Figure 1: View of the new pavilion from the West.
The Massachusetts Department of Conservation and Recreation (DCR), formerly the Metropolitan District Commission (MDC), is charged with maintaining the Bernie King Pavilion. Built in the late nineteenth century, this open timber structure provided shade on sunny days and shelter from occasional storms to generations of beachgoers on Nantasket Beach in Hull, Massachusetts (Figure 2). The structure’s west wall formed a partial enclosure, with varying low and full-height in-fill panels. A low-slope roof rose from the west eave to a ridge line very close to the structure’s east edge. There, a steep pitch brought the roof line back down to the ocean-facing eave. The pavilion’s east side was fully open, with a view of the beach and the Atlantic Ocean beyond (Figure 1).
Figure 2: Hull, Massachusetts - Locus Map. (Arial Photo © MassGIS).
As the MDC architect and project manager examined the condition of the structure, he realized that the timbers and their connections had suffered irreparably from a century of exposure to New England weather. Repair would be impractical. The MDC made the difficult decision to raze the existing building and replace it with a historically faithful reproduction. What began as a structural evaluation of the existing timber structure became an assignment to design a structure and all of its connections to meet the demands of current building codes, and to satisfy the aesthetic objectives of a passionate historical society.
Description of the Structure
The structure is a timber frame, with heavy posts, beams, trusses, and knee-braces. It measures 180 feet long north to south, and 58 feet east to west. Four rows of posts align north-south. At the north end of the building, a concession stand fills three bays (Figure 1). To the south, a storage room occupies a single bay with a small element that extends beyond the column grid.
The Massachusetts State Building Code requires that this structure withstand wind loads from a maximum fastest-mile speed of 90 mph. With its open exposure and scale, the resulting forces on the structure are significant. For example, the net lateral force acting at the top of a typical east-west frame is 5.6 kips. Because of its gently sloping roof, the net uplift is 17.4 kips.
The building lies below the 100-year flood plain. Consequently, its design must also account for water that can rise 2.75 feet above the building’s concrete floor. The applicable FEMA flood map indicates a base flood elevation (BFE) 2.75 feet above the floor at the seaward side, but only 1.75 feet above the floor at the landward side. The structure is designed to accommodate breaking waves that could reach the higher depth, cross the floor, and strike the knee walls along the building’s west wall. The Massachusetts Code requires that these walls be open at their base to allow water to pass through and that they be designed to collapse or break away, thereby preventing wave loads from threatening the structure overall.
Hull lies within Massachusetts snow load Zone 2, with a required design snow load of 30 psf. Individual girders and trusses support significant snow loads, and the shallow knee braces that help support the trusses resist substantial forces (Figures 3, 6, and 7).
Figure 3: Column during construction. Conventional straps and hangers will be hidden by sheathing.
Preliminary analysis revealed that wind loads overstress the timber posts in bending. Post sizes would have to increase to meet current-code wind load demands, but this meant departing from the strict requirement for historical reproduction. The client and the historical society agreed that larger timbers would have only a minor aesthetic impact. With this substitution, updated loading requirements had forced the first design change. More would follow.
The original structure included simple bolted connections, many of which had failed, and none of which met current load requirements. Furthermore, with snow and wind loads dictated by Code, demands on the knee braces are significant both in compression (snow and lateral forces) and under wind loads (lateral forces and uplift). With bolts installed near the end grain of each brace, the original knee braces were likely capable of resisting loads only when they were in compression. As the knee braces must also resist tension forces, their connections to girders and columns had to be designed accordingly.
Under both snow load and wind-induced uplift, the girders themselves were likewise inadequate on their own to resist induced bending within allowable stresses. The shallow knee braces could help resist uplift forces, but their shallow angle magnified the load in each. With uplift loads translating into significant tension forces, knee braces and truss diagonals now required substantial connectors. The design changed again (see Figure 8). New, steel-reinforced connections transfer tension forces and also reduce local crushing when the connections are loaded in compression.
Aside from the substantial forces that each connection must transfer, the design objective of the historical society demanded that the connections be as close as practical to their original appearance. Originally, bolt heads and washers were visible, but all connections had been made without the benefit of supplemental hardware. New connections required steel straps, ties, and braces.
The client approved a system of connectors made of ¼-inch thick steel plates that were shop fabricated and galvanized. Bolts and shear plate connectors would "hide" between the timbers, with only their ¼-inch thickness visible in many places (Figure 5). "Off-the-shelf" connectors supplemented the custom hardware. Commercial connectors were especially useful where wall sheathing eventually covered them (Figure 3) and where they were hidden behind decorative brackets (Figure 9).
Figure 4: A shear plate is imperfectly hidden by the knee-brace connector.
Figure 5: Through ties employ off-the-shelf connectors "hidden" from view.
Design of wood structures is guided by the National Design Specification® for Wood Construction (NDS®), published by the American Forest & Paper Association. The NDS includes allowable values for shear plate connectors, and these formed the basis for connection designs for this project. By employing shear plates, connections are capable of resisting several kips without significantly altering the frame’s appearance. Most of what remains visible are the thin edges of the steel connectors, nuts and bolts, and new knee-brace connectors that proved impossible to hide (Figures 4 and 7).
Figure 6: Framing in progress. Concrete walls in the background form the base of the concession stand.
Reproducing the pavilion, including its original geometry and details, was not possible. However, the final structure reflects a compromise between the competing demands of current building codes and a watchful historical society. The new pavilion represents everyone’s best effort to create a substantial and climate-worthy replacement for a classic structure. Exposed bolts, visible narrow edges of steel connectors, and exposed timber frame all recreate the rustic informality of the original structure. This was accomplished by using off-the-shelf connectors wherever possible, and by combining designs for new connectors with the load transfer capabilities of shear plates. Driven by the desire to match the geometry of a nineteenth-century structure, the shear plates represent twentieth-century technology combined with historic timber engineering to meet twenty-first-century design loads.▪
Figure 7: Mid-building posts support trusses to the east, west, and above.
Figure 8: Structural design of frame to resist code-required lateral loads.
Figure 9: Brackets cover steel tie-down connectors at the top of posts.
This article was originally published in Wood Design Focus, December 2006 and has been reprinted here by permission of the author.