Fundamentals of Highway Bridge Demolition

Fundamentals of Highway Bridge Demolition Matthew Barsottelli, P.E., M.ASCE1 and Onur Avci, Ph.D., P.E., M.ASCE2 1 Project Manager, AECOM USA Inc, 20 Exchange Place, 12th Floor, New York, NY, 10005, USA. Email: matthew.barsottelli@aecom.com 2 Assistant Professor, Civil Engineering Department, Qatar University, P.O. Box 2713, Doha, Qatar. Email: onur.avci@qu.edu.qa ABSTRACT As the transportation infrastructure in the United States is aging and expanding rapidly, demolition and replacement of the existing systems has increasing importance. Highway bridge demolition has become increasingly significant as the bridges reaching their service design life require rehabilitation or replacement. Highway widening to meet capacity increase also involves partial or total removal of highway bridges. This paper presents the fundamentals of highway bridge demolition by explaining the systematic deconstruction, crane usage, and the engineering involved with this process. Systematic deconstruction is the art and science of preventing unplanned potential energy release during a demolition process. Engineering is needed for the deconstruction process in order to protect the lives of the workers and in order not to damage any other structure in the close proximity of the structure to be demolished. In addition, the expensive machinery used for the deconstruction process needs to be protected. As a result, nearly as much engineering analysis is needed to demolish a structure than to initially build it. While the stereotypical image of demolition is a wrecking ball or an explosive event, these are not typically used in highway bridge demolition. Explosive demolition is rare, and is used occasionally for very large bridges (as well as other large structures outside of this scope). For a typical highway overpass bridge hydraulic excavators sit on the deck removing concrete with a hydraulic hammer or shear, and then excavators or cranes placed on grade remove the girders. The bridge structure is analyzed for different excavator positions and different stages of the demolition. The excavator body weight and the hammer tip weights are considered separately for different cases of demolition sequence. The engineering work required for cranes involves support design for crane bases (tracks or outriggers), pick plan and pick weights (pick sequence, radii, clearances, 1 center of mass, lay-down area), and rigging design (wire rope slings, stability of structure while being picked, local failures of the object being picked or the rigging components). INTRODUCTION The engineered deconstruction and removal of an existing structure is called demolition. More theoretically, demolition is systematic deconstruction such that no potential energy is released in an unplanned way. There are two kinds of demolition: • • Explosive Demolition: it is conducted by very specialized engineering firms (depicted in Figure 1a). Conventional (non-explosive) Demolition: It is taking apart a structure and lowering its components to grade so there is no potential energy that can be released (Figure 1b). a) Explosive Demolition b) Non-Explosive Demolition Figure 1 - Types of Demolition The vast majority of highway bridge demolition is a conventional, utilizing excavators and cranes. There have been a limited number of publications found in the literature on bridge demolition. Abudayyeh et.al (1998) and Singh et.al (2008) are the one that can relate to the scope of this paper herein. The general sequence in a highway bridge demolition typically starts with the removal of the concrete deck. Concrete can either be hammered to the ground or panelized and moved away from the bridge. Sometimes shielding is placed under the bridge so that concrete debris does not fall into a waterway, traffic, or property below. After concrete is removed, the girders are removed from the structure with a crane or excavator (equipped with grapple or shear attachment) and lowered to grade. It is not mandatory to remove the concrete first. However, the concrete weighs so much relative to the steel on a typical highway bridge that one would need a much 2 larger crane to remove both concrete and steel at the same time. This is typically impractical and uneconomical. WHY IS ENGINEERING NEEDED? For the demolition process there is much structural analysis that goes into the removal sequence. The structure must be checked for strength and stability at every planned component removal, as the structural system, loads, and the load path will change. The removal of lateral bracing diaphragms between girders becomes very important for the demolition operation. It is more convenient for the contractor to remove as many braces as possible prior to supporting a girder to be removed with a crane or excavator. Thus, the engineer needs to analyze and determine how many braces can be removed so the structure remains safe for the work crews. Engineering is required because: 1) The work is performed by people, and their safety is of utmost importance: Every stage during the process needs to be analyzed in detail, and the important information clearly presented in the demolition plan drawings to be used in the field. 2) The structure being removed is usually located near other structures: Often to-be-demolished highway bridge structures are in close proximity to other structures (Figure 2). Sometimes, the structure being removed is in the direct vicinity of the travelling public. The demolition sequence and stability of the structure at every stage should be analyzed and the safety of the proximity structures and the people should be ensured. a) Traffic Under the Bridge b) New and Old Bridges Next to Each other Figure 2 – The Structure Removed is in Proximity to Other Structures. 3) The equipment being used in the demolition is very expensive, and a wellengineered demolition plan assists the contractor to choose equipment capable of the task. 3 MISCONCEPTIONS REGARDING DEMOLITION AND DEMOLITION ENGINEERING “What’s so difficult, don’t you just explode stuff?” Nearly as much engineering analysis is needed to demolish a structure than to initially build it. There is much less detailing regarding connections, maintenance, and serviceability requirements; however there are different kinds of detailing and experience needed in order to take the structure apart. Also, there is seldom the luxury to take something apart the way that it was constructed, so in some ways it can be like a puzzle that needs to be solved. When a large bridge or building is demolished using explosives it is on the local and national news. That’s why the general public equates demolition with explosives. “Wrecking Ball!” Stereotypical image of demolition (besides explosives) is a wrecking ball. This demolition technique is still used on buildings, is relatively rare, and is not an option for the demolition of highway bridges. This technique does not require a lot of engineering and is used primarily on brittle structures like masonry. DEMOLITION ECONOMY Generally the demolition engineer is hired by the demolition contractor, and the demolition submittal is reviewed by the State or City Department of Transportation (D.O.T). The contractor typically prefers the heaviest possible equipment (excavator) to be placed on the structure, as larger (heavier) equipment completes the operation much faster and more economically since it delivers higher impact energy. The engineer’s role is very important to help the contractor determine the largest equipment that can be used on that particular bridge while maintaining structural safety.The design truck weight is 72,000 lbs in AASHTO and the hydraulic excavator options usually range from 30,000 lbs to 250,000 lbs (Figure 3). In some cases the equipment on the bridge during deconstruction can be heavier than the bridge was designed for. It is an interesting aspect since the bridge will see loads potentially it has not seen during its operation. On the other hand, if a contractor tries to demolish a bridge with very tiny equipment on the bridge (e.g. 5000 lbs), there may be no need for an engineer. However it would be so uneconomical to the contractor that they would never prefer this option. Using larger equipment brings greater need for engineering. 4 Figure 3 – The AASHTO Design Truck and Hydraulic Excavator Hydraulic excavator is shown in Figure 4. Typically they are equipped with a hydraulic hammer or shear to remove the concrete deck. A hydraulic hammer weighs typically 6,000 to 18,000 lbs for highway bridge demolition applications. There are also hydraulic hammers weighing up to 30,000 – 40,000 lbs but they are not typically used in highway bridge demolition. The controlling load case for a bridge is when an excavator is hammering the deck since it imposes concentrated loads and dynamic effects of the hammer impacts. This case is shown below in Figure 4, known as the “leverage position”, where the operator pushes down on the boom to apply more load on to the hammer tip to break the concrete more efficiently. By doing this, the front tracks lift off the pavement and the bridge is subjected to a reaction at the hammer tip and reactions at the back of each excavator tracks. This controls for the deck design, and most often the substructure design. Figure 4 – Controlling Limiting Load Case for an Excavator 5 For equipment located on grade (like a crane) it is more economical for the contractor to use a smaller crane. It is important for the engineer to correctly determine pick weights and crane radius / geometry since the full extent of the allowable crane chart may be utilized but never exceeded. CRANES IN HIGHWAY BRIDGE DEMOLITION Generally defined, a crane is a power operated machine for lifting or lowering a load and moving it horizontally which uses wire rope in which the hoisting mechanism is an integral part of the machine. The girders for conventional demolition is typically picked and removed by cranes (in some cases a hydraulic excavator is used instead). There are two main types of cranes: Crawler cranes (Figure 5) and hydraulic cranes with outriggers (Figure 6). Crawler cranes have large tracks called crawlers, a main body, a counter weight, and the boom of the crane. Due to its large size, crawler cranes have to be driven to the site on separate trailers and assembled at the site. Hydraulic cranes with outriggers; can be driven to the site since it is its own mobile unit. Typically the counterweight is transported separately. It has four outriggers that support the entire weight of the crane while performing picks (the wheels are not designed as such). For girder removal with a crane sitting on grade the contractor and engineer must determine the smallest crane (for economy) that can safely perform all picks. Figure 5 – Crawler Crane 6 Figure 6 – Hydraulic Crane with Outriggers What Engineering Services Are Needed For Cranes? Support of Crane Base Design (Tracks or Outriggers): For the determination of crane loads imposed on the crane support structure, there are programs available to find reactions under the outrigger or the tracks for every lift. A heavy lift at a large radius produces higher loads in one outrigger or one crawler track (in various load patterns). The crane sits either on soil, pavement, a building, or a substructure (a city street that’s above a subway) so it is important to determine that the imposed loads during the maximum lift can be supported safely. The loads of the critical picks are translated down to what supports the crane, and deflection is a very important parameter. If a crane boom is for example, 300 ft in length with a pick radius of 200 ft, a small settlement deflection at the support will produce a huge deflection at the end of the boom, likely toppling the crane. In an attempt to distribute track or outrigger loads, “dunnage” may be used. A dunnage goes underneath the tracks or outrigger and it spreads the load laterally so that more of the soil or the structure is engaged to support the load. A crawler with dunnage is shown in Figure 7. Sometimes steel plate sits on wood but typically wood goes directly under the tracks or outrigger without the steel. An outrigger is shown in Figure 8; the wheels of the crane are off the ground. Four outriggers support the crane. 7 The engineer is especially needed when dunnage has to span over a structure, or is located directly over an underground utility. Figure 7 - Dunnage under a Crawler Crane Figure 8 - Hydraulic Crane with Outriggers 8 Engineered Pick Plan Design: Anytime something is lifted with a crane, it is called a “pick” in the demolition and crane use terminology. Determination and presentation of pick sequence, lift radii, load charts, crane configuration, geometry (clearances of the boom and all crane components to adjacent structures or equipment), center of mass of each pick, dunnage details, rigging, and lay-down area are all very important parameters that need to be considered in the pick plan design. These items are to be shown on an engineered pick plan. At all stages of the design the demolition engineer must work closely with the contractor to choose the best crane location and pick sequence. Lay-down area: When something is lifted with the crane, it needs to be decided where that piece is going and it has to get there without exceeding the capacity of the crane (if the lay-down area is at a greater radius than the initial pick). Determining the geometry clearances: Often there is a structure in the way during crane operations, and for small pick radii it may be the piece being lifted. It is very important the boom doesn’t touch anything during the entire operation. The crane boom is basically a column (subject to compression and bending) that is assumed to have zero capacity to be hit or tapped. Four feet (minimum) is typically used for the clearance distance as a standard rule of thumb between the boom and another structure. Crane Manufacturer’s load charts: Shows the possible configurations of the crane and the corresponding lift capacities at particular pick radii. There are many ways to set the crane up with different booms, attachments, and counter weights. The pick plans specifically indicate all of these critical configuration parameters. Rigging design: The wire rope (or synthetic material) that attaches the pick to the crane “hook” (called a block) is rigging. The rigging forces are determined based on geometry and statics, and compared with the industry / manufacturer standards. Other aspects of rigging design include preventing local failures of the object being picked. Rigging geometry can impose inward pulling forces on the component being picked and this needs to be incorporated in the design. REFERENCES Abudayyeh, O., Sawhney, A., El-Bibany, H. and Buchanan, D. (1998). “Concrete Bridge Demolition Methods and Equipment”. Journal of Bridge Engineering, ISSN 1084-0702/98/0003-0117-0125. Singh, S., Mirzakashani, M., Hagh, A., (2008). “Dual Deconstruction”, ASCE Civil Engineering Magazine, Volume 78, Number 4, 60-67. 9