Holdowns and tension ties represent key components that comprise a continuous load path. In light-frame construction, holdowns are typically used to resist uplift due to shearwall overturning or wind uplift forces. In panelized roof construction, holdowns are used to anchor the concrete or masonry walls to the roof framing.
Holdowns can be separated into two categories — post-installed or cast-in-place. Cast-in-place holdowns, such as the STHD holdowns or the PA purlin anchors are installed at the time of concrete placement and attached to wood framing with nails. Cast-in-place holdowns are an economical anchorage solution with allowable loads up to 5,300 lb.
After the concrete has been placed, post-installed holdowns are attached to anchor bolts during wall framing. They are attached to the wood framing with nails, Simpson Strong-Tie® Strong-Drive® SD Connector screws and Strong-Drive SDS Heavy-Duty Connector screws or bolts and have allowable loads ranging from about 850 lb. up to nearly 20,000 lb.
The Holdown Selector is a simple web application that selects holdown solutions based on design loads.
Idealized Force Diagram on Full-Height Shearwall Segment
Methods of Providing Overturning Restraint
Holdown and Tension Tie General Notes:
Allowable loads have been increased for earthquake or wind load durations with no further increase allowed. Reduce where other loads govern.
To obtain LRFD values for cast-in-place holdowns (STHD and PA), multiply ASD seismic load values by 1.4 and wind load values by 1.6 (1.67 for 2015 IBC). For post-installed holdowns, multiply allowable loads by 1.4.
Use all specified fasteners.
The Designer must specify anchor bolt type, length and embedment. See SB and SSTB anchor bolts and PAB anchor bolts. Refer to technical bulletin T-A-ANCHORSPEC for anchor solutions for wind and low seismic applications.
Anchor bolt nut should be finger tight plus 1⁄3 to 1⁄2 turn with a hand wrench, with consideration given to possible future wood shrinkage. Care should be taken not to over-tighten the nut. Impact wrenches should not be used.
Post or beam by Designer. Minimum no. 2 or better. Tabulated loads are based on a minimum 3 1⁄2"-wide post (in a 3 1⁄2" wall), unless noted otherwise. Posts may consist of multiple members provided they are connected independently of the holdown fasteners. See common post allowable loads.
Holdowns are for use in vertical or horizontal applications.
Tension values are valid for holdowns installed flush or raised off the sill plate.
Deflection at Allowable Tension Load is determined by testing on wood posts and includes fastener slip, holdown deformation and anchor rod elongation for holdowns installed 6" above top of concrete (4 1⁄2" for HTT). Holdown deflections may be linearly reduced for design loads less than the allowable load.
At 1.5" max. offset anchor bolt, holdowns may be installed raised up to 18" above the top of concrete with no load reduction provided that additional elongation of the anchor rod is accounted for.
Tabulated loads for bolted holdowns may be doubled when holdowns are installed on opposite sides of the wood member. Designer must evaluate the allowable load of the wood member and the anchorage.
Tabulated loads for nailed or screwed holdowns may be doubled when holdowns are installed on opposite sides of the wood member. Member must be thick enough to prevent opposing holdown fastener interference or the holdowns are offset to eliminate fastener interference. Designer must evaluate the allowable load of the wood member and the anchorage. See common post allowable loads.
Structural composite lumber columns have sides that show either the wide face or the edges of the lumber strands/veneers known as the narrow face. Values in the tables reflect installation into the wide face. See Technical Bulletin T-C-SCLCLM for load reductions due to narrow face installations.