Apex Plaza – Mass Timber High-Rise

Mass timber construction has gained momentum throughout North America over the past ten years. For example, Apex Plaza is an approximately 300,000 square foot, mixed-use office/retail/ residential high-rise building in Charlottesville, Virginia, with six stories of mass timber framing over a four-story concrete podium parking structure. At the time of its completion in April 2022, Apex Plaza became the largest mass timber-framed building on the east coast. 

Recent development in Charlottesville has focused on congested sites near the University of Virginia and the city’s downtown. The site for Apex Plaza has all the challenges associated with urban development, such as property line and existing building constraints on all sides, a high water table, and a limited laydown area for construction.  

Apex Clean Energy is the building’s primary tenant, which led the project team to establish sustainability as a guiding principle. William McDonough + Partners (WM+P), the project architect, engaged Simpson Gumpertz & Heger (SGH) as the structural engineer. The project team incorporated features to help to satisfy the sustainability goals, including:

  • Mass timber framing using Forest Stewardship Council (FSC) and cradle-to-cradle certified sources for the upper levels.
  • Steel-framed canopies support photovoltaic (PV) arrays at the Level 3 terrace and the roof.
  • Vegetative roof assemblies at the Level 3 terrace.

A Code Modification

Preliminary design began in early 2018 under the 2012 Virginia Construction Code (VCC). Unfortunately, the scale of the proposed construction (Type IV timber frame over Type IA podium; Figure 1) did not conform to the VCC’s prescriptive construction requirements, prompting a code modification under VCC Section 106.3. The building official agreed to the code modification, but with stipulations impacting the structural design, including:

  • Three-hour fire separation between the podium and timber structure was achieved by an increased cover of reinforcement on the bottom mat. 
  • Vertical transportation cores enclosed by reinforced concrete walls designed as shear walls. 
  • Timber design per the 2015 National Design Specification® for Wood Construction (NDS®). 
  • Timber elements sized to comply with the International Building Code (IBC) Table 602.4 – Wood Member Size Equivalencies and designed for a one-hour fire-resistance rating.
Figure 1. Timber construction over concrete podium

Podium Structural Design

The podium structure is used for parking, retail, and limited office space. Foundations are primarily shallow spread and continuous strip footings bearing on native soils with combined footings at the perimeter to accommodate the eccentricity created by the foundation at the property line. Rammed aggregate pier foundation improvement was implemented in one area with unsuitable fill soil. The transportation cores share a thick mat foundation for shear wall stability.

The office floor timber grid was not aligned with the parking layout throughout the footprint, so post-tensioned and conventionally reinforced concrete girders at the top level of the podium transfer loads from ten timber columns to the podium columns. The podium structure is primarily comprised of conventionally reinforced cast-in-place concrete slabs, with drop panels in select locations. 

Timber Frame Design

The upper levels of the building are occupied with office spaces. Floors 4 through 8 and the roof are framed primarily with multi-span cross-laminated timber (CLT) slabs and glued laminated timber (glulam) purlins, girders, and columns. Seven-ply CLT slabs span about 20.75 feet, and glulam beams span up to 27 feet. Floors are topped with gypcrete over an acoustical mat to control sound transmission through the levels as required by the VCC. Floor vibration due to walking was evaluated according to the empirical design approach in the 2nd Edition of the Canadian CLT Handbook.

The timber fabricator and supplier, Nordic Structures (Nordic), was engaged early in the design phase of the building. Nordic’s engineering team served in a design-assist role and became a valued contributor to the design. Their suggestions enhanced fabrication efficiency, economy, and constructability.

Most of the timber framing is installed as slabs and beams, allowing long unobstructed views of the exposed CLT from inside and outside of the building. To minimize beam depths, the beams are paired side-by-side in depths up to 27 inches and widths up to about 25 inches per pair. 

Cutting penetrations through glulam beams would have significantly reduced the fire-rating for the upper framed levels, which could have threatened the project’s viability. Instead, the framing is installed in purlin and girder configuration near the building core, with the girders dropped in elevation to allow for M/E/P distribution into the slab and beam framed areas. 

The mass timber connection design was delegated to Nordic by performance specification. The beam-to-column connections were designed as bearing-type connections and consisted of notched columns and beams (Figure 2) that fit together without relying on mechanical fasteners to transfer load. This provided economy for the connection design, accommodated cantilevers, and simplified fit-up for columns narrower than the beams. 

Figure 2. Beam-column bearing connection at braced frame.

Compared to conventional dimension lumber construction, wood shrinkage is more easily managed in post-and-beam mass timber systems since vertical loads from above can be transferred parallel to grain. Wood shrinkage is largest tangentially and radially to the grain and is significantly less parallel to grain. The connections between columns at Apex Plaza allowed the columns to stack on top of each other, transferring all vertical load directly in the columns, making shrinkage less of a concern. 

Lateral-Force-Resisting System

The building’s lateral-force-resisting system primarily comprises reinforced concrete shear walls at the vertical transportation cores extending from the foundation up to the penthouse roof.

At the upper, timber-framed levels, timber braced frames were introduced on the east and south sides of the building to provide additional lateral strength and stiffness at the ends of the building away from the core. Timber braced frames are not a prescriptive seismic force-resisting system included in ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. From a literature search and review of provisions in the Canadian codes and standards, a design approach utilizing a seismic response modification coefficient of R = 3 for the braces and concrete shear walls was implemented. Timber brace connections were designed utilizing an overstrength factor of 2. Dowel-type connection strength was designed for ductility utilizing yield modes III and IV per the NDS. Additional discussion about the brace design can be found at: www.youtube.com/watch?v=E4Or7m1N0AU.  

The braced frames are exposed in most locations at the upper levels, creating a unique architectural feature but a connection challenge. The brace connections at columns and beams reduce their cross-section and the members’ fire resistance. This could be addressed by increasing the size of the members, but, in the case of Apex Plaza, this would result in inordinately large members. Instead, connections were wrapped with gypsum to provide passive fire protection (Figure 3).

Figure 3. Timber braced frame with gypsum wrapped connection.

The design and construction teams recognized the potential for conflicting construction tolerances between concrete and mass timber. Mass timber is fabricated to tolerances much less forgiving than tolerances for reinforced concrete construction. Alteration of the timber framing in the field can be difficult and hinders erection progress.   As the concrete cores were erected well in advance of the timber, there was sufficient time to coordinate timber fabrication with the as-built geometry of the cores.

Building Construction

The delivery date for the project was fixed by the needs of Apex Clean Energy which was consolidating its headquarters’ operations from several sites to Apex Plaza. Additionally, Nordic had a dedicated window to fabricate the timber and deliver it to the site. However, the structural design considered several iterations, including a change from five to six stories of timber and various transportation core configurations during the preliminary phases. These iterations put the project at risk of missing the fabrication window. Delaying fabrication would have meant the loss of several months in the schedule and missing the project deadline. Therefore, the team looked at options to expedite construction to allow exterior wall and interior construction to proceed to complete the project on time.

The construction team evaluated options and proposed erecting the building using vertical phasing. Rather than constructing the entire building conventionally floor-by-floor as planned, the project was separated into multiple adjacent, full-height phases. Phase 1 involved constructing the concrete podium full-height on the east side of the building. Once the podium was topped out at this location, the timber installation would begin over the completed portion of the building, while podium construction began on the west side. This sequence continued on the west and south sides of the building until construction was complete. Allowing the concrete and timber construction to coincide effectively extended the timber fabrication schedule. 

Overall, this approach required detailed coordination between the design and the construction teams. Despite numerous logistical challenges, the project was completed on time for occupancy, with the Grand Opening of the building in April 2022 (Figure 4). 

Figure 4. Completed construction at the grand opening ceremony.

Lessons Learned

Through the design and construction of this project, a few key lessons were learned and reinforced:

  • Close coordination between the A/E/C team is essential to success.
  • Early engagement of the timber fabricator is crucial to secure the fabrication window to meet the project construction schedule.
  • Integration of the timber fabricator into the design process offers tremendous opportunities to streamline the project’s design, fabrication, and erection.
  • Interface tolerances between different materials must be considered in the design and specifications.
  • Multi-span continuous CLT slabs perform substantially more effectively than simple spans for strength and serviceability; however, continuity induces loads on interior supports up to 25% larger than simple tributary loading.

Looking Forward

Apex Plaza is an example of the emerging trend in large timber buildings throughout North America. With many jurisdictions soon to adopt the new tall mass timber provisions in the 2021 IBC, many more exemplar buildings are expected and can benefit from and build on the lessons learned at Apex Plaza.■ 

Project Team

Development Team: Hourigan Group & Riverbend Development
Strategic Partners: Apex Clean Energy & acac Fitness & Wellness Centers
Structural Engineer: Simpson Gumpertz & Heger
Architect: William McDonough + Partners 
MEP Engineer: Staengl Engineering
Timber Supplier: Nordic Structures
General Contractor: Hourigan Construction Corp.

About the author  ⁄ Eric R. Ober, P.E., S.E.

Eric R. Ober, P.E., S.E., is a Senior Project Manager at Simpson Gumpertz & Heger’s Washington DC office. Eric can be reached at erober@sgh.com.

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