Quantification of Aluminum Increase Factors for Curtain Wall Design Using Finite Element Methods

Abstract

In this paper we present structural analysis of an aluminum-backed curtain wall system to inform blast-resistant structural design practice. This analysis focuses on the expected material response of an aluminum curtain wall support to a set of typical blast loadings used in U.S. government criteria for domestic projects. Dynamic experiment test results of aluminum samples are used with a three-dimensional finite element hydrocode to account for over-strength, dynamic strengthening, and postfailure response of aluminum. The analysis shows that a curtain wall member under high-rate (blast) loading that is prescribed to fail using the recommended values of yield strength in the criteria documents, actually has very little evolved plastic strain. Further analysis shows that the combination of both the measured quasi-static yield over-strength and dynamic yield strength of specific aluminum evaluated exceeds the design recommendations prescribed in government design criteria in excess of 30% for the 6063 aluminum evaluated. The additional fidelity provided by the more accurate material properties improved the calculated performance of the structural member to the point that a member upgrade would not be required. BackgroundThis paper quantifies the difference between structural design and analysis of curtain wall systems. Structural analysis focuses on the response that is expected based on test data. In this approach, material properties from tests on experimental coupons are used in the constitutive model for aluminum. These values are readily available in such engineering references such as the Machinery's Handbook and the Mechanical Engineering Design Handbook [5,9]. Structural design is about what is allowable based on a margin of safety and surety. This requires constitutive properties which are conservative and have very high confidence intervals (in excess of 90%). In the absence of building code for blast design, there are technical documents from the DoD, GSA, DOS, and ASCE which are commonly referenced to meet the Standard of Care for blast design. These standards mitigate the occurrence of failures which result from design error, or materials that do not meet the expected strengths. However, there could be instances when a structural element does not meet the allowable design standard, but engineering analysis indicates that it is not expected to fail.