Logged in as: Public User

F-Shape PCB Transition to Rigid Barrier – Thrie Beam Splice

Question
State FL
Description Text

I'm working on our FTB to rigid barrier thrie beam splice drawings and I need to ask you a few questions about the design.  I've attached PDF's of our preliminary drawings:

 

(See attached file: 0715s08 2.pdf)(See attached file: 0715s08 1.pdf)

 

When you get time, please take a look at these drawings and the following questions:

 

1.  What size and type of bolts are used to attach the thrie beam terminal connectors to the barriers?  In our drawing 8 of 9 above we've shown a through bolted connection using five 7/8" dia. HS bolts (ASTM A449 Type 1) with hex nuts and washers under the nuts and heads.  This is what we use for permanent attachments of thrie beam to our concrete traffic railings. In the attached photo you sent me, it looks like some kind of big lag screw (3/4" dia.?) was used for the test installation:

 

(See attached file: DSCN2687.JPG)

 

Can you give me any specs or the name of a manufacturer for these lag screws/bolts?  Do these screws/bolts require the use of an expansion sleeve?  If we wanted to use through bolting only, do you see any problems other than fit up of the bolts and the terminal connectors on the two opposite sides of the barriers?

 

2.  Proper lateral positioning of the FTB relative to the end of the permanent barrier is proving to be a challenge with all the different shape barriers and end transitions we currently use or have used in the past - 32" F shape (shoulder and median), 42" F shape, 32" Jersey shape (shoulder

and median), 32" and 42" vertical face, corral shape, 8' F shape / soundbarrier, etc.  On drawing 8 of 9 above we show two combinations: 1) FTB to permanent median barrier and 2) FTB to an old FDOT style shoulder barrier transition.

 

Depending on the shape and overall thickness of the permanent barrier and lateral positioning of the FTB, potential snag points could be created. We're still looking at all our possible combinations of approach and trailing end configurations.  One combination with a possible snag point is shown on the upper right corner of drawing 8 of 9.  If the direction of

adjacent traffic is right to left on this plan view, the exposed lower end of the FTB could be a snag point.  To reduce this potential I'm thinking of filling the cross hatched area with miscellaneous asphalt.  If the direction of adjacent traffic is left to right on the plan view, the

potential for a snag looks to be minimal.

 

 

Another problem seen in this same plan view is bending and fit up of the thrie beam guardrail on the back side of the barrier.  I'm thinking of using offset blocks on the back side of the FTB between the FTB and the terminal connector so the thrie beam doesn't have to be bent as shown.  A second offset block may be needed at the end of the FTB to engage the middle portion of the thrie beam.

 

When you get time, please take a look at these issues and let me know what you think.

Keywords
  • Temporary Barriers
Other Keywords none
Date October 5, 2005
Attachment 07246.pdf


Response
Response

I got a chance to go through your PCB transition questions this morning. I have written my responses and comments below in red.

1. What size and type of bolts are used to attach the thrie beam terminal

connectors to the barriers? In our drawing 8 of 9 above we've shown a

through bolted connection using five 7/8" dia. HS bolts (ASTM A449 Type 1)

with hex nuts and washers under the nuts and heads. This is what we use

for permanent attachments of thrie beam to our concrete traffic railings.

In the attached photo you sent me, it looks like some kind of big lag screw

(3/4" dia.?) was used for the test installation:


(See attached file: DSCN2687.JPG)


Can you give me any specs or the name of a manufacturer for these lag

screws/bolts? Do these screws/bolts require the use of an expansion

sleeve? If we wanted to use through bolting only, do you see any problems

other than fit up of the bolts and the terminal connectors on the two

opposite sides of the barriers?


The bolts used in the full-scale test were 3/4" dia. by 6" long Powers Fasteners Wedge Bolt Anchors. We have been using these anchors quite a bit lately and have been happy with them. We use them to attach the SAFER barrier to the concrete walls of the race tracks. I have attached the specs for this anchor. It does not use a sleeve. On important note on these anchors is that we used the anchors with a minimum 8" spacing between anchors. The performance of the anchors can degrade under smaller spacings. As such, you may need to change the anchor layout shown on you drawing to use the three outer holes in the end shoe and the two inner holes. This is shown in the picture you emailed me. This layout gives the 8" spacing.


As far as through-bolting the end shoe, that is fine as well. You may encounter issues hitting rebar as you mentioned.


2. Proper lateral positioning of the FTB relative to the end of the

permanent barrier is proving to be a challenge with all the different shape

barriers and end transitions we currently use or have used in the past -

32" F shape (shoulder and median), 42" F shape, 32" Jersey shape (shoulder

and median), 32" and 42" vertical face, corral shape, 8' F shape /

soundbarrier, etc. On drawing 8 of 9 above we show two combinations: 1)

FTB to permanent median barrier and 2) FTB to an old FDOT style shoulder

barrier transition.


Depending on the shape and overall thickness of the permanent barrier and

lateral positioning of the FTB, potential snag points could be created.

We're still looking at all our possible combinations of approach and

trailing end configurations. One combination with a possible snag point is

shown on the upper right corner of drawing 8 of 9. If the direction of

adjacent traffic is right to left on this plan view, the exposed lower end

of the FTB could be a snag point. To reduce this potential I'm thinking of

filling the cross hatched area with miscellaneous asphalt. If the

direction of adjacent traffic is left to right on the plan view, the

potential for a snag looks to be minimal.


Charles, as you mentioned, the snag risk at this location is not significant unless the potential for two way traffic and reverse hits is present. However, we would recommend that the potential be reduced in this situation. Ron and I discussed you suggestion to use asphalt to fill in the hatched area. We believed that this should reduce snag, but we have a couple of concerns. First, it would be important to use the fill asphalt to make a smooth and complete fill of the hatched area. The fill should completely cover the area and any gaps would be undesirable. Second, we are a little concerned about the ability of the asphalt to prevent rim and wheel of an impacting vehicle from gouging or digging into the asphalt and creating a snag hazard or exposing a corner of the barrier. A better method may be to make a simple form and place a concrete fill in the hatched area. This will be more resistant to gouging during the impact and provide better reduction of snag potential.


Another problem seen in this same plan view is bending and fit up of the

thrie beam guardrail on the back side of the barrier. I'm thinking of

using offset blocks on the back side of the FTB between the FTB and the

terminal connector so the thrie beam doesn't have to be bent as shown. A

second offset block may be needed at the end of the FTB to engage the

middle portion of the thrie beam.


You can use offset blocks to eliminate the need for the bending the thrie beam in this situation, but there are some things to consider. First, you will want to offset the beam from the barrier at two locations at a minimum and more would be preferable. You would want to block the area near the joint between the barriers as well as the of the end shoe connection at minimum. It should also be noted that the use of the blockouts may introduce bending loads into the bolts or anchors used in the end connection with the blockout. Thus you may want to check and possibly increase the capacity of these anchors. I would not recommend using the Wedge Bolt anchors in this installation due to lack of bolt embedment and the potential for bending loads. Instead I would require the through-bolt option here.


When you get time, please take a look at these issues and let me know what

you think.


I have a couple of other comments about your CAD with the four types of installations. On the top installation, the median installation, there may be some issues with reverse hits on the non-staked side of the transition barriers downstream of the permanent barrier. I think we have discussed before that impacts with the stakes on the back side of the barrier are very likely to result in increased rotation of the barrier sections and thus increased potential for vehicle instability. As such, we are not very excited about the use of the transition in the median area. However, we realize that you may have no better options at this time. Until we have investigated this issue more thoroughly, the best recommendation I can give is that we are concerned about reverse hits in this installation due to the potential increase in barrier rotation and resulting increase in vehicle instability. Another thing to keep in mind with the median installation is slopes. Do you know what kind of slopes you will be installing the barriers on??


My last comment concerns the fourth installation detail at the bottom of the page. This trailing end transition should be fine for installations only dealing with one way traffic, but on narrow highways with two way traffic, the potential for reverse hits on this installation is significant. In that case we would recommend you install the approach transition here.


Date October 10, 2005


Response
Response

Here is a sketch that shows our old F and NJ shape traffic railing transition, an FTB and a Thrie Beam Splice:


(See attached file: Old Transition.pdf)


This section of traffic railing is supported by the wing wall that is part of the end bent. The bridge would be at the far left side of the sketch.


The old W beam guardrail end shoe would have bolted up to the 2'-6" long "End Post" section. We plan to fill in the recess at the bottom of this section with concrete to reduce the snag potential for traffic moving left to right. For traffic moving right to left, we plan to leave the recess open. The Offset Blocks at the back of the FTB are there to keep the Thrie Beam straight (unbent).


The "Varies" dimension is drawn at 1'-0" but could be any length from 0' to many feet. Our problem shows up at the left end of the "Varies" dimension where the Thrie Beam Terminal Connector straddles the deck expansion joint.


We can't bolt across this joint with the Terminal Connector as thermal movements of the bridge would likely tear the connection apart. Thus we're thinking of using a 25' stick of thrie beam to get the left Terminal Connector (as seen in the sketch) up on the bridge beyond this problem area. The right Terminal Connector would be attached to the FTB at the same location as is currently shown.

Date October 24, 2005
Attachment Old Transition.pdf


Response
Response 1. The schematic you sent me is acceptable even with a 12" gap between the final PCB and the bridge transition piece as long as the there is only one way traffic on the roadway. If traffic is moving from the PCB's towards the bridge in your schematic, then the chance for snagging on the end of the bride is minimal, and the thrie beam sections should possess sufficient capacity to hold the joint between the bridge transition and the PCB together even with the larger gap.


If the traffic is moving the other direction, you said you planned to fill the overlap area with concrete to reduce the snag potential. We agree that this is necessary and quite critical. It may be your best option to fill that area for now, but you may want to think about a redesigned transition section in the future to reduce the snag potential. In general, we would recommend not running two way traffic in this type of installation unless the snag issues can be sufficiently eliminated.


2. As far as the connection to the bridge rail on the downstream end, I agree that you don not want to straddle the expansion joint. A simple fix for this problem would be to shift the thrie beam sections upstream or downstream as needed to prevent straddling the deck joint. Shifting the thrie beam 12"-18" in either direction should not have a large effect on performance, and it would be simpler and cheaper than using a 25' long section to guarantee that you extend past the joint. For the installation in the schematic, you could shift the thrie beam sections upstream a few inches until the end shoe was positioned on the flat face of the end post/transition piece. We would probably prefer this shifting of the thrie beam rather than using the longer section.
Date October 31, 2005


Contact Us:
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
(402) 472-0965
Email: mwrsf@unl.edu
Disclaimer:
The information contained on the Midwest Roadside Safety Facility (MwRSF) website is subject to change without prior notice. The University of Nebraska and the MwRSF is not responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the use or misuse of or reliance upon any such content, goods, or services available on this site.