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Steel Bridge Railing Question

Question
State IA
Description Text

What would be your opinion of installing a steel bridge railing (Illinois 2399 curb-mount) at standard post spacing (6'-3" as tested), but increasing the post spacing at four locations on the bridge in order to accommodate some structural members?  Our consultant feels they can limit the maximum post spacing at these locations to 7'-6".  Do you think allowing the larger post spacing at these locations would be feasible without additional testing, or should we be investigating other options?


There would be only 1 spacing of 7'-6" at each of the four locations on the bridge.

Keywords
  • Bridge Rails
Other Keywords none
Date June 3, 2009


Response
Response

MwRSF feels that increasing the post spacing from 6'-3" to 7'6" in only a few non-adjacent spans is a possible task.  However, the bridge rail must be stronger to accommodate the 20% increase in moment due to the elongated post spacing.  As such, we recommend the following:

 

Replace the 4"x4" bottom tube with another 8"x4" tube (the top tube).  Thus, the bridge rail would consist of 2 8"x4" tubes.  Assuming the top and bottom rail carry equal loads (which it really doesn't " top takes more load), this small change would provide a 30% increase in rail strength - enough to accommodate the 20% increase in moment.

 

This rail combination should be used throughout the bridge to ensure rail continuity and prevent snag points

 

Also, keep the bottom of the lower tube at 14" above the roadway.  Thus the top of the lower tube is 22" above the roadway (2" gap between rails).  This will allow the lower rail to better interact with an impacting vehicle and absorb more of the impact load. 

 

Date June 15, 2009


Response
Response

We have an additional question to follow up the attached email which recommended that an 8" x 4" tube be used on the bottom rail throughout the bridge.


Since this bridge is relatively long, using an 8" x 4" tube for the bottom rail over the entire length would result in a significant increase in the steel quantity and cost. (The length of bridge to receive new rail is about 3,000 feet and the weight difference between a 8 x 4 x 5/16 tube and a 4 x 4 x 1/4 tube is 11.14 pounds per foot. Thus there would be an increase in steel of about 2 x 3,000 feet x 11.14 lb./ft. = 66,840 pounds.) Also, we would like to minimize the additional total dead load that is added to the bridge since the weight capacity of the bridge is an issue. (We are even planning to use lightweight concrete for the curbs on this project.)


In view of this, would it be possible to strengthen the rail at only the few areas where the span would exceed 6' 3"? In order to accomplish this, could the rail be strengthened at just those longer rail spans and any necessary adjacent spans, while using a 4 x 4 x 1/4 tube for the bottom rail throughout the rest of the bridge? The following are some ideas for your consideration to accomplish this:


Increase the wall thickness of the standard top and bottom rails in order to get a 20 % or greater increase in the section modulus (S) for bending. This would result in no change in the outside railing geometry.


Install a tubular member inside of the standard top and bottom rails in order to get a 20 % or greater increase in the section modulus for bending. For example, a 4 x 4 x 1/4 tube has a S of 3.90 inches^3. If a 3 x 3 x 3/16 tube (S = 1.64 inches ^3) were inserted inside of the 4 x 4 tube, the total S for the bottom rail would be increased by 42 %. This would result in no change in the outside railing geometry.


Add another 4 x 4 x 1/4 tube directly above the standand 4 x 4 x 1/4 bottom rail to increase the bending strength. In order to avoid a snag point, this section would need special fabrication at the ends for a transition down to the typical bottom rail.


Replace the bottom rail with a 8 x 4 x 5/16 tube as recommended in the attached email, except fabricate a special transition down to a 4 x 4 x 1/4 tube at the ends in order to avoid a snag point.

Please let us know if any of the above concepts would be acceptable, and if so, we will ask the consultant to investigate further.


Date June 16, 2009


Response
Response

We do feel that we can strengthen the rail in the areas surrounding the extended post spacing only. With a 3,000 ft bridge, using the increased rail size for the entire system would be wasteful. Comments on the proposed solutions are discussed below.


(1). Using a thicker / stronger rail in certain areas will result in abrupt stiffness transition points at the connections between the two rail types. These stiffness transitions could lead to vehicle instabilities or snagging.


(3) & (4) Altering the shape of the rail in these locations can lead to more vehicle interaction problems (snagging, instabilities, wedging, etc...). As such, we do not favor the option of transitioning between different rail geometries without testing these transitions.


(2) MwRSF does like the tube-in-a-tube idea for strengthening the rail. The inserted tube should fit relatively snug inside the original tubes, so that the smaller tube develops load before the rail suffers larger deformations. The 3x3 tube inside of the lower rail (4x4x1/4) tube is a good fit. However the upper rail should also be reinforced. The same 3x3 tube could be used if its position could be centered inside the 8x4 (perhaps resting it between the attachment bolts, bolting through the 3x3 tube, or using spacers to position the 3x3 tube inside the 8x4 tube.


The inserted reinforcement tubes should be extended out from elongated spacing, though the adjacent spacing of 6'-3", and to the nearest ¼ spacing. The 1/4 points of the rail are recommended for the stiffness transition to prevent the tube end from occupying a point of maximum deflection / deformation (midspan) or a stress concentration point (at the posts). Thus, the inner tubes should be extended 94 inches past the posts of the longer spacing (6'-3" plus 19"). Total length of the inner tubes would then be 188 inches plus the length of the longer post spacing (approximately 7'-6" from your previous e-mail.

Date June 24, 2009


Response
Response

I've got one more (hopefully the last) request for you regarding our I-74 bridge rail replacement. Apparently our consultant, rather than incorporating your previous advice, has developed an alternate method for spanning the wide expansion joints on the I-74 bridge. This method places specially-designed posts on either side of the joint, spaced 5 feet apart.


Could you please review and comment on the attached drawings showing the proposed design? Just as before, this will be used at a total of four locations on the bridge - on both sides of the road at each of the two suspension towers.


The post spacing varies in order to avoid the vertical stringers located just beyond the edges of the bridge deck.


The consultant felt that he needed to space the corbels (and therefore the posts) in order to avoid the vertical trusses due to the tight tolerances (see the attached picture of the current bridge). The vertical trusses are located approximately 1'-5" behind the face of rail. Would you agree that even if a post were placed at a truss location, that the truss would lie outside the working width of the barrier?


The proposed spacings have not been analyzed. Do you feel the abrupt changes in post spacing throughout the bridge is concerning enough to warrant a possible redesign? If we could somehow reduce the depth of the corbels, perhaps that would allow them to be installed at truss locations?

Date July 14, 2010


Response
Response The full scale testing on the original Illinois steel tube bridge shows a maximum dynamic deflection less than 3 inches. Also, although the working width of the system was not specified in the summary pages, the vehicle does not appear to extend more than 12 inches past the face of the rail. Thus, the 1'-15" of clear space between the face of rail and the vertical trusses provides enough room to minimize the risk of vehicle snag on the truss members. Further, the 17 inches of space matches that of the recommended offset from the head ejection envelope developed in TRP-03-194-07 for the 95th percentile passenger (14 in. + 3 in. = 17 in.).


With a maximum dynamic rail deflection < 3" with the post spacing at a constant 6'-3", I am not concerned about pocketing of this rail when the spacings you have shown are all between 5'-6" and 3'-6". All the extra posts will only stiffen the bridge rail. Therefore, the post spacings you show are acceptable and the splice at the tower locations should work.
Date July 15, 2010


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