Jan 10, 2019 in Analysis

Bridge Demolition Analysis Essay

Bridge Demolition Analysis and Comparison to Traditional Bridge Design

1. Introduction: Importance of Calculations

Although bridge designers do not usually consider the demolition process of the construction before the bridge is actually built, it is evident that bridge demolition process needs to be carefully planned before the construction of the bridge starts since planning will allow performing the process of existing bridge demolition more safely. In order to organize the demolition process in a proper way, one needs to be aware of the overall procedure, construction sequence, and equipment that will be used. As the planning project for demolition design requires deep knowledge of many specific aspects regarding the process and equipment, both contractor and a structural engineer need to be involved in it. 

Every stage of bridge demolition process requires many factors to be considered. Thus, it is important to consider and evaluate the design of every element of the bridge. Moreover, a competent professional needs to inspect the overall process of bridge demolition as long as it lasts so that all regulations, instructions, safety recommendations, and procedures are met. If these aspects are neglected, there is no guarantee that the demolition process will be safe even if its planning is done carefully. 

2. Project Description

The current research is based on the investigation of the calculations for the bridge demolition that was made in the past in order to determine the considerations and calculations needed in the planning process of bridge demolition so that structural adequacy of the construction is guaranteed. At any stage of demolition process, temporary supports that will ensure structural safety are required if there is some structural inadequacy in any portion of the construction. 

The contractor’s project management team provided the researcher with rigging information and the evidence about removal sequence and the equipment used in the demolition process. It was determined that the structural engineer who planned the process needed calculations that indicated the adequacy of every part of the bridge lifted by the crane and the remaining construction. Structural engineer had also provided data regarding the description of every detail in the demolition procedure, design of the temporary supports used, and the reports on the capacity of the equipment involved in the heaviest lifts. Since the demolition procedure can be changed after a thorough structural analysis, project manager and the engineer need to have a discussion in order to talk about all peculiar aspects and potential issues. 

The bridge taken as an example for the research crosses over West Dupage River, situated in Naperville, IL at the intersection of 75th and Washington Street. The demolition of this bridge is planned since the construction has being used for 30 years and the new wider bridge is needed. 

The structure intended for demolition is three-span bridge. The overall span length is 116’-8’’, whereas the individual spans are 29’-0”, 28’-11”, and 29’-0”, respectively, spanning towards the east-west. Skew angle is around 18 degrees, and the total width of the structure is 36’-0”. The overall construction is made of the adjacent precast prestressed box beams (a 3” hick bituminous wearing surface and 27 total beams) and an 8” thick deck over it. The thickness of the sidewalks and median are 11”-12” each. The detailed project drawings are provided in the Appendix C. Appendix also contains the diagrams given in the mid-span and each end of the span. 

As it occurs in most demolition projects, the bridge should remain open to traffic irrespective of the stage of demolition and building of the new construction.  Since the traffic flow during demolition of the old bridge and building of the new one can be maintained only if traffic is rerouted and the lanes of traffic redistributed through the bridge sections while other sections undergo two-staged process (deconstruction of the existing bridge and construction of the new one), careful planning of traffic rerouting and redistribution must be ensured. 

After consideration of available evidence, it was found out that the first stage presupposed removing and then replacing the southern part of the bridge. That half had 12 precast prestressed box beams. Moreover, it contained associated portions of the deck and a surface above these portions. On the northern part of the bridge, the designers decided to add another section in order to make the bridge wider and, therefore, create additional lanes for transport. As a result of careful planning, the additional constructions (sections with added lanes) did not influence the demolition process of the structure that had to be replaced. 

The second stage of the overall demolition and construction process presupposed replacing those sections of the superstructure that remained after the first stage of the reconstruction was finished. The section that was to be replaced was initially situated in the northern part of the bridge, however, after the first stage of demolition and building it turned out to be in the middle section. The remaining 15 box beams, associated portions of the deck, already existing parapet and sidewalk constitute the part of bridge that had to be replaced. 

3. Determination of Construction Loads

The construction loads, including those of the equipment used, additional material needed, and staff involved in the overall process of demolition and new bridge building need to be considered in order to insure the success of the procedure. The calculations under this project consideration are not presented in this research, however, it is important to take them into account since there is a risk of inadequate design of the original construction for the loads related to rerouting of the traffic for the time of section reconstruction. In the evaluated project (the one about the bridge over West Dupage River), the design loads were calculated for the original structure, so the calculations for the new one were not needed. 

Project management team was the group responsible for choosing all equipment needed for the construction. The equipment was given to the structural engineer so that the professional could do the required calculations. The equipment included the CAT 330DL Hydraulic Excavators (used for the second part of the construction), and BobCat 430 Mini Excavator with hydraulic hammer attachment. Appendix B contains the product specifications considering the weights for the equipment and its components. 

It is important to consider the loads from the existing structure as well as the equipment loads. The process of determining the dead loads (loads of the original structure) for each component of the bridge does not differ much from the design loads of a new construction. Cross-sectional area was used for the calculation of the self weight of the beam. This procedure is done first. Along with the beam’s self weight calculation, the assumed weight of the concrete to be 150 lb/ft3 was calculated. After that, the computer design was used in order to create a weight diaphragm. For that, the computed volume was used and a concrete weight of 150 lb/ft3 was assumed. The last step was to compute the weight of wearing. The width of one beam had an equal to the tributary width. This can be explained due to the fact that each beam was adjusted to the one next to the first one without any space. Wearing surface was assumed to have the calculated value of 150 lb/ft3. As there was a certain sequence of the actions in demolition process, it was marked that the wearing surface should be removed before the process of the demolition of the original structure. In this way, the importance of construction procedures consideration in the structural calculations is demonstrated. The load of the bridge was not contributed to during the deconstruction process, even though the weight of the wearing surface was calculated by the computer program. 

 

Dead loads are considered to create fewer difficulties in calculations than the live loads do. The difficulty is in the number of things to be considered. Thus, the equipment load is the main element in the live load. However, it is significant to consider the weights of the precast box beams. The beams will add to the overall load used for the supporting structure when the beams are lifted. Excavators add the load to the structure locally. Hey are not used concurrently in a very close distance. For this reason, the heaviest load must be considered only if it is in the position that creates the greatest load effect. 

According to the specifications concerning the product (in case of the project the product is the equipment) provided by the product’s producer, the weight of the major equipment used in both stages of the construction process (CAT 330DL and CAT H120 Hydraulic Hammer) transmits 79.7 kips of load to the construction. The load in this case is greater than that of the old construction, which is transmitted with the help of that very equipment. If to use BobCat 430 Mini Excavator with hydraulic hammer attachment, there load transmitted to the structure will not be as great as with the use of the equipment described above. The evaluation of the equipment-related load is continued with the analysis of the live load from the use of the CAT 330DL. 

The bridge has 3 spans. Span 1 and span 2 are the longest; their length is 29’-0”. With regard to these characteristics, the greatest difficulty – the heaviest load occurs when the CAT 330DL is placed on the spans 1 or 3 instead of being placed in the span 2. The excavator is placed on the longer spans when it is needed to take away the beams from the central part of the bridge. If to consider the characteristics of the CAT 330DL provided by the manufacturer, the load is estimated to be spread in equal proportions under the tracks. 

It is needed to evaluate two orthogonal directions in order to envisage the worst case of the placement of the CAT 330DL. The tracks of this piece of equipment can be placed parallel to the longitudinal direction of the beams. Another position of the excavator can be perpendicular to the longitudinal direction of the beams. The first position will be more dangerous than the second since fever beams will be involved in sharing the load from the vehicle. 

It is also essential to determine the number of beams and /or strips of the deck that will be used for carrying the load. The assumptions about caring the load are similar to the deck design. The conservative approach suggests that three beams serve to share the load from the excavator. In reality, more beams participate in the process since all elements of the bridge must be designed in the way that ensures their functioning together as a whole and, therefore, sharing the loads.

4. Capacity of Structural Components for Construction Loading

When the designers determined the center of gravity of the excavator, they computed the live load impact factor. After that, they determined the capacity of the bridge beams. The service limit state and strength states of the capacity were checked. The way the capacities of bridge beams are computed for demolition does not differ from the new construction, but there is no need to cover this subject as it was already discussed in the course. The flexural utilization was 75%; the shear utilization constituted 81% of the overall load capacity. For this reason, the beams were appropriate for the planned loading. 

5. Design Connections

The structural adequacy of the bridge concerning construction loading is not the only aspect to be considered. It is also needed to verify the bridge beams and design lifting connections. In the project under consideration, the beams were lifted with the help of lifting brackets. Therefore, these brackets needed to be designed in such a way that they connected the beams. Moreover, they needed to be checked for the loading when they were utilized. 

The brackets for lifting were made of three HP10x42 sections welted together to form letter C shape. The shape was designed to surround the beam from three sides. During the lift, the brackets are placed at each side of the beam. The brackets were supposed to support the beam and their self weight. There are different methods concerning the way brackets are used. They greatly depend on the material of the beam and construction of the bridge. In case of this project, the most appropriate technique was used. 

Each bracket is designed to support the half of the beam as the beam is lifted by two brackets (one at each end) and its own self weight. With regard to that fact that there is a possibility that one bracket will have to support more than a half of the beam because of the angle of the lift, each bracket is designed in such a way that it is able to support 61% of the weight of the beam. The additional 17% of weight is provided for variations in the weight of the beam that the bracket is intended to support. Both shear and flexural capacity of the bracket elements needs to be checked. The checking procedure is usually applied for the steel components that are not initially designed for the bridge constructions. 

The design should be also ensured for the lifting tabs that are situated on the top of the brackets. The designer needs to arrange the tabs in such a way that they had the same vertical force that was assumed for the brackets and the welds at places of joints. As these calculations are typical for different steel details, they will not be covered in detail in this paper. However, the check of the components was needed for a safe demolition. 

6. Additional Considerations

Additional considerations that are needed to be taken into account while dealing with the bridge demolition issue are as follows. When it is needed to remove only a part of a bridge, plate girders and trusses should are of great importance mostly because of some stability issues. Thus, installing stability bracing is necessary. It is required to support heavy machinery by the bridge beams while the removal of the wearing surface. Therefore, the beams for this machinery also require checking, as well as for the inclusion of the weight of the wearing surface.

During the construction of a bridge that was not designed for the traffic flow, the change ion traffic pattern should be taken into account, as well as the weights of the temporary traffic barriers or any other materials stored on the bridge.

7. Conclusions

Taking all the aforementioned facts into consideration, being aware of the constructing methods, procedures, and equipment are of paramount importance while determining accurately the construction loads in any bridge construction project. Contractor’s project management team are typically charged with the responsibility the appropriate methods and equipment. A structural engineer in the bridge demolition planning process is responsible for determining the adequacy for a bridge under the construction loads for each and every stage of construction.

The procedures that follow the check of adequacy of the bridge demolition process are: determining the procedures of installation and positioning of the construction loads, verification of the capacity of the bridge components for the previously calculated construction loads, as well as the loads form the bridge’s own self weight, and, finally, designing the lifting brackets. It is worthy of note that the aforementioned procedure are applicable only, and only in the case under consideration. Evidently, additional design or checking may be required for other bridges, depending on the conditions and circumstances.

Going by the course discussions, calculations for the demolition of a bridge differ from that, that of required for bridge design is a sense that an inclusion is required of particular construction loads. In this regard, the requirements for design of lifting brackets or other elements required for lifting of any bridge components for removal differ as well. Calculations of construction loads are essential and based merely on the weights of equipment in and out of operation, as well as the placement during construction, in cases when the latter is predetermined, or the worst case loading is variable or unknown. Project specifics predetermine the additional structural components required for bridge demolition procedures. In the case under consideration the required additional components are lifting brackets and tabs. A long-term design of the bridge in service is regarded as the point if primary concern. However, it is necessary to take some construction loadings while the bridge is not yet finished. Unlike bridge construction, bridge demolition is not concerned with the long terms for the components need to serve only over a limited period time, namely, during the bridge demolition procedures. Therefore, demolition calculations and analysis are typically attributed to determining the adequacy of the bridge structure, called to sustain temporary but heavy construction loadings. Additional calculations are necessary in order to check the bridge components after certain parts of bridge were removed. In this case, additional calculations are needed only if strength or stability conditions change, affected by missing of specific elements of bridge. Thus, different focus of the two analyses, i.e. different approaches, should be taken into consideration in structural calculations.

Throughout the bridge demolition and calculations, the principles and physics of the procedures are the same. However, different checks should be made. Determining the adequacy for concentrated construction equipment or stocking of materials is important an important factor in the bridges designed for the long-term dead and live loads. Therefore, bridge strength should be verified as well. Apparently, the bridges that exist nowadays were designed with the help of older codes and materials. Moreover, theses bridges have already been serving for a number of years. For this reason, their strength may not be sufficient may not correspond to the modern standards of the so-called proposed loading. In case a bridge is regarded as insufficient for the construction loading, it is necessary to provide temporary shoring or other support systems in order to guarantee safety during the demolition procedures. On the other hand, equipment or positioning of the equipment may be altered from that originally specified in order to reduce the loads on a bridge and maintain safety.

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