Replacing a Wooden Girder with a Steel I-Beam
As level building lots continue to disappear, we’ll need to make the most of sites once thought unbuildable–and rely on technology to help us do it. This blog shows of the construction details of the in-law suite featured in last week’s posting, “An Award-Winning In-law Suite on an Unforgiving Site.”
To recap, the garage structure was cut into a steep, south-facing slope above San Francisco. The lot’s high side bordered a city street, so a garage right on the street became the portal to descending stairs, an elevated walkway, and a dramatic, modern house set downhill about 50 ft. from the road.
In time, the homeowner realized that the cavernous space under the garage could accommodate an in-law suite, so he hired Stephen Shoup, principal of buildingLab in Emeryville, Calif., to design and build it. The great challenge of the project involved transferring the loads borne by the main girder-which supported a garage floor above-to a steel I-beam set flush to the finished ceiling that would span the 22-ft. width of the suite without intervening support posts.
The weights were considerable. The garage floor system consisted of 2×12 douglas fir joists spaced 12 in. on center, a 3/4-in. plywood subfloor, and a 4-in.-thick reinforced concrete slab. The clearances were also quite tight when it came time to cut through the garage floor joists: the slot cut was roughly 3/16 in. wider than the width of the I-beam-allowing 1/16 in. on each side of the I-beam and 1/16 in. of “wiggle room.” The fit had to be that exacting so joist ends could be hung off the 2x12s bolted to the sides of the I-beam.
Thanks to Bob’s Iron of Oakland, California, for allowing me to photograph its crew at work.
Thousands of field-tested tips
This blog was adapted from Renovation 4th Edition, recently published by Taunton Press. Renovation 4‘s 614 pages include 250+ technical drawings, 1,000 photos from the 40,000 I have taken over the years, and thousands of field-tested tips and techniques that master builders have shared with me. I hope you find it useful. –Mike
© Michael Litchfield 2013
Unfinished space under the garage. The 6x12 Douglas fir girder, at top, spanned 22 ft. with the aid of 2x6 beam pockets at both ends and a 6x6 post at midspan. The white sticks about halfway up the wall indicate the height of the subfloor to come.
Cut into a steep hillside, the unused space beneath the garage was a natural for an in-law suite--if some formidable structural challenges could be met and an ingenious floor plan could be devised.
A slot has been cut into the 2x12 ceiling-joist array above, into which the W12x26 I-beam will fit. At this transitional point, the girder and post seen in the first photo are carrying roughly half the load--the joists at left--and shoring, at right, supports the cut-through joists. Once the I-beam is in place, the severed joist ends will hang from the 2x12s bolted to both sides of the I-beam.
Raising a beam of this length and weight should be attempted only by skilled professionals with specialized equipment. A W12x26 I-beam weighs 26 lb./ft., so its total weight is 572 lb.
On the other side of the shoring, another worker uses a ratcheted come-along to pull the beam towards the opening in the wall.
An I-beam on the way up. At left, shoring; upper right, a rectangular hole into which the beam end will fit once it has been fully raised. At either end, posts will be cut to exact length, set into the wall, and bolted to the legs of the I-beam cap, which is an integrally welded assembly of 1/4-in. steel plate. Those posts will transfer loads down to the perimeter foundation.
Renovation 4th Edition contains thousands of field-tested tips and techniques that hundreds of master builders shared with Mike Litchfield, a founding editor of Fine Homebuilding.
After installing the subfloor, the crew erected shoring to pick up half the load of the garage floor-joist array. The other half would be still be supported by the original wooden girder after a slot was cut to receive the steel I-beam.