Foundation Hold Downs for Modular Construction
Hold-downs (HD) that anchor structures to foundations are often overlooked but play a crucial role in structural integrity. No one wants their home to be at risk of wind damage. With various products available, choosing the right one for your project can feel overwhelming. While many hold-down options may technically work, each can impact the construction process differently. Selecting the right option based on your project’s needs and preferences can make a significant difference. The main impacting factors on your project are:
- The capacity of the HD
- The tolerance allowed in module box setting
- The effects on the factory assembly process
- Requirements for additional hardware
- Speed of installation
The Engineer of Record (EOR) determines whether hold-downs meet design demands, which helps narrow down suitable options. Although tolerance during module setting rarely affects EVstudio projects, it can pose challenges for inexperienced crews. Even a half-inch error may make certain hold-down types difficult or impossible to install. Additionally, some hold-downs require extra hardware, ranging from simple flat straps to costly custom steel fabrications. While installation speed is rarely a deciding factor, most hold-downs can be installed after the first-level modules are set and before additional levels are placed.
Cast-In-Place Strap tie Hold Downs
This commonly prescribed option works for projects ranging from single-family homes to large multifamily structures. Cast-in-place hold-downs offer average uplift capacity and a simple post-installation process. An example is the SIMPSON STHD14RJ.
Pros:
- Installation is simple and is complete as soon as modular boxes are set
- Average uplift capacity compared to other options
- Little to no reduction in capacity when near foundation wall edges
- Requires no additional hardware
Cons:
- Requires siding to be left off at HD locations in the factory to allow for the strap to be installed in the field
- Little flexibility to account for box setting errors as the straps will need to be fastened to specific studs in the module
Post installed Cavity anchors
Used for custom housing or projects requiring high hold-down capacity, these anchors suit scenarios where sheathing and finishing are not factory-installed. A common example is the SIMPSON HDU8. The following images are used in a panelized residential housing project.
Pros:
- These Hold Downs are installed after boxes are set so there is no chance of misalignment with the module framing
- No exterior siding needs to be left off in the factory construction process.
- Hold downs have a high capacity
Cons:
- Gypsum/ interior finishing must be completed on site as the wall cavity needs to be accessible in order to install
- Can require an adhesive component that requires additional time to set after the boxes are placed
- Always requires additional hardware that may need to be fabricated
- Reduction in capacity when located too close to foundation wall edges
Post installed Strap tie Hold Downs
This low-capacity option is ideal for residential projects with minimal demands. They are easy to install but less commonly used by our team.
Pros:
- Placement is done in the field after boxes are set so it is impossible to misalign with the module studs
- Requires no additional hardware
Cons:
- Low capacity when compared with other Hold downs
- Siding must be left off in the factory to allow for field installation
Twist Strap Hold Downs
Similar to the previous option, this HD can be utilized in smaller single family residential projects. This option provides the option to have no siding or sheathing left off in the factory assembly process. To accomplish this, additional hardware is used to fasten the modular wall framing to the floor cassette prior to siding being installed in the factory. The floor cassette is then fastened directly to the foundation. Due to the location of these straps, a crawlspace or basement foundation is required and would need to have a serviceable amount of depth in order to be able to install after modules are set.
Pros:
- Can be used in a group capacity to slightly improve capacity.
- Does not require exterior siding or interior gypsum board to be left off during the factory assembly process.
- Installation is done after the boxes are set so there is no chance for misalignment
Cons:
- Low capacity
- Requires additional hardware to be installed
As you can see there is a laundry list of hold-down options and reasons why you could potentially choose one or another. It is an important part of the design process for you to work with your design team to pick an option that best fits the needs of your project.
