For years, the IBC has allowed light wood-frame buildings up to five stories for residential occupancies (and six stories for office use) over a single-level podium. The 2015 IBC evolved to recognize multi-level podiums, which had been permitted in the Seattle Building Code for some time. Across the country, designers began maximizing the value of their midrise projects with 5-over-2 and 5-over-3 configurations; however, for one Sacramento project called 1430 Q, 5-over-2 still wasn’t sufficient to make the investment in pencil out. Sacramento’s competitive building market required that 1430 Q have at least six floors of wood-frame residential units to make the project profitable, in a 6-over-2 configuration. By using the City’s Alternate Means and Materials Request (AMMR) process, the design team was able to successfully achieve the extra height and, in so doing, build the country’s tallest light wood-frame building. Several additional projects in Sacramento, which are currently under construction, have been permitted and are using a similar 6-over-2 approach.

In Washington, six stories of Type IIIA construction are permitted when certain criteria are met. Two example projects which are leveraging these code allowances are called Analog and Moraine, both of which are located in Tacoma.

Ascent - Mass Timber Reaches New Heights in Milwaukee

Photos courtesy of Korb & Associates, New Land Enterprises (left); New Land Enterprises, LCP 360 (right)

This Milwaukee-based mass timber residential tower reaches a record-setting 25-story height and exposes more timber through the use of fire testing while also realizing faster construction time with the extensive use of BIM and numerical coding.

Location: Milwaukee, Wisconsin
Size: 493,000 square feet
Architect: Korb + Associates Architects
Developer: New Land Enterprises LLP & Wiechmann Enterprises
Structural Engineer/Mass Timber Design Program: Thornton Tomasetti
Contractors: C.D. Smith Construction (lead builder) and Catalyst Construction

At 25 stories, Ascent is a record-setting height for hybrid wood construction and is considered the tallest timber tower in the world at the time of its completion. The 493,000-square-foot mixed-use building in Milwaukee’s East Town neighborhood includes 259 apartments, a wellness and fitness center, and a top-floor amenity level. Extensive use of BIM and numerical coding of every mass timber component helped to speed up construction. According to the project’s general contractor, a conventional concrete and steel structure of this kind would typically take eight days per floor to complete—but with mass timber, the project team completed a new floor every five to six days. Due to its expansive use of wood, Ascent represents a carbon benefit equal to removing 2,350 cars from the road for one year.³⁶

Nineteen residential stories are constructed from mass timber above a five-story concrete parking garage. A system of glulam beams and columns supports 5-ply CLT floors while two concrete cores provide lateral stability.

To reach its planned height and expose the mass timber structure allowed by code (50%), the City of Milwaukee required the project team to demonstrate that Ascent could meet the rigorous fire safety standards of a Type I building. Subsequent fire testing proved that the timber structural members not only met, but exceeded, fire rating requirements, clearing the pathway to approvals.³⁷

Incorporating Parking in Light Wood-Frame Mid-Rise

A common configuration in multistory, multifamily wood-frame construction is parking on the first level(s) with three to five stories of wood-frame residential on top. Many designers assume that the parking level is required to be noncombustible and utilize the podium provisions within IBC 510.2; however, there are several opportunities for alternate provisions and/or wood framing within the parking levels that can offer significant cost savings.

Section 406 provides several provisions specific to motor vehicle-related occupancies. Section 406.5 provides requirements for open parking garages. The natural ventilation requirements for a garage to qualify as open are discussed in Section 406.5.2 and generally require openings on at least 40% of the garage’s perimeter, with some exceptions specified. Section 406.5.1 permits open parking garages to be of Types I, II, or IV construction.

Enclosed parking garages are covered by Section 406.6 and require both mechanical ventilation and NFPA 13 sprinklers, per Section 903.2.10. There are no limitations on construction type for enclosed parking garages, indicating that parking garages of Types III, IV or V construction can be used and framed with wood, concrete, steel and all other materials as permitted for each of these types of construction per the IBC Section 602 definitions.

Occupancy separation is a cost-effective design option frequently overlooked when a parking garage is classified as enclosed. This design route utilizes a single construction type for the entire building (i.e., the Type I-A podium of Section 510.2 is not necessary). The parking garage can still be framed with concrete or steel, for example, but there is no code requirement to use Types I or II simply due to the presence of noncombustible materials.

In a building that has an NFPA 13 sprinkler system throughout, only a 1-hour rating is required when separating parking from occupancies such as Group B, M, and R, per Table 508.4. This indicates that a mixed-use building consisting of an enclosed parking area and other occupancies could be completely framed with wood if allowable building size calculations permit the use of Types III, IV, or V construction. Even if other materials are used in the building (perhaps as columns, beams, or walls on lower levels), there is economic value in using Section 602.1.1 to classify the entire building as the lowest construction type allowed.

Utilizing the options presented in Section 510.4 offers the ability to capitalize on an increased number of stories—similar to the horizontal separation provision of Section 510.2—by stacking a Group R building on top of a single-story Group S-2 parking garage, gaining an additional story.

The main benefit of utilizing Section 510.4 is that it provides the benefit of an additional story while not requiring the full podium provision limitations of Section 510.2 (i.e., the lowest level doesn’t have to be Type I-A with a 3-hour podium slab). Specifically, this provision allows the use of a heavy timber-framed parking level (if open) and only requires a 1-hour rating (if the parking level is Type IV) or 2-hour rating (if the parking level is Type I) to separate the parking from adjacent residential units above.

Incorporating Occupied Roof Decks in Light Wood-Frame Mid-Rise

ccupied roof decks are becoming more common in multifamily and commercial buildings as designers and owners seek to increase the marketability of their rental spaces with more amenities. In most instances, these roof decks are open-air, without roof coverings, and have partial-height guards or wall parapets around their perimeter. Designers often wonder if a roof deck of this kind needs to be included as a story when calculating the required construction type, building area, and number of stories. Language was added to the 2018 IBC stating that a roof occupied in full or in part with the same occupancy as the story immediately below does not need to be included in the building area regulated by Section 506. Further, if the building is fully equipped with an NFPA 13 or NFPA 13R sprinkler system and an occupant notification system in accordance with section 907.5, the roof occupancy is not restricted to the occupancy of the story below.

In most instances, an occupied roof deck with a roof covering or walls that are taller than 48 inches is considered a separate story and must be included in building area calculations. If considered a mezzanine or accessory occupancy (Sections 505.2 and 508.2), it may be possible to exclude the occupied roof deck (and its enclosing elements) as a separate story when determining the construction type.

Tall Timber Adds Distinction to Cleveland Locale

Photos courtesy of ImageFiction (left), courtesy Hartshorne Plunkard Architects/Harbor Bay Real Estate Advisors (right).

Just-in-time prefabricated tall wood construction speeds up assembly, while an alternate-means process boosts the amount of timber exposed in this 500,000+ square-foot Cleveland-based residential project.

Location: Cleveland, Ohio
Size: 512,000 square feet
Owner: Harbor Bay Real Estate Advisors
Architect: Hartshorne Plunkard Architecture (HPA)
Structural Engineer: Fast+Epp
Contractor: Panzica Construction
Completion: Spring 2022

When Chicago-based Harbor Bay Real Estate Advisors acquired a coveted corner lot adjacent to Cleveland, OH’s well-known West Side Market, they embraced the opportunity to honor the cultural prestige of their historic surroundings.

Partnering with design firm Hartshorne Plunkard Architecture (HPA), Harbor Bay’s 115-foot tall, 512,000-square-foot, nine-story mass timber complex provides 300 apartments—many with unobstructed lake and skyline views. The resulting mass timber structural system consists of 3,000 cubic meters of blond hem-fir and spruce trees. Eight of the nine stories, set atop a one-story concrete podium, use mass timber.

The design team considered traditional concrete and steel construction but then looked to mass timber as an opportunity to achieve project goals including aesthetics, speed of construction, and environmental performance. According to Harbor Bay, INTRO’s construction time has been about 25 percent faster than typical concrete or steel construction, which was achieved by streamlining the project’s workflow and carefully managing just-in-time delivery and trade coordination.

Nearly half of the building’s interior surfaces feature exposed structural wood, including precisely cut glulam beams and columns, as well as CLT floors and ceilings in the units’ bedrooms and living rooms. This approach was guided by an Alternatively Engineered Design report specifying how the tall wood building could safely include an exposed timber interior.