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Engineering the Conversation Plinth

The hardwood CLT structure is the first in the United States

By Rick Reynolds

Conversation Plinth Bensonwood IKD collaborationThe first hardwood cross-laminated timber project in the United States, dubbed “Conversation Plinth,”  and five other installations will form the centerpiece at “Exhibit Columbus,” an annual celebration of architecture, art, design and community in Columbus, IN. This year’s Exhibit Columbus will run from August 26 through November 26, 2017.

A unique installation, “Conversation Plinth”  is a collaboration between IKD, a Boston-based architectural design firm, Bensonwood, a structural wood engineering expert, SMARTLAM, the first CLT manufacturer in the US, and Clemson University’s Wood Utilization and Design Institute.

One of five Exhibit Columbus winners of the J. Irwin and Xenia S. Miller prize, Conversation Plinth is designed to generate conversation around the use of hardwood CLTs in US construction. While softwood CLTs have been used in North America in the construction of mid-to-high-rise buildings, the hardwood variety has only been available in Europe.

Since the largest cash crop in Indiana is hardwood from local forests, the goal of the CLT project is to encourage the development of low-value hardwoods into a high-value construction material. With only 45% of each log considered of high value to builders and consumers, the remaining 55% of low value hardwood, such as grade three common oak, maple, hickory and ash—some from beetle kill, standing-dead trees—can be used to create a new, high value CLT product for commercial projects.

Like softwood CLTs, hardwood CLTs are quick to build with since they can be CNC milled beforehand, off-site, and efficiently shipped to the build site where they are then rapidly raised into place. Moreover, unlike concrete slabs that need time to cure, hardwood CLT slabs are ready to go the moment they arrive at the site.

Since hardwood CLTs are so new to the US, and their use as slabs, unique, the Bensonwood engineers needed to understand and leverage their benefits, while tackling the challenges of the design.

As for their benefits, the hardwood in hardwood CLTs has a denser, smaller diameter fiber than softwood, and is more resistant to failure due to “rolling shear,” or the shear strains in a plane perpendicular to the grain, as well as to “punching shear,” a type of failure in slabs subjected to high localized forces at column support points. The 80,000 lb. Conversation Plinth installation is supported by 30 steel posts. A “plinth,” for construction nerds and crossword puzzlers, is a term for the lowest member of a base, or subbase, supporting a column, post, or pedestal.

CLTs are unique in the way they bend. The material is “anisotropic,” meaning its properties vary with different orientations, unlike steel, which is “isotropic,” its properties being the same in all directions.

Since the hardwood CLTs in the Conversation Plinth project are used as horizontal slabs, perpendicular to the flow of gravity, two-way bending had to be taken into consideration. In timber beams, one-way bending can be visualized as a smile. In two-way bending, CLT slabs deform like a trampoline with a bowling ball on it.

To engineer the two-way bending of the slabs, special software (RFEM, made by the German-based company, Dlubal, with a module plugin for laminated surfaces) was used by Chris Carbone and the engineering team, to ensure the structural integrity of the installation. In particular, these engineering calculations indicated a modification of the design to avoid over-stressing the CLT floor slab..

Aside from the significance of the first US hardwood CLT structure at Exhibit Columbus, the city has long been a Midwestern Mecca for progressive architectural design. Over 60 public buildings in Columbus, IN—a town of just 44,000 people—have been designed by some of the finest modernist architects of our times: I.M. Pei, Cesar Pelli, Richard Meier, Harry Weese, Robert Venturi, James Polshek, Eliel Saarinen, and son, Eero Saarinen.

Eliel Saarinen built the first modern building in Columbus, IN, in 1942, which marked the beginning of the city’s design-centered movement. His “starchitect” son, Eero Saarinen, later designed a stunning, light-filled, white marble and glass house in Columbus in 1957, and then went on to design such architectural icons as the Gateway Arch in St. Louis, the TWA Flight Center at Kennedy Airport in New York City and the main terminal at Dulles International Airport, to mention a few.

With the potential for hardwood CLTs on display in Columbus, IN, the conversation has started and the sky’s the limit.

For those who would like to experience the 18 installations of Exhibition Columbus, Opening Weekend kicks off with a preview party, Friday, August 25 (buy tickets), and is open to the public, free of charge, from August 26 through November 26. 2017.

Project: Conversation in Plinth: Award Winning Project opens August 26, 2017

Architect: Yugon Kim/ IKD, a Boston-based architectural design firm

Engineer: Chris Carbone and the Bensonwood engineering team

Structural Engineer for Steel: Elizabeth A. Woolf, PE, SE; Katahdin Engineering, LLC

CLT Manufacturer: SMARTLAM

Project Partners: National Hardwood Association, the Indiana Hardwood Lumberman’s Association, the Indiana Department of Natural Resources, the Hoosier National Forest Office, Indiana Society of American Foresters. Supported in part by the United States Forest Service 2017 Wood Innovation Grant.

Material: Indiana Hardwood CLT: Mostly oak, with maple, hickory, and ash/beetle kill (Emerald Ash borer)

All photos and renderings courtesy of Yugon Kim