Simpson Gumpertz & Heger Inc. (SGH) is a privately held ENR 500 engineering firm that designs, investigates, and rehabilitates structures and building enclosures. Their work encompasses commercial, institutional and residential buildings, transportation, water/wastewater, nuclear, science, and defense structure projects throughout the U.S. and over twenty foreign countries. SGH has 500 employees at seven locations in Boston, Chicago, Houston, New York City, San Francisco, Southern California and Washington, D.C. SGH provides services in Commercial, Institutional and Residential Buildings, Transportation, Nuclear, Water/Wastewater and Science/Defense structures. SGH services are supported by technical capabilities, including: Building Envelope Engineering, Structural Engineering, Engineering Mechanics, Building Science and Construction Engineering. Their practice encompasses design, investigation and performance evaluation, repair and rehabilitation of constructed works. SGH implements innovative techniques for design, analysis, and project management. Their laboratories supports design and investigation work with material, mechanical, and environmental testing. The majority of their projects are buildings in diverse markets, including educational, industrial, healthcare, and residential. Founded in 1956 by three former Massachusetts Institute of Technology professors - Howard Simpson, Werner Gumpertz, and Frank Heger - SGH specializes in structural design, historic preservation and evaluation of building exteriors. The firm has a reputation for assessing structural or interior damage to buildings. Nationally, SGH is recognized as an expert on the science and causes of structural failure. For example, when mirror-glass panels began falling off Bostons John Hancock Tower and had to be temporarily replaced with plywood in 1973, SGH was hired to analyze what went wrong. More recently, SGH was hired by the National Institute of Standards and Technology (NIST) to investigate the collapse of New Yorks World Trade Center and assess the Towers' structural response to impact damage and fire after the September 11, 2001 attacks. NIST retained SGH to develop computer models simulating the towers’ structural response to aircraft impact and subsequent fires. Several studies, conducted by NIST and its consultants, provided input for the SGH study, including aircraft impact analysis, fire dynamics and heat transfer models. NIST also tested structural steel recovered from the WTC site to determine its mechanical and metallurgical properties including temperature-dependent thermal expansion, modulus, plastic flow, and creep properties. SGH first developed models of components, connections, and subsystems of the WTC towers and studied their structural response to fire-induced temperatures over time. Using results of such studies, SGH developed computationally efficient global models of the towers and performed FE analyses from initial impact through each tower’s collapse.