Use of tie-beams in piers?

[kckriete]

Some questions have arisen in my office lately about the use of tie beams in piers, advantages, when to use, should they be used in seismic prone areas? I have not had a large amount of experience with the deisgn of tie beam elements and would appreciate some guidance.

The question arose over the use on a seismically designed bridge and the problems it posed in creating workable details for connection of the tie beam to the columns. Especially with spirally reinforced columns.

 

[Qshake]

Kevin,

The use of tie beams in bridge bents can be a very practical method of mitigating the column end moments due to frame action. Furthermore, they help limit or reduce the effective length thereby allowing taller bents for use in bridges which often can have large lateral wind and seismic loads. The latter depending on the location and the former depending more so on the height, which is inherent in bents with tie beams anyway, rather than the location.

All reinforced concrete connections in zones of moderate to high seismicity are necessarily crowded to allow for the required ductility supporting possible inelastic rotations or deformations. One way of dealing with constuction issues is splicing in areas outside of the likely plastic hinging zones. In the actual connection, single hoops are acceptable so long as the confinement of the core is still met and the failure mechanisms addressed properly. In answer to this MoDOT allows the use of the seismic "P" bars with the same pitch as a spiral. These bars meet the confinement criteria since they are hooked 135 degrees around a longitudinal bar thereby addressing the pullout so common with the ties used in the years prior to seismic detailing. The seismic "p" bars may be stacked away from the placement of the main bars and dropped into place once all the main bars are placed.

I thought all special structures guys knew this stuff!!

 

[kckriete]

Qshake

Well a fellow Sverdrupite. The discussion in the office centers around if the tie beam actually reduces the effective length since the frame is not braced in the longitudinal direction.

The use of the P-bars is a good suggestion that we had not considered. The MoDOT manual does not directly address this issue.

 

[Qshake]

I also wanted to mention the Corps has a nice section on design of reinforced concrete beams for seismic design which really details the seismic requirements in the hinging area.

Also, to clarify - if you have an integral endbent, the bridge is braced in the longitudinal direction - you just need to quantify the displacement of the end fill - in most cases the strength of the end fill will not be exceeded thereby giving way to soil deformation. If an open seat endbent is used, then it is not braced.

I wanted to make that point, but the tie beam really helps in the transverse direction, not the longitudinal direction no matter what endbent you have. Since the transverse response of the bridge is stiff and attracts load, the transverse moments can be large. Any attempt to reduce them is good since they are combined SRSS with the longitudinal moments.