difficult if not impossible to eliminate.
- ADVANTAGES
A relatively simple and effective way to stabilize a building's major structural element.
- DISADVANTAGES
Difficult to accomplish in other than basement or crawlspace areas, as columns may have to be placed in inconvenient places and will require some removal and restoration of existing finishes. Beams over crawl spaces may be difficult to access.
2. REINFORCE THE EXISTING BEAMS OR JOISTS.
Existing beams or joists can be reinforced by adding steel or wood reinforcement (sistering) along the existing members to develop additional load-carrying capacity (below). The length and bearing of the new reinforcing beams or joists will depend on the existing conditions and should be reviewed with a structural engineer.
- ADVANTAGES
Can eliminate the use of a new column support; does not affect the space below the beam.
- DISADVANTAGES
May be difficult to insert new support alongside the existing beam if access is a problem (such as in crawlspaces) or if joists frame directly into beam (Below, top image), in which case the existing floor joists would have to be temporarily supported, a new beam installed, and the joists hung from the new beam with joist hangers (below, bottom image).
3. TRANSFER LOAD TO EXISTING JOISTS
If a floor joist has been severely cut to accommodate a large pipe or wiring, it may be impossible to splice on a reinforcement member. In this instance, it may be preferable to transfer the load from that joist to adjacent joists using header joists that are end-nailed across the cut end of the interrupted joist to the adjacent trimmer joist. If the header has a span of 4' or less a single header may be satisfactory (Below, top image). For wider openings (up to 10') headers can be doubled up (Below, bottom images). Consult with a structural engineer or architect to verify.
- ADVANTAGES
Can reinforce floors when other alternatives are not practicable.
- DISADVANTAGES
Not possible where access is a problem.
