Concrete versus Steel for Proposed Parking Garage in Anchorage, Alaska

For the past few months, I have been working with Reid Middleton’s Alaska office on the design for a new, seven-story parking garage in Anchorage. The South Central Foundation Parking Garage will be located in Anchorage Alaska, near the center of Alaska’s Native Medical Center campus. Reid Middleton’s client for this project is the design-build contractor, Neeser Constrction, Inc.

I have a lot of previous experience with underground parking structure design, usually consisting of the lower levels of a mixed-use retail and residential building. Working on this project, I have learned a lot about the construction industry in Alaska, and a lot of new things regarding parking structure design.

For any building structure, during the early stages of the project it is important to discuss possible framing options. Framing types are often determined based the building size and function. Large, multi-story office structures are predominantly steel and concrete. Residential buildings are often wood or metal stud framed. In Alaska, most of the large multi-story buildings are framed from steel. Sometimes, local availability of materials drives the framing decision. For instance, concrete happens to be relatively expensive in Alaska, and this was a key factor in our early design decisions.

Before any of the columns or foundations were laid out, the following pros and cons were considered for the proposed SCF Parking Garage:

Advantages of Steel Framing

A steel framing system is typically much lighter than concrete. This can reduce material costs for the foundations and seismic bracing. In our case, Buckling Restrained Braces (BRB’s) were studied for seismic bracing. When compared with traditional braced frames and shear walls, BRB’s significantly reduce seismic demands.

In Anchorage, steel is more economical when it comes to erection costs. According to RSMeans Building Construction Cost Data 2016, installation of structural concrete is about 17% higher than the national average, while steel installation cost is just 5% above the national average.

Steel may be a more flexible option in Anchorage where the construction season is relatively short, and the low temperature is likely to drop below freezing for 8 months of the year. Special procedures are required to place concrete in near-freezing conditions.

Advantages of Concrete Framing

Sprinkler systems and sprayed fire-proof coatings can quickly drive up costs in a steel parking garage. Concrete of the appropriate minimum thickness does not require any fire-proofing, or sprinkler systems.

Parking garages are often open or partially open structures exposed to the rain, snow, ice, and de-icing chemicals. Concrete mixes can be carefully proportioned to resist corrosion from the elements, while steel structures require special coatings and continued maintenance to keep the steel from corroding. Encapsulated systems are used to protect tendons and anchorage in post-tensioned slabs and beams that could be exposed to weather.

The repetitive floor framing characteristic of parking garages is ideal for concrete framing since the form work and shoring can be re-used for each level.

Conclusion and Current Status

After partially developing a structural steel option for the parking garage, our client ultimately decided to make the switch to concrete framing. Cost estimates indicated that the steel framed garage was nearly $1 million over the original construction budget of about $15.5 million. Reid Middleton has recently completed the foundation design for the concrete framing option, and is currently working on the design of the upper levels. Site work and excavation is under way.

The Alaska Native Medical Center is operated by the South Central Foundation (SCF) under direction of the Alaska Native Tribal Health Consortium (ANTHC). The ANTHC operates multiple organizations throughout Alaska, and SCF is the largest of these organizations.