Greetings from Bridging Solutions!
This installment of “A Bridge Moment” supplements our last one on bridge railings and barriers. The portion of the bridge barrier you see along the fascia of the bridge is important but the part you don’t see is just as critical to the design and performance of the railing or barrier – i.e. the anchorage system.
1. Bridge barriers need to be able to transfer and dissipate the energy from a vehicle crash through the barrier system safely into the mass of the bridge superstructure.
2. In multi-beam and deck type bridges, the crash energy is distributed through the railing or barrier into the deck and the anchorage system distributes the energy (stress) through the deck to the primary support beams.
3. Railing systems consisting of posts and rails impart higher concentrated stresses at the post locations compared to barrier (or parapet) type systems.
4. The AASHTO code provides empirical design equations for the effective length of deck that can be considered in resisting these stresses depending on the type of railing or barrier system.
5. Base plates and anchor bolts transfer the crash energy to the deck on post and rail systems and 90 or 180 degree hook reinforcing bars are used on barrier (parapet) systems. See the typical detail below.
6. The transverse reinforcing steel in the concrete deck transfers the crash stresses to the support beams. If an existing bridge deck does not have adequate transverse steel, engineers may consider using through bolts and external anchor plates at the bottom of the deck.
7. Earth filled bridge structures such as box culverts and arches (e.g. CONSPAN and BEBO) do not have concrete decks to transfer the crash energy. Therefore, cast-in-place concrete moment (or junction) slabs are typically used to resist the crash energy since the typical head walls or spandrel walls do not have adequate mass to absorb the energy. See the photograph of a moment slab for a custom barrier on a BEBO arch bridge.
