Bridge Type
Selection
1. Engineering, architectural (when warranted),
and cost studies shall be prepared for each structure or group of structures.
Where several structures are in close proximity with each other, studies may be
prepared to show possible interaction with each other.
In the consideration of the need for a movable
bridge structure, the long term investment associated with machinery
maintenance, liabilities associated with navigation hazards and staffing the
structure with operators should be considered. Also, the impact of traffic
congestion due to openings should be considered. These issues should be
addressed in assessing the cost and practicality of a movable bridge versus a
fixed bridge.
2. These initial studies should be developed from
a careful appraisal of the site, foundation, drainage conditions, highway
limitations, and environmental impact, both present and future. The structural
types proposed as a result of these studies must be based on the highest
standards of creativity and engineering technique.
3. For a group of bridges in a contract, structure
types should be similar so that similarity of construction details may result
in economy of costs.
4. New materials and developments may be
incorporated in the design of the proposed structures. This is provided that
approval has been given by the Manager, Bureau of Structural Engineering.
5. Economy, aesthetics, maximum safety and
infrastructure security are compatible in the design of structures. For grade
separation structures, in urban as well as rural areas, the absence of shoulder
piers allows for possible future widening of the lower roadway while removing
sight line restrictions and minimizing safety hazards. The resultant
"open" structure usually results in a more pleasing appearance.
6. In planning new bridges, the list of available
structure materials and types of construction should be considered. The use of
High Performance Steel and High Performance Concrete is encouraged. At any
given location, the ultimate selection should be based on suitability and
aesthetics. This is with consideration of the bridge and its site as an entity
and also as part of the surrounding environment.
The character and coloration of the terrain and
the form of nearby structures should all be influences on the aesthetics
proposed for the structure.
7. Superstructures of shallow proportion shall be
strived for; however, stiffness requirements and other design considerations
must be balanced against those of aesthetic appeal. Ordinarily, the
superstructure should be of uniform depth from end to end. Unsightly details,
which present abrupt discontinuities in the bridge profile, should be avoided.
8. In arriving at span proportions, substructure
elements should be positioned well clear of travelled roadways. Minimum lateral
clearances are illustrated therein. Where considerations of economy permit,
abutment faces should be at least 30 feet from roadways. Planning along these
lines should result in proportions which are economical, aesthetic, and that
provide maximum safety for the travelling public.
9. Abutments and wingwalls should be made as
inconspicuous as possible by limiting the exposed height of the abutment
(preferably stub to semi-stub). An appropriately aesthetic type treatment shall
be proposed for all large exposed surfaces.
10.Concrete piers which are built near roadways
should generally be of open-type construction (i.e. column bent piers). When
supporting a multitude of closely spaced stringers, a common and simple frame
consisting of a uniform depth cap beam on circular columns may be suitable.
Often times, frame proportions are enhanced by allowing the cap beam to
cantilever over the exterior columns with a variable depth that tapers to a
minimum beyond the fascia stringer bearing. The slender tee-pier should not be
overlooked for the support of high crossings or narrow structures.
11.New
designs, as well as major rehabilitation work for high level or complex
structures, should include permanent provisions for inspection, such as
catwalks, in order to make bridge members accessible. Bridge design engineers
should ensure that easy and adequate access can be achieved, especially to
pin-hanger assemblies, fatigue prone details and fracture critical members.
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