What to do about bouncy floors
Most problems with floor/ceiling structural assemblies are related toexcessive 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 orshrinkage 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 aplaster ceiling of the space below the floor would crack. While this isgenerally considered adequate to control deflection, some architects, engineers,and designers believe that stiffer floors are necessary for a user's sense ofwell-being, and design to higher deflection limits (such as L/480) or increasethe floor load requirements from 40 psf on the non-bedroom floors to as much as100 psf in the more public spaces such as kitchens, entries, family rooms, andliving 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 belowcurrent requirements. It is not uncommon to find 3x6 and 3x8 joists in pre-1900shousing that, in some cases, have been notched where they frame into girders byas much as one-half their depth. These members may well be split, specially ifthey have been affected by rot or insect damage. Because of the large safetyfactor used in the design of newer floor systems (up to factor of four), floorswill rarely fail structurally, but they may have excessive bounce and feelunsafe.
Techniques for solving problems
1. REINFORCE EXISTING MAIN BEAM BY ADDING SUPPORT.
A house's main beam that is over-stressed, has deflectedexcessively over time, or has been affected by termites can be reinforced byadding 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 aslope in the finish floor above, although long-term settling is often difficultif not impossible to eliminate.
A relatively simple and effective way to stabilize a building's major structural element.
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 woodreinforcement (sistering) along the existing members to develop additionalload-carrying capacity (below). The length and bearing of the new reinforcingbeams or joists will depend on the existing conditions and should be reviewedwith a structural engineer.
Can eliminate the use of a new column support; does not affect the space below the beam.
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, itmay be preferable to transfer the load from that joist to adjacent joists usingheader joists that are end-nailed across the cut end of the interrupted joist tothe adjacent trimmer joist. If the header has a span of 4' or less a singleheader may be satisfactory (Below, top image). For wider openings (up to 10')headers can be doubled up (Below, bottom images). Consult with a structuralengineer or architect to verify.
Can reinforce floors when other alternatives are not practicable.
Not possible where access is a problem.
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