While CLT has been used in Europe for years, it is still relatively new to North America. Fortunately, U.S. building codes are flexible enough to accommodate new materials. Promega’s design team for The Crossroads earned local building department approval by using ANSI/APA PRG 320-2011 standard. The design team discussed the standard with building officials early in the process, and submitted engineering information under the ‘alternate designs’ section of the IBC.

“The building was designed per the 2009 IBC with State of Wisconsin amendments,” says structural engineer Aitor Sanchez-Prado. “The basic criteria for the project consisted of minimum roof snow loads of 30 psf with a maximum snow drift load of 132 psf. The structure was designed for basic wind speeds of 90 mph and seismic design category B. These loads, together with diaphragm chord and tie forces, set the design criteria that the custom-made CLT panels were required to meet.”

In the Earth Sciences Building, under the prescribed fire safety requirements for the British Columbia Building Code (BCBC), having the academic wing built entirely of heavy timber or wood-frame construction was not permitted. In order to allow for the use of wood as the main structural element, a fire hazard assessment was carried out and documented in a Building Code Alternative Solution, which was peer-reviewed as part of the building permit approval process.

The assessment essentially looked at how the wood construction might perform in fire. The Earth Sciences Building was designed with an automatic sprinkler system, and a fire retardant coating was applied where necessary to alter the surface burning characteristics of the wood interior finishes to more closely meet the standard required in a non- combustible building. Further, taking advantage of the highly compartmentalized nature of the wing, a fire risk analysis was presented to show that the risk of fire growth and spread would inherently be limited by the many separate, individual offices and rooms. To address concerns, the wood structure was engineered to account for the charring of the wood members for the required fire-resistance period so that the resistance provided by the reduced cross section (unburned portion) of the wood members would continue to resist the gravity loads expected during the fire.

A key component in the fire safety design of the heavy timber element, which is traditionally not explicitly addressed by the building code, was the fire performance of the steel connections. The minimum fire resistive properties of steel connectors were considered critical since the loss of the load-bearing capacity of the structure at the early stage of a fire would be significant if the steel connectors failed. To ensure the connectors would continue to transfer loads in the heavy timber wood-frame system during a fully-developed fire, concealed connections were specified. Where there were other exposed steel elements that were part of the wood structure’s load path, appropriate ‘passive’ fire protection, in the form of an intumescent coating, was also specified.