Articles & Notes

Part 1

Hold Down Systems:
Key to Shear Wall Performance

Structure Magazine

Tie-Downs connect structures to their foundations. Tie-down hardware usually consists of threaded rod, bearing plates, holdowns, TUDs (Take-Up Devices), nuts, couplers and washers. Rod, plates, holdowns and TUDs are load and elongation rated, while the balance of the components are “grade compatible”. Note: Tie-Down Systems may include plates and/or holdowns to connect to the wood.

Buildings usually shrink or settle from 1/4″ per floor or more. Tie-Down systems with TUDs (Take-Up Devices) self-adjust and keep the buildings tight so the shear panels work during a wind or seismic event. Systems must maintain their strength, elongation, and reliability as the building shrinks and settles. Note: system elongation not to exceed 0.200”. This article outlines the problem and suggests solutions.

Note: The 0.200” system elongation is usually the upper limit for typical standard walls with a wall length of 10 feet or more. Short walls are often to a tighter limit, sometimes a low as 0.100” elongation.  

Part 2

Strap and Tie-Down Systems:
A Critical Evaluation

Structure Magazine

Tie-Down hardware often consists of nailed steel straps and formed steel components. These items are usually rated for strength with an elongation provided at the rated hardware capacity. System hardware must consider the elongation of all items between reaction points and must include the shrinkage and settling between reaction points. I call this “elongation without load”.

This section reviews current hardware and how it addresses shrinkage. 

Part 3

Continuous Tie-Down Systems: A Critical Evaluation

Structure Magazine

Seven different Continuous Tie-Down Systems are reviewed and evaluated. Each system has some advantages and disadvantages. You choose.   

Part 4

Designing Rod
Tie-Down Systems

Structure Magazine

The goal of any tie-down system is to limit the “top-of-wall” movement to a point before the shear panel begins to yield. This varies depending on the shear panel construction (Per the NDS), the size (height/length), and loading. Acceptable uplift movement generally varies from 0.125” to 0.200”. This article outlines some of the typical challenges and problem.  

Part 5

Rod Tie-Down Systems: Inspection

Structure Magazine

Inspection includes reviewing the drawings, calculations and the listed base values of component parts. Compare the material strengths to the listed values and confirm that the elongations match code reports. Note: a common problem is rating rod. Rod strength is rated on the nominal OD (outside diameter) per AISC, while all-thread rod elongation is rated on the net tensile area. Depending on the rod size this varies by 26 to 38%. Be precise and critical!

Inspection continues with a physical inspection of the property with submittals in hand. Look at the system as installed. What are the defects? Are they relevant?

Some of the components shown here are difficult to install, some don’t work well and have been removed from the market. Remember that these systems are the only structural moving parts in a structure. Care in design and installation is needed.   

Part 6

Evaluating Tie-Downs:
A Systems Approach

Structure Magazine

Tie-Down design looks at and evaluates complete systems between reaction points. Each item in series, such as threaded rods, bearing plates, and TUDs are evaluated for strength, elongation and shrinkage to verify that minimums are met. In most cases elongation, not strength, determines the components necessary since the movement controls shear wall performance.

Elongation incudes the movement of each rod, the bearing plate, hold down (if used), shrinkage, and TUD movement (Δa and Δʀ). Multiple story systems may have the same elongation limit on every story – or it may be assigned on a story-by-story basis. Note that some items may have a precise deflection (threaded rod) while others may have a deflection that varies widely (Some TUDs).