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CHAPTER 4: THE CONSTRUCTION PROCESS OF SEGMENTAL BRIDGES - page 43 / 47

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Chapter 4: The Construction Process of Segmental Bridges

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bridge decks, retaining walls, and railings (AASHTO 1996). Seismic design is covered in a separate section.

In particular, the AASHTO Standard Specifications for Highway Bridges provide information on the following actions that may affect the structure: Dead load, live load, impact or dynamic effect of the live load, wind loads, other forces, such as longitudinal forces, centrifugal force, thermal forces, earth pressure, buoyancy, shrinkage stresses, rib shortening, erection stresses, ice and current pressure, and earthquake stresses (AASHTO 1996, p19). However, no reference is given to erection stages in this context. Section 8.15.3 of Division II—Construction is dealing with construction loads and states the following (AASHTO 1996, p493):

“Light materials and equipment may be carried on bridge decks only after the concrete has been in place at least 24 hours, providing curing is not interfered with and the surface texture is not damaged. Vehicles needed for construction activities and weighing between 1,000 and 4,000 pounds [0.454 and 1.814 metric tons], and comparable materials and equipment loads, will be allowed on any span only after the last placed deck has attained a compressive strength of at least 2,400 pounds per square inch [16.55 N/mm2]. Loads in excess of the above shall not be carried on bridge decks until the deck concrete has reached its specified strength. In addition, for post-tensioned structures, vehicles weighing over 4,500 pounds [2.041 metric tons], and comparable materials and equipment loads, will not be allowed on any span until the prestressing steel for that span has been tensioned.”

Finally, the provisions of the AASHTO Standard Specifications for Highway Bridges give actual information on how to treat erection methods (AASHTO 1996, p493):

“Otherwise, loads imposed on existing, new or partially completed portions of structures due to construction operations shall not exceed the load-carrying capacity of the structure, or portion of the structure, as determined by the Load Factor Design methods of AASHTO using Load Group IB. The compressive strength of concrete (f’c) to be used in computing the load-carrying capacity shall be the smaller of the actual compressive strength at the time of loading or the specified compressive strength of the concrete.”

Table 3.22.1A of Division I—Design provides coefficients for different combinations of loads, including the aforementioned Load Group IB for both Load Factor Design and Service Load. The following Table 4-5 gives the values of the aforementioned table particularly applicable for construction loads, i.e. Load Group IB within Load Factor Design (AASHTO 1996, pp30f):

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