INDOT has been reviewing our current standard drawings for bridge rails. We currently detail out a side-mounted bridge rail identified as TS-1, with a NCHRP-350 TL-2. This railing is only used on local roadways (not state or NHS routes). This railing uses a structural-tubing post (TS 6"x3"x1/4"), see the attached PDF (TS-1 INDOT Details) that includes all three sheets of our standard drawings. This bridge rail is very similar to SBT01a, see the attached PDF (SBT01a) found in the Guide to Standardized Highway Barrier Hardware. INDOT, as you can see on sheet 3 of the attached details, lengthened the anchor bolt with a piece of structure tubing (spacer) on either side of the post. The test level of TL-2 was given based on the fact that it was indicated as an SL-1 railing in NCHRP-239 (Multiple Service Level Highway Bridge Railing Selection Procedures). According to a document, Bridge Railing Design and Testing, also attached (Document), in a FHWA memo date August 13, 1990 any bridge railing indicated as SL-1 in NCHRP-239 could also be considered equivalent to a PL-1 . Further in the same document it mentions the Railing Level Equivalency Table which allows all PL-1 tested rails to be considered as NCHRP-350 TL-2 with no further testing required. So with all this said, we have a few questions.
1. Does the addition of the structural tube spacers at the anchor bolts affect the design? See attached figure SBT01a and attached figure TS-1 INDOT Details.
2. Is SBT01a still in the Guide to Standardized Highway Barrier Hardware? We only have a hard copy of an older version. In a later version of the guide I could only find SBT01b which uses a steel beam rather than a structural tube.
3. In the crash test I believe a post spacing of 8'-4" was used, our standards allow for spacing to vary between 6'-3" and 8'-4" is this ok?
4. The transition, as you can see in the attachment TS-1 INDOT Details, page 3, INDOT uses a different length and post spacing for the transition than shown in the crash report, see attachment TS-1 Crash Test Detail. Are both transition ok? If not which should we use?
5. Do you think we could replace the symmetrical thrie-Wbeam transition with an asymmetrical thrie-Wbeam transition to accept MGS w-beam? If so, what would be the best way to transition the splice location to midspan?
Thank you for your time.
I will start by addressing your questions in order of submitted:
1. The additions of the spacer tubes should not alter the performance of the post attachment to the side of the deck
2. The current version of the online Hardware Guide is a mess. It has many errors and omissions. In fact, most of the drawings attached at the bottom of the page for SBT01b are for the SBT01a system. I am not aware of issues resulting in the system being taken down, so I think you are ok to continue using it.
3. I don’t see any issues with using a tighter spacing of 6’-3”. Tighter spacing would only increase the stiffness and strength/capacity of the bridge railing. A 6’-3” spacing makes more sense in terms of rail-to-post attachments being at standard hole punch distances anyway.
4. I am not aware of any transition to this bridge rail being tested or evaluated. The one shown in NCHRP 239 was not tested as part of the bridge rail evaluation. I would recommend using the TL-2 guardrail transition developed and tested at TTI. It is short and connects 31” MGS to thrie beam utilizing the asymmetric transition segment. Currently, it is the only MASH TL-2 transition that I’m aware of, so it is probably the best system to utilize here. I have attached a PDF of the report on this system.
5. See notes above and the attached PDF for TL-2 transition design details.
From the details you have sent, it looks like INDOT has lowered the location of the anchor rods (base-plate-to-deck attachment) with respect to the post-to-base plate bolt. The original system had a vertical offset of 1.25” between the post bolt and the anchor rods, but your drawings show a 4” vertical offset. I’m not sure why this was done, but perhaps they were shifted downward to center them within the spacer tubes. This increased offset distance (increased moment arm in the plate) will likely result in the base plate bending away from the deck as the post is loaded laterally. This behavior could significantly alter the stiffness and performance of the bridge rail. Thus, if you desire to continue using this system, the anchor rods in the deck should be shifted upward to return to a vertical offset of 1.25” between the post bolt and the anchor rods.
Although the system you are proposing may work fine for TL-2 applications, I suggest utilizing a new railing. MwRSF recently developed and crash tested a side-mounted, MASH TL-2 bridge rail for Nebraska DOT. This railing utilizes S3x5.7 posts spaced at 6’-3” and standard W-beam rail. Thus, it would result in a significant reduction in materials/cost. Similar to the MGS bridge rail, it can be directly connected to standard MGS with a 6’-3” post spacing between bridge post and MGS post, so it does not require a guardrail transition at the ends of the bridge. I encourage you to consider using this bridge railing based on improved performance, simplified design, and reduced installation costs. The report is still being written, but I have attached a drawing set for the tested system. If you would like more information on the new bridge railing, let me know.
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