8.3 Substructures: Piers and Abutments
8.3.1
8.3.1 The substructure shall be designed for seismic forces calculated based on seismicanalysis method specified in Chapter 6Chapter 6. The conceptual design & geometry of the substructure shall be based on the provision given inChapter 4Chapter 4. The calculation of time
Guidelines For Seismic Design of Bridges
foundation and connection between substructure & superstructure. The effect of both horizontal & vertical component of seismic needs to be investigated for all possible combination as per the provision given inChapter 5Chapter 5 “General Design Provisions”.
8.3.2
8.3.2 Force Force Transfer Transfer mechanism mechanism from from bearing bearing to to abutment abutment and and pierpier
The transfer of force through connection between substructure & superstructure is an important aspect in design of substructure. The connections between supporting and supported members shall be designed in order to ensure structural integrity and avoid unseating under extreme seismic displacements. The piers shall be designed to withstand shear forces corresponding to the pier’s plastic hinge capacity. The maximum induced shear in the piers shall be limited to the plastic hinge moment (or moments) divided by the height of pier as ascertained inChapters 4 & 7Chapters 4 & 7
For Seismic Zone IV & V, use of elastomeric bearings for resisting horizontal seismic actions by shear deformation, shall not be permitted. In such cases PoT, POT Cum PTFE & Spherical Bearings shall be adopted over elastomeric bearings for resisting seismic loads.
In seismic design, the fixed bearing shall be checked for full seismic force along with braking / tractive force, ignoring the relief due to frictional forces in other free bearings. The structure under the fixed bearing shall be designed to withstand the full seismic and design braking / tractive force.
8.3.3
8.3.3 Load Load CombinationCombination
For design of substructure, the seismic force shall be assumed to act from any horizontal direction. For this purpose, two separate analyses shall be performed for design seismic forces acting along two orthogonal horizontal directions. The design seismic force resultants at any cross-section of a bridge component resulting from the analysis in the two orthogonal horizontal directions shall be combined as perClause 4.1.2Clause 4.1.2
8.3.4 Vertical component of seismic action 8.3.4 Vertical component of seismic action
The effect of the vertical seismic component on substructure and foundation may be omitted in zones II & III. The vertical accelerations should be specially considered in bridges with large spans, those in which stability is the criteria of design and in situation where bridges are located close to epicentre.
However, the effect of vertical seismic component is particularly important in the following components/situation and needs to be investigated for bridges in all seismic zones.
a) Prestressed concrete components, b) Bearings and linkages,
c) Horizontal cantilever structural elements, and
Guidelines For Seismic Design of Bridges
8.3.5 Seismic Design Force for substructure 8.3.5 Seismic Design Force for substructure
The seismic design of substructure and transfer of seismic forces shall be adopted as given in Chapters 7.Chapters 7. The bridge substructure shall be conceptualised in such a manner to ensure that intended configuration of plastic hinges should avoid the brittle failure mode of the structure under seismic action. This can be achieved by using capacity design principal.
Effects of abutment flexibility shall be considered in the seismic analysis and design of all bridges. Bridge inertial forces shall be based on its structural capacity and the soil resistance that can be reliably mobilized.
Skewed abutments are highly vulnerable to damage during seismic actions. For bridges with skew angles more than 30 degrees, the skew angle at piers and abutments shall preferably be reduced, even at the expense of increasing the bridge length if possible. For bridges having skew angle ≥ 30 degree & horizontally curved radius ≤ 100m, special studies shall be performed and provision givenTable 2.1Table 2.1 shall be followed
The energy dissipation capacity of the abutments should be considered for bridges whose response is dominated by the abutments.
Provisions given in Chapter 9Chapter 9 for ductile detailing of members subjected to seismic forces shall be adopted for design of supporting components of the bridge. Further, the design shear force at the critical sections of substructures shall be the lower of the following:
a)
a) Maximum elastic shear force at the critical section of the bridge component divided by the response reduction factor for the components as perTable no-4.1Table no-4.1
b) Maximum shear force that develops when the substructure has maximum moment that it can sustain (i.e., the over strength plastic moment capacity) in single column or single- pier type substructure,
c) Maximum shear force that is developed when plastic moment hinges are formed in the substructure so as to form a collapse mechanism in multiple column frame type or multiple-pier type substructures, in which the plastic moment capacity shall be the over strength plastic moment capacity.
8.3.6 Verification of strength of piers and abutments under Seismic Load Combination 8.3.6 Verification of strength of piers and abutments under Seismic Load Combination
Serviceability Limit State (SLS) checks need not be performed for Seismic Load Combinations. Under ULS, strength and stability are to be ensured under Seismic Load Combination.
For checking the equilibrium of substructure, and structural strength different load factors are specified for different set of load combinations, for which Table B.1 and B.2 of Annex Table B.1 and B.2 of Annex – – B of IRC: 6 shall be referred.
Guidelines For Seismic Design of Bridges
In general, the compliance criteria stated above aim explicitly at satisfying the non-collapse requirement in conjunction with certain specific detailing rules, the same criteria are deemed to cover implicitly the damage minimization requirement as well.