Bridge Demolition Engineering: Best Practices

Demolition activities often occur near other structures, infrastructure, and public spaces. Poorly planned demolition activities have caused property damage resulting in additional costs for the contractor and the owner to repair or replace the damaged property. A properly engineered demolition plan considers adjacent structures to minimize risk and impact and provides protection where warranted. Bridge demolition is typically performed using controlled methods, following a bridge demolition plan that adheres to necessary code or specification requirements and considers unique loading conditions, stability concerns, and safety issues. The analysis methods used in performing demolition engineering must be appropriate for the type and configuration of the structure being analyzed. Demolition projects may require a detailed step-by-step analysis, as both loads and load paths are modified from their as-built configuration. Bridge demolition is an industry that requires highly technical operations to be executed near the traveling public or other public spaces.

Most new bridge projects and bridge rehabilitation projects require partial or complete demolition of existing bridge structures. Government owners are increasingly adopting bridge demolition criteria, as documented in Bridge Demolition Practices. Private owners, such as the railroads and the National Park Service, have some documented requirements for bridge demolition and removal. This chapter lists several available references and guidelines for bridge removal. There are no current design codes that specifically address the analysis of bridges for or during demolition operations. As such, it is a common industry practice to extrapolate from existing design codes using the guidance of sound engineering judgment for analysis. There are two AASHTO documents that are typically used for bridge demolition: the AASHTO LRFD Bridge Design Specifications and the AASHTO Manual for Bridge Evaluation. AASHTO also has a construction specification, which provides limited requirements for demolition. At least 26 states have some requirements related to bridge demolition that are mentioned in the states' standard specifications.

The tools or equipment required for a demolition project vary widely depending on the structure type, site conditions, and project schedule. This chapter includes an overview of standard equipment used for bridge demolition projects. Standard equipment typically performs a wide range of operations common to most demolition jobs. These tasks can include structure removal (cutting, shearing, and rubblizing), material processing, and hauling. A section on specialty heavy lift equipment has also been included. This type of equipment may be required on complex demolition projects owing to access restrictions, schedule requirements, or sensitive environmental areas. Bridge demolition may require the cutting of bridge elements into manageable components that equipment can handle. Debris protection systems are often needed on bridge demolition projects to prevent debris from falling or migrating into areas outside the demolition work zone. Certain bridge demolition projects require the use of additional external supports, bracing, and/or temporary members.

Bridge demolition analysis is like erection analysis in that both must consider the effects from dead, live, and environmental loading. Dead loads include all permanent components of the structure, such as primary and secondary load carrying members, decks and overlays, barriers, railings, and ancillary structures. Live loads include traffic, pedestrians, construction personnel, equipment, shielding, and all other transient loading that may be on the structure during the demolition process. In general, environmental loading is controlled by wind, especially as the structure is partially deconstructed, but loading such as ice, snow, and seismic events should be considered on a project-specific basis. Whereas modern bridge design is typically performed using load and resistance factor design, allowable stress design is often preferred for bridge demolition because unfactored loads are required for calculating deflections and determining pick weights and pick locations. During demolition, contractors have a wide variety of equipment at their disposal to aid in various removal methods.

The use of explosives in bridge demolition has been undertaken for decades. The intent of using explosives for bridge demolition is to bring the structure quickly and safely to the ground where it can be processed with conventional equipment. The use of explosives to demolish a bridge is a highly technical process. Significant up-front engineering to determine the charge locations and analyze the existing structure for pre-blast activities must take place. The design work is typically divided between two experts: the explosive engineer and construction engineer. Explosive demolition is typically subcontracted to a specialty explosives contractor. There are several reasons to consider the use of explosives for a demolition project: structure type, safety, condition of the existing structure, cost, and schedule. Pier removal with explosives is typically only justified in special circumstances. Large concrete piers may collapse on themselves (implosion), and steel frame piers can be brought down with external-shaped charges (explosion).