Overall Strategies and Features of the Structural System
The overall strategy for this structural system was based on an original design goal that I set — that being, a wood-framed structure for sustainability. The general idea of the structural system includes wooden beams and joists, connecting to large wooden-columns, which end at a spread footing. Metal beams were used to support the balconies, as I did not like the aesthetic of having large wooden beams hanging underneath. Overall, I made a fairly symmetrical and simple structural system to allow for easy constructibility.
Material and Framing System Selected
- Joists: 4”x12” LVL (Laminated-Veneer-Lumber) @ 2’ o/c spacing
- This was used to create the beam-system. Considering the relatively large span, large LVL joists were selected. The joists were off-set -5” to account for the concrete floor with metal deck placed above.
- Beams: 18”x24” Mass Timber Beams
- These beams were selected to create the support beams. The joists were tied in directly. While smaller beams could have been used (if metal or concrete were chosen), I wanted to stay true to my original design goals. In addition, I like the aesthetic of having the large exposed wooden beams throughout the building. For this reason, the mass timber beams were not embedded in the walls and were free-standing. The beams were off-set -5” to account for the concrete floor with metal deck placed above.
- Columns: 20”x20” Mass Timber Columns
- Similar to the beams, these were selected to support the beams and deliver the load to the spread footings. Once again, these are exposed as a part of the aesthetic of the design.
- Footings: 8’x6’x1.5’ Rectangular Concrete Spread Footings
- Standard footings were used below the ground level of every column.
- Balcony Beams: W14x61 Steel Beams
- As discussed above, the balcony beams used a long-span metal cantilevered frame. This was chosen because in order to use wood, very large beams would be used and I did not like the look of them from the side profile. In addition, the steel beams may be better when exposed to the elements.
- Cross-Bracing: 18”x24” Mass Timber Beams
- Serving both as a functional and aesthetic feature, these cross braces used large cross-section mass timber members.
Locations of Structural Columns and Major Framing Elements
The structural framing was chosen so that it interfered as little as possible with the interior layout and function of the building. In terms of spacing, a maximum span length of 30’ was selected, although most spacing was 25’. As seen by the second floor layout, most of the columns/gridlines were placed between rooms with the exception of one column in the middle of the exhibition center. In addition, the 2-floor atrium in the lobby remained intact by using two story columns. In order to accommodate the elevator and stairs, intermediate beams and joist-cutouts were used.
Special Structural Challenges
Some special considerations that had to be made for this design included supporting the cantilevered balconies, framing the atrium space, and using large cross-bracing without interfering with the design of the building too largely. Altogether, these elements came together nicely and helped fulfill some of the design goals. It is likely that the “green roof” would likely require further work, and perhaps some additional lateral bracing would be required.