Chapter 3. DESIGN PHILOSOPHY
3.3 Performance Evaluation
As a guide for evaluating performance criteria at a specific port, the relationship between degree of damage and the design earthquake motion is illustrated in Fig. 3.2. The curves in this figure form the basis for the performance evaluation procedure. This figure is based on the specification of performance grades in Table 3.2. The curves in Fig. 3.2 indicate the upper limits for the acceptable level of damage over a continuously varying level of earthquake motions, including the Design Philosophy 27
Table 3.3. Performance grade based on the importance category of port structures.
Performance Definition based on seismic effects on structures Suggested importance
grade category of port
structures in Japanese code
Grade S jCritical structures with potential for extensive Special Class loss of human life and property upon seismic
damage
kKey structures that are required to be service-able for recovery from earthquake disaster lCritical structures that handle hazardous
materials
mCritical structures that, if disrupted, devastate economic and social activities in the earthquake damage area
Grade A Primary structures having less serious effects Special Class or for j through m than Grade S structures, or Class A
n Structures that, if damaged, are difficult to restore
Grade B Ordinary structures other than those of Class A or B Grades S, A and C
Grade C Small easily restorable structures Class B or C
designated L1 and L2 motions. Each curve in this figure is defined by two control points corresponding to the upper limits of the level of damage for L1 and L2 motions defined in Table 3.2. For example, the curve defining the upper limit for Grade B should go through a point defining the upper limit for damage degree I for L1 motion, and another defining the upper limit for damage degree III for L2 motion. The shape of the curves may be approximated by line segments through the controlling points or may be refined by referring to typical results of non-linear seismic analysis of port structures.
The vertical coordinates of Fig. 3.2 are converted into engineering parameters such as displacements, stresses or ductility factors specified by the damage crite-ria discussed in Chapter 4. This conversion allows direct comparison between required performance and seismic response of a structure. The seismic response of a structure is evaluated through seismic analysis over L1 and L2 motions and plotted on this figure as ‘seismic response curve.’ As minimum requirement, analysis should be performed for L1 and L2 earthquake motions. For example, if the structure being evaluated or designed has the seismic response curve ‘a’ in Fig. 3.3, the curve is located below the upper bound curve defining Grade A.
Thus, this design assures Grade A performance. If an alternative structural con-figuration yields the seismic performance curve ‘b’ in Fig. 3.3 and a portion of the curve exceeds the upper limit for Grade A, then this design assures only Grade B performance.
Fig. 3.2. Schematic figure of performance grades S, A, B and C.
Design Philosophy 29
Fig. 3.3. Examples of seismic performance evaluation.
Damage Criteria
In performance-based design, the acceptable level of damage, i.e. the damage cri-teria, should be specified in engineering terms such as displacements, limit stress state, and ductility/strain limit based on the function and seismic response of the structure. The damage criteria may be established based on Table 3.1 in Chapter 3 by a group of design engineers with assistance and advice from the users and owners of the structure/facility. The guidelines for establishing damage criteria for typical quay walls are described in Sections 4.1 through 4.4. The proposed dam-age criteria discussed in this chapter have been established for the following specific conditions:
1) The operational damage criteria in Table 3.1 in the previous chapter dictate the Degree I, whereas the structural damage criteria in the same table allow for the Degrees II through IV.
2) The quay walls considered are those where the seismic damage poses no threats to human life, no hazardous material is handled, without cranes on rails, and the sea space in front of the quay wall is unlimited.
Additional guidelines are offered in Section 4.5 for quay walls with cranes on rails. Damage criteria for breakwaters are described in Section 4.6.
4.1 GRAVITY QUAY WALLS
(1) Seismic response of gravity quay walls
A gravity quay wall is made of a caisson or other gravity retaining structure placed on the seabed. Stability against earth pressures from the backfill soil behind the wall is maintained by the mass of the wall and the friction at the bot-tom of the wall. For gravity quay walls on firm foundations, typical failure modes during earthquakes are seaward displacements and tilting as shown in Fig. 4.1(a).
For a loose backfill or natural loose sandy foundation, the failure modes involve overall deformation of the foundation beneath the wall, resulting in large seaward displacement, tilting and settlements (see Fig. 4.1(b)). A wall with a relatively small width to height ratio, typically less than about 0.75, will exhibit a predom-inant tilting failure mode rather than horizontal displacements.
On the conceptual level of design, several options may be available for increasing the stability of the gravity quay wall. An obvious measure to reduce the earth pressures against the wall is to use backfill material with a large angle of internal friction. Rockfill and similar materials should be preferable to loose sand for such use. The risk of overall deformation as well as of liquefaction would be minimized if such practice could be followed. Other possibilities for increasing the stability of the quay wall include drawing landwards the centre of gravity of the wall and thereby increasing the stabilizing moment, and imposing a higher friction coefficient between the base slab and the foundation layer. In concept, the former may be achieved by allowing for appropriate offsets of the precast blocks along the vertical face of the structure or by filling the rear cells of caissons with heavy materials such as metal slag. The latter has indeed been achieved by using an asphalt or rubber mat beneath the caisson wall. The latter may alternatively be achieved by providing a foundation slab of concrete with a special anti-skid shape or by designing for inclined, rather than horizontal contact surfaces at foundation and higher levels of the block wall.
Horizontal displacement and uniform vertical settlement of a gravity quay wall may not significantly reduce the residual state of stability, and may be generally acceptable from a structural point of view. However, tilting of the wall can significantly reduce the residual stability and result in an unacceptable structural stability situation. Case histories show an overturning/collapse of concrete block type walls when tilting occurs. This type of wall needs careful consideration in specifying damage criteria regarding the overturning/collapse mode.
(2) Parameters for specifying damage criteria for gravity quay walls
Seismic performance of a gravity quay wall is specified based on such service-ability considerations as safe berthing, safe operation of wheeled vehicles and cargo handling, flooding, and level of structural damage in the form of displace-ments and tilting (including relative displacedisplace-ments between the blocks of a concrete block type quay wall). Parameters that may be used for specifying 32 PIANC
Fig. 4.1. Deformation/failure modes of gravity quay wall.
(a) On firm foundation.
(b) On loose sandy foundation.
damage criteria include displacements, settlements, tilting, and differential displacements along the face line of a wall, and the deformations at apron includ-ing settlement, differential settlement at and behind apron, and tiltinclud-ing (Fig. 4.2).
Damage criteria should be established by choosing and specifying appropriate parameters from those mentioned above. More specifics are given in the next subsection.
(3) Damage criteria for gravity quay walls
Provided the conditions mentioned at the beginning of this chapter are applicable, the damage criteria may be established by referring to Table 4.1. The criteria shown in Table 4.1 are minimum requirements. Thus, in evaluating seismic performance by referring to damage criteria based on a number of different para-meters, the highest damage degree should be the final result of the evaluation.
4.2 SHEET PILE QUAY WALLS
(1) Seismic response of sheet pile quay walls
A sheet pile quay wall is composed of interlocking sheet piles, tie-rods, and anchors. The wall is supported at the upper part by anchors and the lower part by embedment in competent soil. Typical failure modes during earthquakes depend on structural and geotechnical conditions as shown in Fig. 4.3.
Structural damage to a sheet pile quay wall is basically governed by structural stress/strain states rather than displacements. It is important to determine the preferred sequence and degrees of ultimate states that may occur in the composite sheet pile quay wall system.
Fig. 4.2. Parameters for specifying damage criteria for gravity quay wall.
(2) Parameters for specifying damage criteria for sheet pile quay walls
Seismic performance of a sheet pile quay wall is specified based on serviceability, similar to that for a gravity quay wall, and in terms of structural damage regard-ing stress states as well as displacements. Parameters for specifyregard-ing damage cri-teria are as follows (refer to Fig. 4.4).
Displacements:
– sheet pile wall and apron: refer to the parameters for a gravity quay wall;
– anchor: differential settlement, ground surface cracking at anchor, pull-out displacement of battered pile anchor.
Stresses:
– sheet pile (above and below mudline);
– tie-rod (including joints);
– anchor.
Damage criteria should be established by choosing and specifying appropriate parameters from those mentioned above.
The preferred sequence to reach ultimate states with increasing level of seismic load should be appropriately specified for a sheet pile quay wall. If a damaged anchor is more difficult to restore than a sheet pile wall, the appropriate sequence may be given as follows (refer to Fig. 4.5).
1) Displacement of anchor (within the damage Degree I).
2) Yield at sheet pile wall (above mudline).
3) Yield at sheet pile wall (below mudline).
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Table 4.1. Proposed damage criteria for gravity quay walls.
Level of damage Degree I Degree II Degree III Degree IV Gravity Normalized residual Less than 1.5~5% 5~10% Larger than
wall horizontal 1.5%** 10%
displacement (d/H)*
Residual tilting Less than 3~5° 5~8° Larger than
towards the sea 3° 8°
Apron Differential Less than N/A*** N/A N/A
settlement on apron 0.03~0.1 m
Differential Less than N/A N/A N/A
settlement between 0.3~0.7 m apron and non-apron
areas
Residual tilting Less than N/A N/A N/A
towards the sea 2~3°
* d: residual horizontal displacement at the top of the wall; H: height of gravity wall.
** Alternative criterion is proposed with respect to differential horizontal displacemt less than 30 cm.
*** Abbreviation for not applicable.
4) Yield at anchor.
5) Yield at tie-rod.
If a damaged sheet pile wall is more difficult to restore than an anchor, the yield at anchor should precede the yield at sheet pile wall.
(3) Damage criteria for sheet pile quay walls
Provided the conditions mentioned at the beginning of this chapter are applicable, the damage criteria for a sheet pile quay wall may be established by referring to Table 4.2. The most restrictive conditions among displacements and stresses should define the damage criteria. Structural damage to the embedded portion of a sheet pile is generally difficult to restore, and thus necessitates higher seismic resistance. Brittle fracture of a sheet pile wall, rupture of a tie-rod, and the collapse of anchor should be avoided.
Fig. 4.3. Deformation/failure modes of sheet pile quay wall.
(a) Deformation/failure at anchor.
(b) Failure at sheet pile wall/tie-rod.
(c) Failure at embedment.
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4.3 PILE-SUPPORTED WHARVES
(1) Seismic response of pile-supported wharves
A pile-supported wharf is composed of a deck supported by a substructure con-sisting of piles and a dike/slope. The unsupported pile length above the dike/slope surface is variable. When rockfill suitable for construction of the dike is uneco-nomical, a gravity or sheet pile retaining structure is also constructed to replace a portion of the dike. The seismic response of pile-supported wharves is influenced to a great degree by complex soil-structure interaction during ground shaking.
Typical failure modes during earthquakes depend on the magnitude of the inertia force relative to the ground displacement (Fig. 4.6). In particular, the failure Fig. 4.4. Parameters for specifying damage criteria for sheet pile quay wall.
(a) With respect to displacements.
(b) With respect to stresses.
Fig. 4.5. Preferred sequence for yield of sheet pile quay wall.
Damage Criteria37 (when an anchor is more difficult to restore than a wall).
Level of damage Degree I Degree II Degree III Degree IV
Residual Sheet Normalized residual Less than N/A N/A N/A
displacements pile wall horizontal displacement 1.5%**
(d/H)*
Residual tilting Less than 3° N/A N/A N/A
towards the sea
Apron Differential Less than N/A N/A N/A
settlement on apron 0.03~0.1 m
Differential settlement Less than N/A N/A N/A
between apron and 0.3~0.7 m non-apron areas
Residual tilting Less than N/A N/A N/A
towards the sea 2~3°
Peak response Sheet Above mudline Elastic Plastic (less than Plastic (less than Plastic (beyond the
astresses/ pile wall the ductility the ductility factor/ ductility factor/strain
strains factor/strain limit strain limit limit above mudline)
above mudline) above mudline)
Below mudline Elastic Elastic Plastic (less than Plastic (beyond the the ductility factor/ ductility factor/strain tstrain limit limit below mudline) below mudline)
Tie-rod Elastic Elastic Plastic (less than Plastic (beyond the
the ductility factor/ ductility factor/strain strain limit for tie-rod) limit for tie-rod)
Anchor Elastic Elastic Plastic (less than Plastic (beyond the
the ductility factor/ ductility factor/strain strain limit for anchor) limit for anchor)
*d: residual horizontal displacement at the top of the wall; H: height of sheet pile wall from mudline.
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associated with the ground displacement shown in Fig. 4.6(c) suggests that appro-priate design consideration is required of the geotechnical aspects.
Among various structural components in the pile-supported wharves, attention should be paid to the application of batter piles. Batter piles remain the most effi-cient structural component for resisting lateral loads due to mooring, berthing and crane operation. However, the batter pile-deck system results in a much more rigid frame than one with vertical piles. As a result, large stress concentrations and shear failures of concrete batter piles have been observed during earthquakes (e.g.
1989 Loma Prieta earthquake, and 1994 Northridge earthquake in California, USA). This mode of pile failure demonstrates that the structural design of batter piles used in regions of high seismicity must account for displacement demand and appropriate ductility.
When using the concrete piles in the areas of high seismicity, it is becoming more common to design the pile-supported wharves with vertical piles only. This Fig. 4.6. Deformation/failure modes of pile-supported wharf.
(a) Deformation due to inertia force at deck.
(b) Deformation due to horizontal force from retaining wall.
(c) Deformation due to lateral displacement of loose subsoil.
structural system resists the earthquake-induced lateral loads/displacements by bending of the piles and associated moment resistance. Moderate structural damage to vertical piles in bending may be unavoidable during strong earthquake motions. However, bending failure at the pile head is considered preferable to shear failure at the batter pile-deck connection because the former is substantially easier to repair (Buslov et al., 1996).
Innovative structural design and detailing of pile-deck connections has allowed for the continued use of batter piles for pile-supported wharves. The design concepts incorporate a repairable structural ‘fuse’ at the pile cap. The fuse serves as a weak link or load-limiting device that is robust enough to withstand lateral loads during routine port operation, but yields during strong earthquake motions, thereby precluding damage to the wharf deck itself. The load-limiting device may be designed as a component that relies on sliding friction (Zmuda et al., 1995), or as a structural ‘fuse-link’ that yields in shear and can be easily replaced (Johnson et al., 1998).
In the areas of high seismicity such as in Japan, it is common practice to use steel piles for pile-supported wharves.
Structural damage to a pile-supported wharf is governed by stress/strain states rather than displacements. It is important to determine the preferred sequence and degrees of ultimate states to occur in the composite pile-supported wharf system.
(2) Parameters for specifying damage criteria for pile-supported wharves
The structure of a pile-supported wharf consists of a deck/pile system and a dike/slope/retaining wall. The seismic performance of the dike/slope/retaining wall may be specified based on the same criteria as for a gravity or sheet pile wall.
The effects of ground displacements, including the movement of the retaining wall and the dike/slope below the deck, should be carefully evaluated for the per-formance of the deck/pile system.
The seismic performance of a deck/pile system is specified based on service-ability and structural damage. Parameters for specifying damage criteria for deck and piles are as follows (refer to Fig. 4.7).
Displacements:
– deck and piles: settlement, tilting, differential displacements;
– apron (deck and loading/unloading area behind the deck): differential settle-ment between deck and retaining wall, tilting, fall/fracture of bridge.
Stresses:
– piles (pile top and below dike/slope surface or mudline);
– deck (deck body, pile cap);
– bridge.
Damage criteria should be established by choosing and specifying appropriate parameters from these parameters.
With respect to ease of restoration after an earthquake, the preferred sequence to reach ultimate states with increasing levels of seismic load may be specified for a pile-supported wharf as follows (refer to Fig. 4.8).
1) Pile cap (portion of pile embedded for a pile-deck connection).
2) Pile top (just below the pile cap – concrete beam connection).
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Fig. 4.7. Parameters for specifying damage criteria for pile-supported wharf.
(a) With respect to displacements.
(b) With respect to stresses.
3) Deck and pile below dike/slope surface or mudline (within allowable ductility factor).
In the bridging area between the wharf deck and the retaining wall, structural details such as a fail-safe device to prevent fall-down or an easily repairable structure are also important, and, if applicable, a structure to absorb the dis-placements from the retaining wall can be introduced. This could lead to further development of energy absorption devices for pile-supported wharves.
(3) Damage criteria for pile-supported wharves
The criteria for the retaining wall of a pile-supported wharf may be established by referring to those for a gravity or sheet pile quay wall. Damage criteria for the dike/slope under the deck are less established because of the complex soil-struc-ture interaction between the piles and the dike/slope.
Provided the conditions mentioned at the beginning of this chapter are appli-cable, criteria for the piles and deck of a pile-supported wharf may be established by referring to Table 4.3. The most restrictive condition among displacements and stresses should define the damage criteria.
Structural damage to the embedded portion of a pile is generally difficult to restore and has the potential to trigger collapse of a pile-supported wharf. Thus, a more restrictive ductility factor should be used for the design.
Examples of ductility factors/strain limits for use in practice are found in Ferritto (1997a), Ferritto et al. (1999) and Yokota et al. (1999). More details can be found in Technical Commentaries 5 and 7. No case histories have been reported of brittle fracture of steel piles during earthquakes. However, the case histories of brittle fracture of thick steel columns during the 1995 Kobe earthquake indicate that it is still necessary to study this aspect in steel piles.
Fig. 4.8. Preferred sequence for yield-ing of pile-supported wharf.
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Table 4.3. Proposed damage criteria for pile-supported wharves.*
Level of damage Degree I Degree II Degree III Degree IV
Residual Differential Less than 0.1~0.3 m N/A N/A N/A
displacements settlement between deck and land behind
Residual tilting Less than 2~3° N/A N/A N/A
towards the sea
Peak Piles** Essentially elastic Controlled limited Ductile response near Beyond the state
Peak Piles** Essentially elastic Controlled limited Ductile response near Beyond the state