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As noted above, adding a girder stiffens a springy floor and , in so doing, reduces some of the loading on perimeter foundation walls. Although the surest test of that springiness is daily use, you probably need a girder if your joists exceed these spans, especially if joists lack blocking or bridging:
Your engineer will recommend a girder size; Table 10.1 shows maximum spans for built-up beams in two-story houses.
TABLE 10.1: Maximum spans for built-up wood beams in basements, cellars, and crawl spaces, two-story houses:
Source: Adapted from Canadian Wood-Frame Construction, with permission of the National Research Council, Ottawa, Canada,
Graded in conformance with 1971 “NLGA Standard Grading Rules for Canadian Lumber” published by the National Lumber Grades Authority, Vancouver.
(2) These tables provide maximum allowable spans for main beams or girders which are built up from nominal 2 in. members in the species, sizes, and grades indicated. Allowable spans for solid wood beams, glued-laminated wood beams, or built-up beams in sizes or grades other than shown shall be determined from standard engineering formulas.
Supported joist length means ‘4 the sum of the joist spans on both sides of the beam.
For supported joist lengths intermediate between those shown in the tables, straight-line interpolation may be used in determining the maximum beam span.
Beams for 1 1 houses shall be taken from the table for 2-story houses.
The 2-in, members shall be laid on edge and fastened together with a double row of common nails not less than 3 in. in length. Nails shall be spaced not more than 18 in. apart in each row with the end nails placed 4 in. to 6 in. from the end of each piece.
Where built-up wood beams are employed over a single span, the length of each individual piece used to fabricate the beam shall equal the length of the beam.
Where built-up wood beams are continued over more than one span and where lengths of individual pieces are less than the total length of the complete beam, the location of butt joints shall conform to Articles 23H13) and 23H141 of this Code.
In the sequence immediately following, we’ll assume that joists are not sagging badly and so don't need to be jacked. The steps additionally required for jacking are summarized at the end of the section.
Locate the new girder in the middle of the joist span. Plumb down to locate pads and posts, which should be placed every 8 ft. and beneath girder joints, should any exist. There should also be posts at each end of the girder or, failing that, pockets in the foundation wall to receive those ends.
Pads are typically 2 ft square and 6 in. deep, with a single layer of ½ in. steel rebar tied together: one bar running around the perimeter, 3 in. from the edge; and within the pad, rebar every 6 in.
Line the forms with sheet plastic so that the water in the concrete won’t wick into the soil and weaken the pad. To make sure that the pad is level, level the form boards carefully and simply screed off the concrete to them. As the pad starts to set, set the pier block in and rock it gently so that it settles down in about 1 in. Check its top for level. Allow concrete to set before putting weight on it: 3 days minimum, 7 days optimum.
Figure 10.7 Raising a girder. (A) By raising one end of the girder as high as possible and then tacking it up with metal straps, you’ll find that the other end will be easier to raise. Note, however, that this is only a temporary setup so you can get the two end posts in quickly. (B) A metal cap will prevent posts from drifting. Plumb all posts.
TABLE 10.2: Maximum spans for steel beams in basements, cellars, and crawl spaces.
Raising the Girder
When the concrete has cured, bring in the girder. Whether it's solid or is to be laminated from two pieces of 2-in, stock and a -in. plywood core, note its crown and install it crown up. (Plywood joints should occur over posts.)
Precut posts now, measuring down from the bottoms of joists to the tops of the pads, subtracting the height of the new girder. Raising a girder is much easier with many helpers, but you can raise a small one solo if you lift one end at a time and nail it to joists, using a metal connector (e.g., a hurricane tie) nailed to each side. Nailed with just one nail the tie acts as a kind of hinge, allowing the girder to swing freely. Raise the other end of the girder and nail it up temporarily with metal connectors, too.
With the ends of the girder hanging a few inches from the underside of the joists, installing posts is straight forward. Place the two end posts first, beating them into place with a 3-lb hand sledge. To be a little gentler on finish surfaces above, you can also cut posts in. short and drive in shims. Plumb end posts and nail them off, then raise others in between.
To prevent migration, pre-nail post caps to the tops of posts before raising them. Once all the posts are plumbed, pull the few nails holding the hurricane ties and re-nail the connectors properly. Simply toe-nailing the girder to joists is also adequate. Finally, toe-nail the bottom of posts to the wood blocks inset in the top of the concrete piers. If your crawl space has had moisture problems, treat posts to fore stall rot.
If headroom or clearance under your house is limited, steel beams provide a lot of strength for not much depth. They are, however, very expensive, and cumbersome to work with; hire a specialist to install them. Not only are they heavy, they’re problematic to attach to wooden members, requiring holes drilled in or connectors spot-welded to steel. Although your engineer will furnish exact specifications, Table 10.2 gives a rough idea of the size beam you’ll need.
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