Seismic Design of Tunnels - Parsons Brinckerhoff

hinges (or regions of inelastic deformation) should be identified and prevented
(Monsees, 1991; see Figure 21 for example).
• The strength reduction factor, f , used in the conventional design practice may be too
conservative, due to the inherently more stable nature of underground structures
(compared to surface structures), and the transient nature of the earthquake loading.
• For cut-and-cover tunnel structures, the evaluation of capacity using Equation 2-1
should consider the uncertainties associated with the loads E1 and E2, and their worst
combination. For mined circular tunnels (Equation 2-2), similar consideration should
be given to the loads EX and H.
• In many cases, the absence of live load, L, may present a more critical condition than
when a full live load is considered. Therefore, a live load equal to zero should also be
used in checking the structural strength capacity using Equations 2-1 and 2-2.
Operating Design Earthquake (ODE). For the ODE, the seismic design loading
combination depends on the performance requirements of the structural members.
Generally speaking, if the members are to experience little to no damage during the lowerlevel
event (ODE), the inelastic deformations in the structure members should be kept low.
The following loading criteria, based on load factor design, are recommended:
For Cut-and-Cover Tunnel Structures
Ê
ˆ
U = 1. 05 D +1.3L +b1ËE1 + E2 ¯ +1.3EQ
(Eq. 2-3)
where D, L, E1, E2, EQ, and U are as defined in Equation 2-1.
b 1 = 1.05 if extreme loads are assumed for E1 and E2 with little uncertainty.
Otherwise, use b 1 = 1.3.
For Mined (Circular) Tunnel Lining
Ê
ˆ
U = 1. 05 D +1.3L +b 2 ËEX + H ¯ +1.3EQ
(Eq. 2-4)
where D, L, EX, H, EQ, and U are as defined in Equation 2-2.
b 2 = 1.05 if extreme loads are assumed for E1 and E2 with little uncertainty.
Otherwise, use b 2 = 1.3.
22