Densifying Mixed-Use

Brett Goodman

As Salt Lake grows, many of us are working with owners that wish to get more out of their property.  Usually this means designing vertically and densifying the available area.  Parking is pushed under the building to free up more space, and it’s common to enter schematic design with multiple options that vary the number of floors.  Countless designs include building four or five stories of wood (Type III or V) on a concrete podium over one story of non-combustible construction (Type 1).  Many do not realize there are other design solutions to meet the owner’s needs.

Changing The Standard

A new option comes from a change in the 2015 International Building Code (IBC).  Podium construction is built around provisions in the IBC, allowing a change to the building construction type at a horizontal separation.  Requirements can be found in section 510.2. The building may be any type above the separation; however, below the separation, Type I construction is required.  

The 2012 IBC stated, "The building below the horizontal assembly is not greater than one story above grade plane.“ This has been struck from the 2015 code. The elimination of this sentence in the current code opens up the opportunity for densification, making it possible to build seven to eight stories above grade. As a structural engineer, we work with the architect and a cost estimator or contractor to find the size that meets the owner’s pro forma.  

There are several possibilities the team can review. Going from four to five stories of wood means Type III construction.  In Type III construction, wood is still used but the wood in rated shafts and exterior walls need to be fire treated.  This comes with additional cost.  We can now add an extra one to two stories of Type I construction below the horizontal separation. This may eliminate the need for Type III construction. BHB collaborated with a project team recently on an eight stories above grade mixed-use building (five stories of wood and three stories of Type 1).  This outcome was made possible by the 2015 code update.

Load-Bearing Metal Stud

Another option that is often overlooked in our market is using load-bearing metal studs as the structural system.  This method is commonly used in Denver and many areas across the nation.  There is no need for a horizontal separation, as the full building is non-combustible.  Load-bearing metal stud systems are successfully utilized on many project types, specifically assisted living centers.  

It is important to keep the locale’s seismic requirements in mind when reviewing various manufacturer load-bearing metal stud structural system. For example, there are systems manufactured back east that use tie rods as the lateral system.  This does not work well with the Wasatch Front's high seismic requirements.   The best option for shorter buildings is a steel plate shear wall system.  A 22 gauge 4 x 8 sheet of light gauge steel is screwed to the side of studs, similar to a wood shear wall system.  As these building get taller, we start to use CMU or concrete shear walls.  CMU walls are often located at the shaft and other locations that minimize floor plan impacts. We find it effective to frame the floors with concrete over long span metal deck spanning between metal stud bearing walls.  This makes the structural floor assembly 6-¼ to 7-¼ deep.  Concrete over long span deck minimizes the floor-to-floor height, minimizes skin cost and speeds up construction, which saves the owner money.

Load-bearing metal stud are a good option for mixed use.  Going load-bearing eliminates secondary steel beams and columns.  The noncombustible nature of metal stud means a potentially safer building in a fire.  This also open up the type of tenant to include assisted living and other higher-end tenant types.  

Post Tension Floor Systems
Utah is seeing a revival of post tension (PT) concrete floor systems. For tall buildings, there are few systems that can beat out concrete PT.  PT is often used in residential towers, such as 99 West in City Creek.   It is used in office buildings throughout the valley and is cost-effective due to the shallow structural depth, saving skin costs.   This system is used on the lower parking decks and podium in classic mixed-use projects.  Consider using a full concrete PT structural system when your building is over eight stories.

Mass Timber

A future cutting-edge option for taller mixed-use projects is mass timber construction.  There are a number of tall structures being proposed with this type of construction, such as a nine story building in London that was built in 17 weeks. This should not be confused with Type IV timber construction.   This is a new system using extremely thick cross-laminated timber.   At this point these buildings are being proposed as performance-based buildings, meaning they do not have to meet the IBC requirements for fire.  The new NDS wood design references have design guidelines for this cross-laminated timber. We will see this type of building construction become more wide-spread in the near future, and it’s a solid option for mixed-use project construction.

Mass timber is seen as a more sustainable way of building buildings.   The large amount of wood required create a lighter carbon footprint.  The panels have high shear values for lateral design   Many our clients have heard about this option in TED talks and articles and are interested in incorporating this into mixed-use projects.  

Mixing it Up in Mixed-Use Projects

There are many good options to densify their mixed use projects, and there’s no reason to limit the number of stories below the horizontal separation, if the project team is open to using different systems.   Load bearing metal stud is commonly used in our area and is a viable option in midrise mixed use.   The flexibility and low cost makes post tension concrete hard to beat for taller structures.  Mass timber may be the wave of the future.

As each owner’s needs vary it is important to keep all options in our tool belt.  A good structural engineer and contractor or cost estimator can help the architect to work through options in schematics. The goal is to match building sizes and materials to densify the property and find the best fit for the owner.