July 2024
by Steve Petracek
Site Adapted Prototypes
On the design side of a project, everyone loves a prototype. Okay, maybe not everyone, but making a project or its details can be prototypical, the more efficiently a project can be designed. However, adapting a prototype to a specific site requires careful consideration. Questions may include: How does the building integrate with the site’s layout? How do planning, zoning, and energy codes influence both aesthetics and functionality? And where are the utilities situated, considering they often don’t align with the optimal building corners?
On the structural side of the project, The main elements affecting structural design at any site are variable loadings: snow, wind, and seismic forces. Foundations can vary as much as the loading types, each presenting unique challenges. The complexity of foundation design increases with soil types like clay, silt, sand, or any combination thereof, not to mention site leveling and frost depth. In conclusion, addressing these variables with a well-adapted prototype ensures structural integrity and cost-efficiency.
Snow
Snow is effectively a variable live load, and while many of us enjoy it, it can significantly impact a structure’s design. For reference, a standard roof live load is 20 pounds per square foot (PSF) across the country. In regions like Florida, southern Texas, and southern California, there is no snow loading. However, places like Maine and Alaska can experience ground snow loads of up to 100 PSF. Site-specific locations in mountainous regions, such as Colorado and Idaho, can have localized loads well over 100 PSF. Consulting the building official in these areas is highly recommended but not always sufficient. Building officials often suggest contacting local fabricators to determine the typical design load used in the region.
Wind
Wind pressure can be applied as a broad loading, or in other words, one design covers a significant area of the country. Almost everywhere in the country, anytime you step outside, you can feel a breeze. For wind to be considered severe in the US, it must reach a speed of 58 mph with most storms only reaching 39 to 46 mph per the National Oceanographic and Atmospheric Administration (NOAA). Therefore, one design may be able to cover 90% of the prototype locations. Unless the project is in a hurricane zone, the code wind speed in the US is predominately 115 mph or less for a Risk Category II structure. For a Risk Category III, the wind speed jumps to 122 mph, well above all but the most severe cases.
Seismic
Location is significant in seismic design, with forces determined by the structure’s weight. Plate tectonics and the Ring of Fire in the Pacific significantly affect the west coast and Alaska. But one can’t forget the Madrid fault in southeast Missouri that in 1811 it shook and was felt in Hartford, CT and Charleston, SC. The lighter the structure, the smaller the seismic force. For most lightweight structures built with wood or cold-formed steel framing, 90% of the designs are completed with a single approach since the structure is relatively light. However, some variations make seismic design unique at each site compared to wind. Beyond the previously mentioned weight concern and building height, framing type plays a significant role in the design effort. Concrete shear walls, moment frames, and braced frames are in a long list of framing options that require different levels of detail, depending on the maximum considered earthquake. And then there is the soil type unique to each site that plays a significant role. Solid or soft, each one resists the load in a different way.
Geotechnical
Geotechnical information varies significantly across the United States, impacting building costs. To break this variation down, I will address them as two foundation types: shallow and deep. Shallow foundations are the typical spread footings located approximately in the upper 10′ of the soil. Deep foundations are some version of piers or piles and extend deeper than 10′. Depending on the structure’s mass, a conservative value for spread footings that work in most places may add avoidable additional costs.
An assumed bearing pressure of 1,500 psf compared to an actual bearing pressure of 2,000 psf could reduce the footing size and material costs by 25%. The nice thing about spread footings is that an adjustment to them can be made easily and quickly, depending on the complexity of the foundation system.
There is no point in making the effort for deep foundations to create a prototype design. There are too many variables. Better results will be achieved by designing deep foundations on individual projects.
Grouping
For a prototype design, the larger the grouping, the quicker the updates or design adjustments can be made. Of course, the design envelope size will vary, depending on the framing material and type. Structural steel and concrete typically have extra capacities based on code minimum or functional requirements. For example, an HSS2x2 post may work, but an HSS4x4 is used because of the connection preferences. Cold-formed steel and wood are typically not loaded to their maximum capacities, and higher loads can be applied to allow for a larger design window.
With all this said, prototype design can be very efficient for a client and a time saver for the designer. There will always be a design element in every project, but if the layout and framing of a structure don’t change, design effort is reduced, saving time and money.
The body having jurisdiction has the final say. Each municipality is different, and many have unique requirements, so even with the best planning, changes could still be possible.
Conclusion
Adapting prototypes to specific sites involves a careful balance of design efficiency and compliance with local codes and environmental conditions. By understanding the impact of site layout, planning, zoning, and structural variables such as snow, wind, and seismic loadings, architects and engineers can create designs that are both functional and cost-effective. Effective communication with local building officials and fabricators is crucial to ensure that the design loads and foundation requirements are accurately addressed.
Ready to optimize your next project with site-adapted prototypes? Contact our team of experts today to ensure your design meets all site-specific requirements efficiently and effectively. Let’s bring your vision to life!
Steve Petracek, Principal
