MIDWEST STATES POOLED FUND PROGRAM
Increased Span Length of the MGS Long Span
Sponsoring Agency Code
TPF-5(193) Supplement 56
The current MGS long-span guardrail system provides the capability to span unsupported lengths up 25
ft. While this span length has many useful applications, many culvert structures fall outside the span
length of the MGS long-span system. Other solutions for mounting guardrail to culverts exist, but
mounting hardware to culverts can also cause difficulties. If the long span can be adjusted to
accommodate longer spans, the difficulties associated with mounting hardware to the culvert can be
Long-span guardrail systems have been recognized as an effective means of shielding low-fill culverts.
These designs are popular due to their ability to safely shield the culvert while creating minimal
construction effort and limiting culvert damage and repair when compared to other systems requiring post
attachment to the top of the culvert. However, previous long-span designs were limited by the need to use
long sections of nested guardrail to prevent rail rupture and the need for providing large lateral offsets
between the barrier and the culvert headwall. The MGS long-span guardrail, as shown in Figure 1,
eliminated those two shortcomings by applying the benefits of the Midwest Guardrail System to a longspan
design. The MGS long-span allowed for increased vehicle capture and stability through increased
rail height, limited the potential for pocketing and wheel snag through the use of CRT posts adjacent to
the unsupported span, and greatly increased the tensile capacity of the rail through the movement of
splices away from the posts and the use of shallower post embedment. These features allowed the
system to be developed without the use of nested guardrail and with reduced barrier offset which places
the back of the guardrail posts in line with the front face of the culvert headwall.
Two full-scale crash tests were conducted on the MGS long-span according to the TL-3 MASH
requirements for test designation no. 3-11. The first test, test no. LSC-1, was designed to evaluate the
structural capacity of the system by selecting a CIP that maximized the potential for pocketing, wheel
snag, and rail rupture. In test no. LSC-1, a 4,991-lb pickup truck impacted the MGS long-span system
8.15-ft downstream of post no. 13 at a speed of 62.4 mph and an angle of 24.8 degrees, and the vehicle
was safely redirected. A second test, test no. LSC-2, was conducted to evaluate the potential for vehicle
instability by selecting a CIP that maximized the interaction of the left-front wheel of the pickup truck with
the wing wall of the culvert. In test no. LSC-2, a 4,984-lb pickup truck impacted the MGS long-span
system 41.24-ft upstream of post no. 14 at a speed of 61.9 mph and an angle of 24.9 degrees, and the
vehicle was safely redirected. The MGS long-span guardrailâ€™s ability to perform safely without nested rail
and a minimal barrier offset made this new barrier a very functional and safe option for protection of lowfill
The use of unsupported lengths longer than 25 ft was not recommended following the original research
project without further analysis and full-scale crash testing. However, the excellent performance of the
MGS long-span system in the full-scale crash testing program suggests that longer span lengths may be
possible with the current design. In addition, it may be possible to modify the barrier system for
significantly longer unsupported span lengths, if so desired. However, this may require substantial and
costly changes to the barrier system.
The objective of this research effort is to design and evaluate the MGS long-span design for use with
unsupported spans greater than 25 ft. The research effort could be focused in one of two directions. The
research could focus on determination of the maximum unsupported span length for the current longspan
design or it could focus on evaluating potential modifications that may allow for significantly longer
unsupported spans. The increased unsupported span design would be designed to meet the TL-3 safety
criteria set forth in MASH.
This research effort will begin with a review of previous long-span designs and other similar systems for
extending unsupported guardrail on bridges and culverts. Next, MwRSF will use LS-DYNA computer
simulation models to investigate the use of longer unsupported spans with the MGS long span. As noted
previously, this effort could be focused in one of two ways. The first option would use LS-DYNA to
simulate the current MGS long span to determine the maximum potential unsupported span length
without modification other than increasing the span. A second option would involve selection of a desired
span length and designing modifications to the original long span to meet the desired unsupported length.
LS-DYNA would then be used to evaluate the design modifications to the long span and determine their
feasibility. LS-DYNA would be used in both cases to determine CIP locations for the full-scale testing of
the increased unsupported spans.
Following the design and computer simulation analysis, two full-scale crash tests would be required to
evaluate the MGS long span with increased span length to the TL-3 criteria in MASH. The first test would
be designed to evaluate the structural capacity of the system by selecting a CIP that maximized the
potential for pocketing, wheel snag, and rail rupture. The second test would be conducted to evaluate the
potential for vehicle instability by selecting a CIP that maximized the interaction of the front wheel of the
pickup truck with the wing wall of the culvert. Both tests would be conducted with the 2270P vehicle
impacting the CIP of the respective test at a speed of 62 mph and an angle of 25 degrees. The tests
would be conducted on a simulated culvert headwall and vertical drop-off.
Following the full-scale crash testing, a summary report would be complied detailing the design and
analysis efforts, the full-scale crash testing, and recommendations for the new MGS long span. The MGS
long span with increased unsupported length would be submitted for approval to FHWA.
The MGS long-span has the ability to perform safely without nested rail and with a minimal barrier offset.
These features make the barrier a very functional, efficient, and safe option for protection of low-fill
culverts. Development of an increased unsupported span length for the MGS long-span system will add
to the flexibility of the design and provide for improved protection of culvert headwalls and vertical dropoffs.
Snapshot of Recent Developments
130 Whittier Research Center
2200 Vine Street
Lincoln, Nebraska 68583-0853
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.