Earthquake Defense mechanical connections require a lot of planning.
With the 2007 CBC codes addressing Earthquake Defense more progressively, we’re feeling more like Carpenters of Steel than carpenters of wood! The amount of steel in new buildings in California for seismic structural engineering is changing the way carpenters frame. For production framing here in California, we used to use the words “blow and go” a lot. But those days are over. Now all anyone talks about is “mechanical connections.”
There was a time when all a carpenter needed was a set of nail bags, a Skill saw, a 100-ft. cord, and a couple of hand saws. Top carpenters showed up with a cats paw! All of that stuff fit easily into the trunk of a VW Bug. Trust me, I know. But framing today is completely different.
As you can see from this partial list of the structural hardware in the 5,600-Sq. Ft. residential building we’re currently working on, framing today is more about mechanical connections than pulling out your hammer and banging together walls and rafters.
- 900+ A35 Anchor Clips
- 400+ LTP4 Lateral Tie Plates
- 130+ Hold-Downs
- 12 HFX Hardy Fames
- 500+ Straps
- 2 Steel Moment Frames
- 2 Bent Steel Rafters
According to the 2007 CBC codes, “Any change in the plane of the structural frame has to have a mechanical connection.” The mechanical connection could be something as simple as an A35 anchor clip attaching the frieze blocks to the top plates, or HDU hold-downs for uplift. The A35 anchor clip isn’t new to framing in the United States, but with 500+ A35 anchor clips in most residential buildings, it’s no longer a simple installation. When A35 clips are required 4.5 in. O.C., most of the time we set our joist beams and joist rim, and install the A35 anchor clips before the TJI joists are installed. Yes, the A35 anchor clips are touching each other at 4.5 in. O.C.
If we install the TJI joist first, then we use a palm nailer to install the nails in the A35s. We buy boxes of Simpson A35s like we used to buy 6-packs of beer. We were always running out of beer, and now we’re always running out of Simpson A35s!
All beams must have a mechanical connection to the top plates. Toe-nailing beams down to wall top plates is no longer allowed. Since every beam has a mechanical connection to the top plates, we often use a router to dado out either the bottom of the beam or the top of the top plates, especially when the mechanical connection is an MST strap positioned horizontally for a drag strap.
The tools we use today are mind-boggling. I never would have imagined it twenty years ago. Now, instead of one saw, we carry three or four—with left-hand and right-hand saws for cutting rafters, Big Foot saws, and chippers (see below). And we even carry routers! For framing! It used to take 3-4 weeks—TOPS—to frame a house. Now it takes a year.
The templates we use for routing the MST straps are made out of 1/2-in. or 3/4-in. plywood with 1x2s attached with screws. The template width between the 1x2s is the width of the router plus the width of the MST straps.
|MST60 strap dadoed into top plates. With the MST strap dadoed into the beam or top plates, the beam sits flat on the plates.|
I live thirty miles from Simpson headquarters. Lately, I’ve given some thought to moving farther away. It seems the closer you get to Simpson, the more hardware you have to put in a building.
We use a lot of hold-downs for mechanical connections to prevent beam uplift.
The PHD2 5/8-in. all-thread hold-down rod is drilled through the 6×12 header, and a 3 1/2-in. x 3 1/2-in. x 5/16-in. plate washer is installed on the bottom of the 6×12 header.
We use a Protool drill guide—with an 18-in. Wood Owl Ultra Smooth Tri-cut Ship Augers bit—to drill double stacked 4×8 truss rafters, as well as drilling out the holes for the PHD hold-down all-thread rods through 6×12 headers.
A lot of planning is required for the 50 hold-downs that are in this basement slab (see photo, left). About 30 of these hold-downs are for the uplift mechanical connections at the ends of the basement shear walls. The other 20 hold-downs are for post-to-beam uplift mechanical connections that transfer up to the roof sheathing for the continuous load path.
Before we start any job, we spend a lot of time looking at the plans for the continuous load paths that require uplift mechanical connections. A lot of the connections—per the structural engineering drawings—are un-constructable. If we see any un-constructable uplift mechanical connections, we send an RFI to the structural engineer. Or we send the structural engineer a replacement drawing or suggestion. The structural engineer then responds with a “no objection” or “objection” to our replacement suggestion…and the job or delay goes on….
When we have to epoxy in new hold-down all-thread rods, we use a rebar-cutter SDS drill bit—Bosch or Relton—to drill/cut through the rebar with our Bosch or Hilti rotary hammer. The hold-down all-thread rods require precise placement in the concrete slab, so we use an SDS drill bit—which is 1/8-in. wider than the all-thread rod—and drill the hole until we hit the rebar in the concrete. Then we switch bits, and drill with the rebar cutter until we’ve cut through the rebar. We finish off the hold-down hole with the standard SDS drill bit. Per the 2007 CBC, all epoxy hold-downs must be inspected by a special inspector. The new hole for the epoxy must be brushed out, then blown out with an air nozzle that reaches all the way to the bottom of the hole, and brushed out again, and then blown out again with an air nozzle.
|A lot of these mechanical uplift connections are side by side! The MST72 and HDQ8 have a 6x post below each of them. At the bottom of the post, another hold-down secures the 6x to the basement slab.|
|There are 5 hold-downs in this picture! And no, I didn’t mock this up as a joke. The two HDQ8 hold-downs at the end of this shear wall are for the shear wall connections. The other three HDU4 hold-downs are for post-to-beam uplift connections.|
Sometimes there is more metal in the walls than wood! So we don’t have to cut studs loose, we install all the hold-downs as we frame the walls.
When we’re not planning out the location of the uplift connections—which transfer load from the basement slab up through the roof sheathing—we’re installing horizontal collector straps. A chipper that I used for cutting rafter seat cuts in the ’70s and ’80s is now used to cut dadoes in the subfloor for these straps.
All these dadoes consume a lot of subfloor thickness. The plans called for 3/4-in. subfloor—imagine what would be left! I up-sized the subfloor to 1 1/8 in., so our flush-cut CMST14s would not weaken the plywood subfloor.
On steel I-beams, it’s the carpenter’s job to layout all of the mechanical connections, like these HFX Hardy Frames with 1 1/8-in. all-thread rods, and the location of the web stiffeners on each side of the 1 1/8-in. all-thread rods, too (below).
When framing with this much steel, it’s important to keep an eye on Moisture Content. That’s something no framer in the ’70s or ’80s thought much about. Code requires that all the lumber be dried to at least 19%, but even that wet, it shrinks. And steel doesn’t. Keep in mind, we have to put 6x material for nailers on the I-beams, so excessive shrinkage can cause major problems in floors and walls (bumps and high spots in floors, cracks in walls, etc.). Luckily, on this building, it sat for so long that most of it was dry. Which also gave us a chance to go back and tighten all the bolts. Think about that. What happens when the frame is completed and dried in before you have an opportunity to tighten the bolts?
A lot of planning and calculations went into making this 67.5° roof axis point work (below). The 7:12 to 26:12 roof axis points were tough enough to calculate, let alone the 67.5° 26:13 pitch-bent steel hip rafter!
In the photo to the right, Erik is drilling 11/16-in. holes in the 1/2-in. steel plate using the Hougen Portable Magnetic Drill. After that, the 1/2-in. thick steel plate is bolted to the concrete wall, and an ECQ column cap base is welded to the plate for the joist beam uplift connection.
The first time I drilled holes in steel I-Beams back in the ’70s, it took about an hour to drill an 11/16-in. hole for the 5/8-in. bolt or all-thread rod—that was using a standard twist-drill bit. With the Hougen Portable Magnetic Drill, it only takes about 1 or 2 minutes per hole. There are two switches on the tool, one for the magnet and one for the motor. First, you position the drill bit on the hole, then you switch on the electromagnet. After that, running the tool is like operating a drill press—you just crank the bit into the steel.
We had a job in SF 1 1/2 years ago where I spent $700 for drill bits to drill a-holes through the I-beams so we could attach backing. With the magnetic drill, and using Annular Cutters, I can drill 100+ holes, instead of the 2 or 3 that you get out of a twist-drill bit.
All of the roof beams have a mechanical connection to either the walls below, or the joist beams below. “Mechanical connection” translates to mechanical fasteners, like bolts, base caps, straps, or hold-downs. Any change in the plane of the structural frame has to have a mechanical connection, which includes every common rafter and hip rafter. No more toe-nailing rafters to the ridge beams!
After we install the fascia, we re-check the building with Stabila levels, so we know where we need to add more structural hardware.
When I first saw the plans for this building, I thought it was going to be a lot of fun, but the structural steel took all the joy out of it. On the main house, we had to first install the 26/12 rafters, then sheet the roof, then cut the radius rafter tails, and scribe the bottom side so it laid on top of the roof sheathing. Finally, we had to cut a hole through the sheathing so we could put an MSTI26 from the rafter tail across the rafter. And that was for every single rafter on the main house—200 radius rafter tails, and every one had an MSTi26 strap.
Sometimes I worry about installing all those straps. With all the engineered walls we build into homes, it’s no surprise that the other trades get us into trouble. One time, the heating and air guys came in and butchered a shear wall—they notched out the top plate. The engineer required us to install a 6-ft. long 1/4-in. x 4-in.-wide steel strap, with 16d nails 1-in. on center! I said to the engineer: “Hey, we’re just going to split out all the wood! What’s the point?” His answer: “Okay, then pre-drill the holes.”
|Just when you think you’re done with all of the mechanical connections, you have to screw the siding and trim to steel Hardy Frames!|
“Blow and Go” = Fun
Mechanical Connections = Frustration and a lot of un-constructable engineering.
Sim Ayers is the owner of SBE Builders, a commercial and residential framing company, located in the San Francisco Bay Area, which was established in 1988. He uses empirical knowledge, gained by means of observation, experience or experiment, to frame buildings from the ground line (Z1), to roof axis (A1), to the bring-back line for scribing (B1).
Sim is a second generation carpenter. He is passing on the family tradition to his two sons Brian and Erik. As a typical California production roof cutter and stacker in the 1970s and 1980s, Sim keeps a sharp eye out for new information on roof framing geometry, or for writing online scripts that use a tetrahedron to show the relationship of geometric framing angles for use with the carpenter’s steel framing square. His online tools can be found on the web at www.sbebuilders.com/tools.