Flat roofs are all alike; every sloped roof is sloped in its own way. With apologies to Tolstoy for butchering his masterpiece, the turn of phrase speaks to the uniqueness of the roof at Northeastern University’s new EXP building, a 350,000-square-foot state-of-the-art research center. Any structural engineer can design a flat roof; most have designed a pitched roof; some have even designed a twisting roof. But how often are engineers confronted with all three, not to mention all three at once? To engineer this roof and achieve curves in all three primary directions, careful planning and manipulation of the structural steel system was necessary.
Overview of Northeastern EXP Image Credit: Payette
First, the pitch – There is a roof terrace at Level 8 of EXP, which is adjacent to a dining space, multi-function rooms, and the University president’s suite of offices. The lowest portion of the sloped roof is over the multi-function rooms and president’s suite, which provides soaring views of Northeastern’s campus and the cities of Boston and Cambridge. As the roof continues to climb, Level 8 moves to the more purely functional – housing the mechanical systems integral to a functioning laboratory building. Resiliency practices in coastal cities such as Boston recommend placing building operations high above flood levels, which was a boon for this dense urban site with no space to store bulky systems at grade. Steadily, the roof climbs higher still, and underneath are structural floors (Levels 9 and 9-Mezzanine, respectively) that provide further support systems without the demands of multi-story high volumes.
The support and compatibility of façade elements complicated the transition from comparatively low to high roof spaces. At the roof’s lowest portions, the façade elements only need to be about 20 feet tall. In transition regions where the roof pitches upward from Level 8, façade elements hit a maximum height of 40 feet before an interstitial floor intrudes to provide additional support, resetting the height to 20 feet. Rigorous deflection control of the supporting structure for gravity and lateral loads was required to ensure that these 40-foot-high façade panels perform compatibly with adjacent 20-foot-high panels.
EXP’s roof encloses some very large volumes.
Next, the roll – The roof is sloped primarily in pitch, but there is a significant roll to help direct water to discrete drain locations during rain events. Maintaining a simple pitch to the roof would result in water flowing down and catching the inside edge of the low parapets, eventually pushing them outwards.
Finally, the yaw – Encompassing the west, north, and east sides of EXP, the roof figuratively draws its arm around the building, focusing attention on the south-facing roof terrace. Beyond creating sculpted massing and an outdoor space for building tenants, enclosing only a portion of the total volume of the building footprint to its highest point reduces the demand for conditioned space and results in a more functional and efficient building.
When confronted with these interacting geometries, early design decisions informed document preparation, gravity and lateral detailing, constructability, and ultimately cost. The primary decisions were how to arrange the roof framing in plan and how to orient the individual beams rotationally. The pitch and roll were contemplated primarily as a contour map, with girders perpendicular to the pitch (parallel to the roll) and purlins parallel to the pitch (perpendicular to the roll). Another key decision was to orient the webs of all beams vertically (instead of rotating to match the slopes), which challenged roof deck securement but prevented rotational deformation from shear flow under loading. To allow for deck bearing and securement, a representative detail, shown below, allowed for the trimming of miscellaneous steel pieces to augment the unique geometry of each beam between its top flange and the underside of the steel deck. In addition to merely supporting the steel deck for gravity loading, the continuity of the diaphragm created by the contiguous roof deck created a load path from lateral loads to be collected by the diaphragm and directed to the lateral force-resisting system.
Roof framing and roof deck detail to accommodate continuously variable slopes
Communicating sufficient information to be able to frame the roof required elevation information for each end of each piece of steel along with the plan rotation of each beam so that where the beams connect to girders could be precisely defined. The traditional “equal spacing” notes would have been woefully ineffective on this roof.
Sample part roof plan with dimension and angle references for each piece of steel
All these structural design elements came together to create a cohesive roof surface that provides support for vertical loads on the roof (snow, rain, wind), lateral loads at the roof plane (seismic, wind, and components of the snow load), gravity support of the surrounding façade panels and deflection compatibility for lateral movement of the façade elements. Making decisions early in the design about how this sloped roof differs from all other sloped roofs saved unnecessary complications, costs, and constructability challenges as the project came to life.