As a field representative for Simpson Strong-Tie, I’m often called to job sites to resolve issues with metal connectors, usually because a building inspector has flagged a mistake. Installation errors can affect the load capacity of connectors and — if not corrected — cause long-term problems. In this article, I’ll discuss some of the most common problems I encounter and what you can do to avoid them.
Anchor Bolts in the Way
Typically a standard cut washer and hex nut are all that’s needed to attach mudsills to embedded anchor bolts. In seismic regions, though, code requires 3-inch-square bearing plates .229 inch thick. Installed in place of the washer, these plates are intended to keep sills from splitting during earthquakes.
When anchor bolts are not carefully located, the bearing plates can extend beyond the edge of the sill or conflict with framing. To address this problem, some carpenters simply notch the rim, joist, or stud around the plate and anchor bolt, but this is a crude fix that weakens the framing and can lead to a failed inspection. A better solution — and one allowed by code — is to use a slotted plate that can be shifted out of the way.
Another acceptable option is to use straplike mudsill anchors instead of anchor bolts. These anchors are cast into the foundation, field-bent around sills, and then fastened in place with nails. Their low profile eliminates framing conflicts, and they have a lower installed cost than anchor bolts. In most cases they can be spaced about the same distance apart as 1/2- and 5/8-inch anchor bolts.
Spalling at Embedded Straps
Shear walls must be anchored with hold-downs to prevent wind or seismic loads from overturning them. In many cases, the anchors are strap-tie hold-downs embedded in the concrete foundation and nailed to the framing. If the hold-down is wet-set or bent out of the way while the concrete is green, it can cause spalling in the concrete.
Small spalls — less than an inch tall — will not affect the load capacity of the hold-down. However, spalls between 1 and 4 inches tall reduce the hold-down’s capacity by 10 percent. If a spall of this size is present, the building designer can make a quick determination as to whether the hold-down can still handle the required load.
No data exists for spalls more than 4 inches high, so when spalling is severe it may be necessary to retrofit an epoxied threaded rod or a mechanical anchor. Given recent changes in the building code, it’s best to have an engineer specify the appropriate anchor.
Strap-tie hold-downs are attached either directly to the framing or on top of the structural sheathing. When they’re installed over the sheathing, it’s sometimes necessary to bend them horizontally — especially if the mudsill hangs past the edge of an out-of-square foundation. Only a small offset is permitted — up to 5/8 inch. Easing or lightly notching the panel edge and nailing from the bottom of the strap upward will prevent the strap from bulging and keep wall movement to a minimum. More than one 90-degree bend is not allowed.
Hold-Down Bolts Too Short
When an anchor bolt is set too low in the concrete, there’s not enough exposed thread to properly connect the hold-down. To achieve full strength, the nut must be threaded a minimum of one full bolt diameter, so that the bolt is flush with or projects beyond the top of the nut. In this connection, the bolt is too short; extending it with a coupler nut and threaded rod and raising the hold-down above the sill may be a good fix. (Check with the manufacturer to make sure a specific hold-down is approved for this use.) A simple way to make sure there will be adequate thread is to use an anchor bolt that has the embedment depth clearly marked on it.
Misaligned Hold-Down Bolts
Hold-down bolts frequently end up in the wrong place, because of layout mistakes or last-minute changes to the plans. If a bolt is too close to the post, it may have to be abandoned and a new anchor retrofitted. If a bolt is too far away, it sometimes can be salvaged by extending it with a coupling nut, then gradually offsetting it to meet the raised hold-down. The usual rule is that the rod should be within 5 degrees of plumb (no more than 1/4 inch of offset for every 3 inches of additional height), but it’s best to check with the hold-down manufacturer.
The most common fastening problem I see is that hardware is missing nails. If nail holes are left unfilled, the connector won’t fully resist the loads it was designed to handle and may deflect, resulting in damage to floor and ceiling finishes. Worse yet, the connector may fail. Unless otherwise noted on the plans, it’s best to fill all the holes in the framing connector. The truss hanger shown here would support an additional 500 pounds if the four triangular holes (two in each side) had been filled.
I sometimes see hardware installed with the wrong size and type of nails, often because someone decided to save money by using standard collated nails. This reduces the load the hardware can carry; smaller-diameter fasteners are less resistant to shear, and shorter ones are less resistant to withdrawal.
When a nail misses the factory-punched hole and makes its own hole — as happened to this hurricane connector — it reduces the shear capacity. If you install a lot of framing hardware, invest in a gun designed to accurately place nails in the factory-punched holes.
The collated nails designed for use in dedicated hardware nailers come in reduced lengths — 2 1/2 inches versus 3 or 3 1/2 inches for hand-driven nails. In most cases, these lengths are okay as long as the nail achieves the minimum required penetration into the framing (not counting the sheathing). For example, a .148 x 2 1/2-inch nail needs about 1 1/2 inches of penetration.
The excessive dimpling on this connector is the result of overdriven fasteners, which can weaken the hardware by fracturing the steel around the nail hole. To check for excessive dimpling, lay a straightedge across the nail head and the metal on both sides. If there’s any gap between the top of the nail head and the straightedge, it’s time to lower the gun’s driving depth.