Rivet removal is an important task in the restoration or rehabilitation of a historic metal structure and is often overlooked in restoration specifications. Rivet removal by an inexperienced construction worker can seriously damage original historic metals. There are several methods for removing rivets, the most popular being the oxy-acetylene cutting torch with a regular cutting tip or a rivet washing tip. The oxy-acetylene method used by an experienced operator can be an effective means for removing rivets. Another tool to remove rivet heads is the “rivet buster,” a pneumatic tool designed to shear off rivet heads quickly. Chisels and punches of various sizes and styles are available to be placed in the rivet buster to punch out rivets after the rivet heads are removed.
The air carbon arc process is another method that is very effective in removing rivets without damage to the parent metal. The air carbon arc torch is connected to an electric welder and air compressor. The process is an arc-cutting process that severs or removes rivet metal by melting it with the heat of an arc struck between a carbon-graphite electrode and the base metal. A stream of compressed air blows the molten rivet head from the surface of the base metal. All the cutting processes mentioned above, except for the oxy-acetylene cutting tip, require a second operation with a pneumatic hammer to punch out the rivet after the head is removed. If the rivet cannot be punched out with the pneumatic hammer, the oxy-acetylene torch with a regular cutting tip is used to pierce the center of the rivet and cut out as much of the rivet as possible while avoiding the edge of the hole.
These processes for removing rivets were demonstrated and thoroughly explained during the Workshop, and participants had an opportunity to handle the equipment during the hands-on activities. [View video: Methods for Rivet Removal]
Pack Rust Removal
Pack rust has always been an issue with engineers when doing a structural analysis for a historic metal structure, particularly in analyzing metal truss bridges. In some cases, an entire bridge may be deemed unsalvageable because of the presence of pack rust. In the absence of effective methods of pack rust removal, engineers have not had the option to remove pack rust and re-use these bridge components.
Pack rust forms between rivets in the joint where a plate and a structural shape (channel or angles) come in contact. A process for removing pack rust was developed during the restoration of historic bridges for the Calhoun County Historic Bridge Park using a rivet hammer and an oxygen-fuel heating torch with a heating tip. The process begins by heating a buckle with a heating torch at 800 to 1,000 degrees Fahrenheit; a plate with a handle is placed over the buckle, the rivet hammer hammers the buckle, and the rust breaks up and is driven from the buckle. The buffer plate prevents scarring or any possible case hardening of the plate. Sequencing the heats and hammering helps prevent distortion; heating a few buckles in one area and then moving on to another buckle further away keeps from concentrating too much heat in any one area. [View video: Pack Rust Removal from historic metal structures]
Three top chord sections from an 1895 historic bridge were brought in from the Calhoun County Historic Bridge Park storage yard and set up at Lansing Community College for the pack rust removal demonstration during the Workshop. Participants saw firsthand how the method works on components of a historic metal structure. Discussions included examples of restoration projects in which pack rust was removed and the metal component re-used in the restored structure, as well as examples where the pack rust was severe enough to use a different approach to restore the structure (such as replacement of top chord plates with new plates riveted on the original channel or angle). In these examples, the presence of pack rust did not result in the automatic rejection of the structure for historic preservation, an important message for the Workshop participants whose work involves making decisions about the feasibility of restoration for a particular metal structure.
Heat straightening of structural steel sections has been used for years to remove distortion within steel. “Use of the oxyacetylene flame in performing difficult – even seemingly impossible – tasks in steel work is often underestimated. There are many jobs that beginners as well as old-timers pass up because they are not acquainted with the advantages offered by contractive forces which set in after heat has been properly applied.” This article by Joseph Holt appeared in the October 1955 issue of the Welding Engineer. These processes are well established in the steel fabrication industry, but they are not well known or understood by bridge preservationists, engineers or General Contractors. Lack of this knowledge can lead to the specification of inappropriate preservation techniques for bent or otherwise distorted components of a historic metal structure, sometimes resulting in the increased cost of the restoration project. During the Workshop the heat straightening method was demonstrated and participants informed of the ways in which steel and wrought iron are dealt with slightly differently in this process.
For the heat straightening demonstration Dan Garijo, part owner of National Bridge Company, provided Workshop participants with work-related applications of straightening damaged steel bridges and other steel structures. In some steel the application of heat is all that is needed, and the addition of hydraulic or pneumatic jacking is not required (in fact, in some structural codes not permitted). In straightening wrought iron, on the other hand, a combination of heating and jacking is required. This is necessary because, unlike steel (which is a homogenous composition of iron, carbon, and other elements), wrought iron is a two-component metal made up of iron silicate (a type of glass-like slag) and high purity iron. [View Video: Heat Straightening for steel and wrought iron]