Seismic Design of Tunnels - Parsons Brinckerhoff

Comments on Loading Combinations for ODE
• The structure should first be designed with adequate strength capacity under static
loading conditions.
• For cut-and-cover tunnel structures, the evaluation of capacity using Equation 2-3
should consider the uncertainties associated with the loads E1 and E2, and their worst
combination. For mined circular tunnels (Equation 2-4), similar consideration should
be given to the loads EX and H.
When the extreme loads are used for design, a smaller load factor is recommended to
avoid unnecessary conservatism. Note that an extreme load may be a maximum load
or a minimum load, depending on the most critical case of the loading combinations.
Use Equation 2-4 as an example. For a deep circular tunnel lining, it is very likely that
the most critical loading condition occurs when the maximum excavation loading, EX,
is combined with the minimum hydrostatic water pressure, H. For a cut-and-cover
tunnel, the most critical seismic condition may often be found when the maximum
lateral earth pressure, E2, is combined with the minimum vertical earth load, E1. If a
very conservative lateral earth pressure coefficient is assumed in calculating the E2,
the smaller load factor b 1 = 1.05 should be used.
• Redistribution of moments (e.g., ACI 318) for cut-and-cover concrete frames is
recommended to achieve a more efficient design.
• If the “strength” criteria expressed by Equation 2-3 or 2-4 can be satisfied based on
elastic structural analysis, no further provisions under the ODE are required.
• If the flexural strength of the tunnel lining, using elastic analysis and Equation 2-3 or 2-
4, is found to be exceeded, the structure should be checked for its ductility to ensure
that the resulting inelastic deformations, if any, are small. If necessary, the structure
should be redesigned to ensure the intended performance goals during the ODE.
• Zero live load condition (i.e., L = 0) should also be evaluated in Equations 2-3 and 2-4.
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