Conclusiones

In this section, all bridge systems presented in Chapter 2 are evaluated and assessed in light of the performance criteria presented in Chapter 3. The performance criteria discussed in Chapter 3 are grouped into four criteria. Each criterion consists of several categories with an assigned numerical rating. The total rating for each criterion is the summation of the numerical ratings for each category. The total score from all four criteria determines the ranking of the described systems. The four criteria with the associated sub-topics along with the numerical rating are presented below:

™ Criteria I : Unit Configurations and Aesthetics (30%)

• Aesthetics (15%)

• Unit Configurations (15%)

Unit configuration is judged by the cost (5%) and the ease of fabrication (10%).

™ Criteria II : Construction and Erection (25%)

• Number of Joints (10%) - Transverse (5%) - Longitudinal (5%)

• Joint Details (5%)

• Ease of Erection (10%)

™ Criteria III : Design Considerations (25%)

• Fatigue Resistance (10%)

• Joints Durability (5%)

• Span Length (10%)

™ Criteria IV : Future Maintenance (20 %)

• Access and Inspection Efforts (10%)

• Protective Coatings (10%)

Tables 4.1 to 4.4 show the results of the evaluation for each stated criteria.

Table 4.1 Comparison of Unit Configurations and Aesthetics Criteria I : Unit Configurations and Aesthetics

Unit Configurations Bridge Type Aesthetics

(15) Cost

(5)

Ease of Fabrication (10)

Total Score (30)

Temporary and permanent truss systems

7 4 10 21

Railroad Flatcar 12 4 8 24

Composite Space

Truss 15 2 6 23

Steel Girders and

Concrete Deck 12 5 9 26

Under-Slung Truss 7 3 7 17

Cold-Formed Steel

Plate Box 13 3 7 23

Table 4.2 Comparison of Construction and Erection Criteria II : Construction and Erection

Number of Joints Bridge Type Transverse

(5)

Longitudinal (5)

Joint Details

(5)

Ease of Erection

(10)

Total Score (25) Temporary and

permanent truss systems

3 3 3 9 18

Railroad Flatcar 5 5 5 9 24

Composite Space

Truss 3 5 3 6 17

Steel Girders and

Concrete Deck 5 5 4 9 23

Under-Slung Truss 5 5 4 7 21

Cold-Formed Steel

Plate Box 5 5 4 8 22

Table 4.3 Comparison for Design Considerations Criteria III : Design Flexibility and 75 Years Service Life

Bridge Type

Fatigue Resistance

(10)

Joints Durability

(5)

Span Length

(10)

Total Score (25) Temporary and

permanent truss systems

4 3 8 15

Railroad Flatcar 7 5 6 18

Composite Space

Truss 7 5 9 21

Steel Girders and

Concrete Deck 8 5 9 22

Under-Slung Truss 7 5 7 19

Cold-Formed Steel

Plate Box 5 5 6 16

Table 4.4 Comparison of Future Maintenance Criteria IV : Future Maintenance

Bridge Type Access and Inspection Efforts

(10)

Protective Coatings

(10)

Total Score (20) Temporary and

permanent truss systems 4 4 8

Railroad Flatcar 7 7 14

Composite Space Truss 7 9 16

Steel Girders and

Concrete Deck 8 8 16

Under-Slung Truss 6 7 13

Cold-Formed Steel Plate

Box 3 8 11

Table 4.5 shows the overall numerical scoring of all bridge systems.

Table 4.5 Numerical Comparison of Existing Bridge Systems Bridge

Type

Unit Configurations

and Aesthetics (30)

Design Flexibility and 75 Years

Service Life (25)

Construction and Erection

(25)

Future Maintenance

(20)

Total Score (100)

Temporary and permanent truss systems

21 15 18 8 62

Railroad

Flatcar 24 18 24 14 80

Composite

Space Truss 23 21 17 16 77

Steel Girders and Concrete Deck

26 22 23 16 87

Under-Slung

Truss 17 19 21 13 70

Cold-Formed Steel Plate Box

23 16 22 11 72

It can be seen from the results that the top three bridge systems are the Steel Girders and Concrete Deck, Railroad Flatcar and Composite Space Truss in the order of ranking. In order to closely examine each of these systems it is important to study the advantages and disadvantages of each system. The results from this comparison will help in formulating the new bridge concepts. Following is an examination of these three systems:

The rating values shown in the tables are subjective and are based on the experience of

the research team with consultation with consulting engineers, bridge fabricators, and

contractors. While rating of the established criteria might slightly vary, the conclusions

will remain the same.

1. Steel Girder / Precast Concrete Deck System

The steel girders with cast-in-place or precast decks are the most common elements in steel bridge construction.

Advantages:

¾ Improved efficiency with lighter steel beams (INVERSET brand system).

¾ Uses standard rolled shapes and welded plate girders.

¾ Economical / average construction costs.

¾ Can be fabricated with exact camber & skew to meet existing site requirements.

¾ Top of deck can be textured for riding surface.

¾ Easy & rapid erection & construction.

¾ Cast-in-place concrete not required at joints.

¾ Suitable for use as continuous spans.

¾ Durable since cast in controlled conditions.

Disadvantages:

¾ In the case of the INVERSET type system, units are cast in “Upside-Down”

Position and must be turned over in manufacturing plant after casting.

¾ Weight limit for transportation may limit the use in long spans.

2. Railroad Flatcar System

As described earlier, the railroad flatcar system has been used in the construction of bridges on low-volume roads. The system relies on the availability of used and discarded railroad flatcars which limits the general use of this system. Therefore, this system is not promoted for widespread use; however, there are several aspects of the system, such as use of the flatcar as a stay-in-place form and details of the longitudinal joints, that can be implemented in a new general concept.

Advantages:

¾ Modular system.

¾ Easy and rapid erection & construction.

¾ Economical – about 2/3 cost of new bridge.

¾ Can be used on existing abutments & piers.

¾ Requires low maintenance if detailed properly.

¾ Effective, easily fabricated and constructed y longitudinal joints.

Disadvantages:

¾ Unknown fatigue resistance.

¾ Use only allowed on low-volume roads.

¾ Limited availability of usable flatcars.

¾ Supports must be at flatcar axle locations.

¾ Limited span lengths.

¾ Not suited for use as continuous spans.

3. Composite Space Truss System:

The composite space truss system scored the highest in aesthetics when compared to all other systems.

Advantages:

¾ High stiffness / weight ratio.

¾ Alternate / redundant load paths.

¾ Aesthetically pleasing appearance.

¾ Durable precast concrete deck slabs.

¾ Has great potential for modular design.

¾ Has potential for greater span lengths.

¾ Cast-in-place concrete not required at joints.

¾ Possible low maintenance if detailed properly.

¾ Suited for use as continuous spans.

Disadvantages:

¾ Current high cost due to complex fabrication.

¾ Complex welded ”K & Y” joint fabrication.

¾ Large number of transverse joints between panels.

¾ Possible need for riding surface preparation.

¾ Erection more difficult than other systems.

¾ Critical longitudinal connectivity of modules.

This system was closely examined by the research team due to its aesthetically pleasing appearance. The system could be modified as shown in Figure 4.1 to expand its potential. Figure 4.1 shows a bridge concept consisting of several modular units tied together to form a bridge. Each modular unit consists of a fully prefabricated concrete deck supported by a space truss. The concrete deck can be prefabricated at the factory and transported to the bridge site or prefabricated close to the bridge and lifted into place.

Figure 4.1 Modified Composite Space Truss System

The research team contacted several steel bridge manufacturers inquiring whether the tubular truss system can be easily manufactured with their existing equipments and fabrication techniques. All contacted fabricators voiced reservation on the practicality of this system and on their willingness to move into this type of fabrication. This fact alone will hinder the acceptance and widespread use of such system in the United States.

Therefore, while at first glance this system offered the best option, the research team

believes that it does not meet the objectives of the project.

In document Núm. 14 (2016) (página 102-106)