Iron & Steel Preservation 

September 16, 2015

Lansing Community College welding lab technicians assisted in demonstrating some basic concepts in the study of metal. Roger Morrison (center), Aaron Kavanagh (right) and James Whaley (left).
Tension , a pulling force
Compression, a balanced pushing of forces
Brittle Fracture: Cast iron is hard and rigid...
...and will easily break if dropped.
Ductility:
The ability of metal to bend, or stretch before fracturing.
Elasticity and plasticity are terms often used in articles written about the failure of metal. Roberto Ballarini and Taichiro Okazaki, in their article "The Infamous Gusset Plates" prepared for "The City, The River, The Bridge" on the collapse of the I-35W bridge, provided an excellent illustration of the concept of elasticity and plasticity. Excerpts from this article, illustrated with photos by Vern Mesler in place of the original diagrams, is shown below.

Like us on Facebook
Greetings,  
 
A hammer in the hands of a skilled metal craftsman can form metal into precise shapes. For an engineer working on the repair, rehabilitation, and restoration of metal structures, knowledge of metals (especially historic metals) and of advances made in metal research, arc welding processes, and metal fabrication all contribute to a successful metal preservation project.
It is essential that those making the decisions to repair, rehabilitate, or restore historic metals select the services of civil/structural engineers with a record of accomplishment in the preservation of metals -- a record that includes projects where public safety is ensured, cost control is maintained, and the craftsmen's record is preserved.
An issue engineers often encounter in the preservation of historic metal structures is the presence of fracture critical members. Dr. Frank Hatfield, Professor Emeritus, Michigan State University, has provided the Iron & Steel Preservation Newsletter with an introductory article on fracture critical members. Also available on-line are articles written by Dr. Robert Connor of Purdue University on the subject of fracture critical members in new construction.
Dr. Connor will be presenting at the Iron & Steel Preservation Conference & Workshop, May 18-20, 2016 at Purdue University.

Vern Mesler
Iron & Steel Preservation Coordinator
Lansing Community College 

SNAP AND CRASH: THE PERIL OF FRACTURE-CRITICAL MEMBERS
Steel is valued for tensile strength far superior to other construction materials, and for ductility, which is the ability to stretch before fracturing, possibly shifting force to unimpaired members and causing deformation that may be recognized as a warning of imminent failure. However, both strength and ductility can be compromised, as evidenced by the broken hulls of World War II liberty ships continuously battered by frigid North Atlantic waves. In addition to low temperature and fatigue due to repetitive loading, other factors that reduce resistance to fracture include material with inadequate notch toughness (a combination of strength and ductility measured by capacity to absorb strain energy) and stress risers (e.g., cracks, nicks, dents, pits and section loss due to corrosion, re-entrant corners, holes, welds, and abrupt changes in cross-section.)

Silver Bridge Point Pleasant, West Virginia

Ship builders improved their product, but bridge designers gave minimal attention to the possibility of brittle (i.e., non-ductile) fracture until after December, 1967, when the Silver Bridge at Point Pleasant, West Virginia, suddenly collapsed into the Ohio River, killing 46 people. Investigators concluded that the forty-year-old bridge complied with design standards when it was built and was not overloaded when it failed. Examination of the wreckage disclosed that collapse was precipitated by brittle fracture of a single steel eye bar. Bridge engineers promptly fixated on the problem of fracture-critical members.

Fracture-critical member (FCM)

A fracture-critical member (FCM) is a tension member or component the fracture of which would cause the bridge to collapse. Typical FCMs are lower chords of trusses, diagonal truss members that slope downward toward mid-span, and bottom flanges of bridge girders if there are only two. Eye bars are usually fracture-critical, but not all tension members are.
  
A redundant tension member is not fracture-critical because other members can accept its force if it fails. The cables of a cable-stayed bridge exemplify redundancy. Since the Silver Bridge disaster, bridge designers incorporate redundancy to minimize the use of FCMs, which are now subject to stricter requirements and consequently are more expensive than redundant tension members.
  
FCMs abound in trusses, and FCMs in older bridges may not comply with current standards. Typically, material notch toughness is not known, and material strength may be uncertain. There is no guarantee that tested samples represent the entire bridge. Visual inspection for stress risers is necessary but unreliable because flaws may be tiny, hidden, or internal to members. More thorough inspection methods are costly and require disassembly.    

Historic Bridges, compliance with current standards

Bringing an older bridge into compliance with current standards may require reducing forces in members. The reduction for FCMs will be greater than that for redundant tension members. Member forces are caused by load, and both can be reduced by installing lighter decking, eliminating a traffic lane, or narrowing the deck of a pedestrian bridge. Deficient components can be replaced with new ones. Strengthening existing members by attaching new material is an option, but riveting or bolting is inadvisable unless existing holes can be reused. Like holes, welds are stress risers. Nevertheless, welding can be used to repair flaws or attach new material, even on FCMs, if the welds are properly designed and appropriate procedures are followed. It may be possible to remediate a deficient member by adding a new member parallel to it. A system of cables paralleling existing truss members can provide redundancy, removing tension members from the FCM category and its concomitant requirements.
Respect Historic Integrity
Historically significant bridges that remain in service are particularly problematic because repair and remediation can compromise original features that should be preserved. Nevertheless, safety is paramount. The services of a civil/structural engineer are necessary, preferably one with a portfolio of restoration projects that respect historic integrity.
 
Dr. Frank Hatfield  
Customized Arc Welding Courses
Customized arc welding courses are being offered at Lansing Community College, West Campus, Lansing, Michigan. These courses can be designed for individuals or groups for either introductory or advanced training in the Flux Cored Arc Welding (FCAW), Metal Cored Arc Welding (MCAW), and Shielded Metal Arc Welding (SMAW) processes.
  
Depending on the client's needs, customized courses can include lectures, demonstrations, and hands-on activities with instructions on the essential variables (voltage, amperage, and other operating parameters) of the arc welding processes. 
Contact Vern Mesler at meslerv@gmil.com for more infomation
Presentations and Tours

Interested in arranging a guided tour of the Historic Bridge Park in Battle Creek, Michigan, or attending a presentation on topics related to the preservation of iron and steel? Contact Vern Mesler (meslerv@gmail.com) to discuss details of setting up a tour or presentation for your group.

 

Suggested honorarium: $100

Additional costs: Travel expenses for presentations outside the mid-Michigan region  

Past Iron & Steel Preservation Newsletters

An Iron & Steel Preservation update

July ISP Newsletter: Purdue & LCC Partner for 2016 Conference 

June ISP Newsletter: Wrought Iron

May ISP Newsletter

April ISP Newsletter : Customized Welding Courses

Sponsors



Want to become a sponsor?
Contact Vern Mesler for more information at meslerv@gmail.com

Lansing Community College
5708 Cornerstone Drive
PO Box 40010 , MC 4100W
Lansing, MI 48917
(517) 614-9868 |   meslerv@gmail.com