Steel-to-timber connection, United States
A new residence located in Martha’s Vineyard, MA, USA. The site is within a high wind region and the building sits on a steep bluff overlooking the ocean, placing significant challenges on the design of the lateral load resisting system. These criteria combined with an open architectural design and requirement for a thermally broken, unpropped balcony led to the framing for the ocean-side elevation being proposed as a combined moment frame and cantilevered system. Though originally conceived in steel, the client and architect’s aspiration was for a predominantly wood solution.
The two-story moment frame consists of two spans of 21 ft and 14 ft, with an average floor height of 10 ft. Deep, multi-LVL timber girders intersect with orthogonal flitch beams forming the cantilevers and backspans to the balcony. The resulting steel node joint is designed to resist forces and moments in the two orthogonal axes. A central hollow section hub was proposed but was found to be unable to resist the high moments from the moment frame girder due to insufficient out-of plane stiffness in the box section walls as well as being prone to flexural failure. The steel joint was redesigned with a wide flange section to maintain continuity in both axes whilst ensuring a direct transmission of forces and moments to the center of the hub. Because the balcony members require steel to run continuously through the thermal line of the building, a thermal break pad is included to reduce heat loss and condensation risk. We were able to create a customized material for the thermal break pad and include it in the analysis within IDEA StatiCa.
Framing members were analyzed and designed using Dlubal RFEM. For the design of the intricate connections in multiple materials, IDEA StatiCa was an ideal supplementary tool. The software has the flexibility to create custom materials and elements with complex geometries and templates for wood-to-steel connections that greatly reduced the time required to set up the model. Although code checks for wood members is not supported by IDea StatiCa, the bolt force outputs were easily transferrable to in-house spreadsheets for verification.
Through our use of IDEA StatiCa and close collaboration with the architect, thermal consultant and fabricator, we were able to evolve a full steel frame into a hybrid wood solution, thus reducing the quantity of steel required and keeping the structure of the building more in line with traditional construction for the locality.