Response Modification Factor for Inelastic Design of Low-rise Steel Concentrically Inverted-v-braced Frame Buildings Subjected to Extreme Wind Loads
Author: Josef K. Jacobs
Publisher:
Published: 2020
Total Pages: 169
ISBN-13:
DOWNLOAD EBOOKThe current practice in wind resistant design in the United States is to proportion and detail the lateral force resisting system to remain elastic, even for extremely-rare large-magnitude windstorms. The objective of this study is to determine the applicability of inelastic design and a corresponding wind response modification factor for low-rise buildings subjected to extreme wind loads that use a steel concentrically braced frame in an inverted-V configuration for the lateral force resisting system. The hypothesis is that controlled inelastic behavior of the steel braced frame and ductile detailing could justify a reduced design force for wind applications, similar to the approach used for seismic applications. The reduced design force is computed by dividing the force that would be generated in a structure behaving elastically by a response modification factor. To test the hypothesis, archetype buildings (a 3-story office building and a 1-story industrial/retail “Big Box” building) were designed for four wind speeds, 110-mph, 127-mph, 156-mph, and 220-mph. This study focuses on the 1-story building. Six types of inverted-V (chevron) braced frames were designed for each wind speed: (1) a conventional “non-ductile” braced frame with a weak beam, (2) a braced frame with a moderately-strong beam, (3) a braced frame with strong beam, (4) a braced frame with a moderately-ductile brace and a weak beam, (5) a braced frame with a highly-ductile brace and a weak beam, and (6) a braced frame with a highly-ductile brace and a strong beam. A nonlinear finite element model of the 1-story building was developed using a hybrid distributed-concentrated plasticity approach and a large strain, large displacement, two-dimensional analysis. The static behavior of the building was determined using monotonic and fully-reversed cyclic wind pushover analyses. The dynamic behavior of the building was determined using dynamic response history analyses of the building subjected to wind loads derived from wind tunnel tests of a small-scale model of the building. The wind speed in the dynamic analysis was increased incrementally until collapse. The results of the static analysis indicated that the system over strength was larger for lower design wind speeds compared to higher design wind speeds. For the braced frame designed for 110-mph the over strength was equal to 2.1, and for the braced frame designed for 220-mph the system over strength was equal to 0.6. The results of the dynamic analyses lead to three main conclusions. First, steel braced frames with a strong beam produced a building with the same or higher collapse safety, compared to a conventional “non-ductile” braced frame. Second, steel braced frames with highly-ductile braces, defined as sections with width-to-thickness ratios that satisfy the highly-ductile requirements for seismic design, produced a building that could safely be designed using a response modification factor is equal to 1.3. Third, steel braced frames with a strong beam and highly-ductile braces produced a building that could safely be designed using a response modification factor equal to at least 4.0. The results confirm the hypothesis and suggest that for the 1-story building with steel concentrically braced chevron frames investigated in this study, controlled inelasticity could justify a reduced design force for extreme wind loads.
