Global Fastener News

2003 FIN – Fastener Engineer: Building Can Take Only So Much

May 13
00:00 2013

April 15, 2013 FIN  – “The structure did far more than we could have asked,” fastener engineer David Sharp declared of the World Trade Center.

The 110-story Twin Towers collapsed September 11, 2001, after terrorists flew Boeing 767 jetliners into them.
Engineers did design for the possibility of such smaller planes as a Boeing 707 flying 180 mph striking an office tower, but today’s large passenger airplanes loaded with jet fuel were not considered in building design, Sharp told the Metropolitan Fastener Distributors Association.
Sharp, vice president of fastener manufacturer TurnaSure LLC, is a consultant for the National Institute of Standards & Technology investigation of the WTC collapse. Investigators calculated that the second tower was “subjected to an impact 11 times greater than its design load,” Sharp reported.
The towers were among the first structures outside of military and nuclear facilities where the potential impact of a jet airliner was considered in planning.
Building Performance Findings
Sharp’s slides showed how engineers took extra precautions to protect the buildings, but there is a limit to what designers can do.
“Just a 600-mph bag of golf clubs can take out pretty much anything,” Sharp noted. “Reliably designing a building to survive the impact of the largest aircraft available now or in the future may not be possible,” Sharp quoted building performance findings.
• Buildings 4 and 5 below the Twin Towers included bolted connections which did not perform during the fires the way designers would have liked, leading to floor collapses within the structure.
• Evidence from the fire-induced collapse of WTC 7 suggests further study of the connections in the huge transfer trusses at the base of the building is warranted.
The Building Performance Team concluded that the performance of bolted connections and their performance in fires deserves further study. Expertise previously held by fire engineers will likely become an important component of structural engineering.
Sharp showed the MFDA slides of fasteners and steel salvaged from the Twin Towers. Included was one slide showing six floors compacted into a few feet.
Groundbreaking for the buildings was in 1966 and tenant occupancy for the first tower was in 1970 and the second tower during 1972. Peak daytime population was 58,000 people in 12 million square feet of office space. The buildings totaled 230,000 tons of steel.
Sharp showed slides from the salvage yards of bolts, which held and those ripped from the steel. He observed that bolts lose strength when heated.
The WTC was built in the “old heart of the New York fastener industry,” Sharp noted. Manufacturers of the WTC bolts included Federal Bolt, Bethlehem Steel, Russell, Birdsall & Ward and Gary Screw & Bolt. There were bolts probably from other manufacturers, such as some with a headmarking of only “2”.
There were premature media reports that a Massachusetts Institute of Technology study blamed the towers’ collapse on faulty “single-bolt connections” in the floor trusses, but those reports turned out to be false. The trusses were attached to the exterior columns with a twin-bolt system supported by a gusset plate and multiple welds.

Excerpts from Initial Studies of the Collapse
• “There is insufficient understanding of the performance of connections and their adequacy under real fire exposures …”
• “Connection performance under impact loads and during fire loads needs to be analytically understood and quantified for improved design capabilities and performance as critical components in structural frames.”
• “Connections are generally not included as part of the assembly tested in traditional fire resistance tests, and most modeling efforts assume that the pre-fire characteristics of a connection are preserved during the fire exposure.”
• “A determination of the combined structural and fire properties of the critical structural connections should be made to permit prediction of their behavior under overload conditions.”
• “Fire protection ratings and safety factors for structural transfer systems should be evaluated for their adequacy relative to …building stability.”
• “The adequacy of current design provisions for members (structural pieces) whose failure could result in large-scale collapse should also be studied.”
• “The issue of connection performance under fire exposure is critical to understanding building performance and should be a subject of further research.”
• “The structural collapse [WTC 5] appeared to be due to a combination of excessive shear loads on bolted connections and unanticipated tensile forces resulting from catenary sagging of beams.”
• “Tensile catenary action of floor framing members and their connections has not been a design consideration for most buildings.” Editor’s Note: David Sharp, vice president of TurnaSure LLC, is based in New York at 57 E. 11th St., 8th Flr., New York, NY 10003. Tel: 646 602-1405 Fax 212 777-7901 E-mail:©2003/2013 Fastener Industry News.
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