Short Span Steel Bridge Secondary Elements

Railing Systems

Railing systems are required to help safely keep vehicles on the bridge structure. Barriers and railing systems are rigidly attached to the bridge and designed to handle impact loads from errant vehicles and redirect the vehicle away from edge of the bridge. This section will specifically look at steel beam rails and precast concrete barriers that are designed to provide safe railing systems to short span modular steel bridges.

Steel Beam Rail


Description

This railing system includes a combination of strong posts and steel beams used to guide errant vehicles back onto the roadway. A common steel section for this type of barrier is a W-beam. Versions of these barriers have proven to be at least a Test Level 3 or better according to the testing system established by NCHRP 350. An example of a bridge using this type of railing can be seen if Figure 53 and a closer look at the connection is provided in Figure 54. [51]


Figure 53 Steel Beam Rail Barrier (U.S. Bridge Tour)


Figure 54 Steel Beam Rail Connection (U.S. Bridge Tour)

Application

Steel beam rails are installed on the bridge in order to provide a protection to the users so as not to allow them to leave the travel way. [51]

Constructability

One method of connecting steel beam rails can be seen in Figure 54, a portion of the railing system is welded to the exterior girders of the bridge. Another method of attaching the railing system is by mounting the posts directly on top of the bridge deck. For either situation, the connection must provide enough strength to resist the force of an errant vehicle collision.

Evaluation

Steel beam rails are lighter than concrete barriers and they impose on the roadway less allowing for a narrower bridge deck. As opposed to concrete barriers, steel beam rails do not have the issue of holding water on the bridge roadway. Connection for this type of railing system may involve on-site welding.

Research Needed


Precast Concrete Barrier


Description

Precast concrete barriers are a common method used to keep errant vehicles from leaving the travel way. There are different shapes of this type of barrier, but the most common two are the New Jersey and F-Shape barriers. Precast concrete barriers are designed to be placed and connected to adjacent sections and provide enough resistance to prevent vehicles from leaving the road. An example of a precast concrete barrier is provided in Figure 55. [47]


Figure 55 Precast Concrete Bridge Barrier [47]

Application

Precast concrete barriers are installed on the edges of a bridge in order to keep errant vehicles from leaving the travelway. [47]

Constructability

These barriers can be connected to one another using an interlocking system. The barrier as a whole can be attached to the bridge deck using a mechanical keyway and a grouting material. Other such systems may utilize vertical reinforcement or other anchorage systems to hold the barriers in place on the bridge. [47]

Evaluation

Precast concrete barriers are attached to the top of the bridge deck instead of being attached to the exterior girders of the bridge possibly causing the need for a wider bridge deck than that needed when steel beam rails are used. This type of barrier does have the potential to cause water retention on the deck which can cause safety issues. With the weight of this type of barrier, the bridge has a larger composite dead load than that of a steel beam railing system.

Research Needed


Cross-Frames and Diaphragms

In the design and construction of steel plate girder bridges, several configurations of cross-frames and diaphragms have been used to provide lateral support to the bridge frame. This section will specifically look at the use of "X" shaped cross-frames, "K" shaped cross-frames and folded plate diaphragms.

"X" Shape Cross-Frame


Description

There are three primary configurations of the "X" shaped cross-frame: simple "X" configuration, "X" shape with a bottom strut and "X" shape with bottom and top struts. [28]

The simple "X" configuration while being the most economical to fabricate, may not provide the most cost-effective bridge overall. For certain bridges it is possible that this type of cross-frame can provide proper support for both lateral loads and cantilever concrete casting loads; but in cases where the braces cannot handle the weight of wet concrete on the overhangs properly, additional bracing will be required. [28]

The addition of a bottom strut to the simple "X" configuration provides a more rigid path connecting the bottom flanges of all the girders. This connection can provide the needed extra support for the overhang loads during construction. This system is assuming that the stresses due to lateral wind loads on the bridge are migrating to the bottom strut. [28]

The "X" configuration with both top and bottom struts ensures the designer that the top and bottom flanges of the girders are braced to resist the lateral wind loads and cantilever overhang loads acting on the bridge. Generally, this system is only needed for deep girders or large diaphragm spacings. [28]


Figure 56 Example of Steel "X" Shaped Cross Frame [10]

Application

"X" shape cross-frames are installed into the gaps between bridge girders in order to provide lateral support to the bridge superstructure. [28]

Constructability

The cross-frame elements are generally bolted to stiffeners that are welded to the webs of the bridge girders. [28]

Evaluation

With the different configurations of "X" shaped cross-frames, the engineer can use this system to provide lateral bracing to nearly any steel plate girder bridge. Generally, "X" shape cross-frames are more economical than "K" shape cross-frames.

Research Needed


"K" Shape Cross-Frame


Description

"K" shaped cross-frames are similar to "X" shaped cross-frames in that they are composed of multiple steel members to provide lateral strength to the superstructure. Where "X" shaped cross-frames are more efficient when the ratio of girder spacing to girder depth is approximately 1, "K" shaped cross-frames are better when this ratio is greater than 1.5. An example of a bridge using "K" shaped cross-frames is provided in Figure 57. [10]


Figure 57 Curved Steel Bridge Frame with K-Shaped Cross Frames [26]

Application

"K" shape cross-frames are installed into the gaps between bridge girders in order to provide lateral support to the bridge superstructure. [10]

Constructability

The cross-frame elements are generally bolted to stiffeners that are welded to the webs of the bridge girders. [10]

Evaluation

"K" shaped cross-frames are not always the most cost effective option for lateral support to a bridge superstructure. As mentioned, for cases where the ratio of girder spacing to girder depth is over 1.5, "K" shaped cross-frames are considered to be the efficient choice.

Research Needed


Diaphragms


Description

Diaphragms, like other cross-frame systems, are included in the steel framework of a bridge to help the bridge resist lateral loads. As opposed to the "X" shaped and "K" shaped cross-frame systems, diaphragms consist of single members performing the lateral bracing. Diaphragms are normally "I", "C" or "T" shaped steel members. An example of a bridge using steel diaphragms is presented in Figure 58. [10]


Figure 58 Bridge with Steel Diaphragms [29]

Application

Diaphragms are installed into the gaps between bridge girders in order to provide lateral support to the bridge superstructure. [10]

Constructability

The ends of the diaphragms are either welded or bolted to stiffener plates attached to the webs of the bridge girders. [10]

Evaluation

A downside to this type of lateral bracing is that inspection becomes difficult unless proper precautions are taken (ex: manholes).

Research Needed