This section establishes the minimum performance standards for structural components. Evaluation procedures for seismic performance, strength and deformation characteristics of concrete, steel and timber components are prescribed herein. Analytical procedures for structural systems are presented in Section 3104F.
This section addresses MOTs constructed using the following structural components:
- Reinforced concrete decks supported by batter and/or vertical concrete piles
- Reinforced concrete decks supported by batter and/or vertical steel piles, including pipe piles filled with concrete
- Reinforced concrete decks supported by batter and/or vertical timber piles
- Timber decks supported by batter or vertical timber, concrete or steel pipe piles
- Retaining structures constructed of steel, concrete sheet piles or reinforced concrete
The following parameters shall be established in order to compute the component strength:
- Specified concrete compressive strengths
- Concrete and steel modulus of elasticity
- Yield and tensile strength of mild reinforcing and prestressed steel and corresponding strains
- Confinement steel strength and corresponding strains
- Embedment length
- Concrete cover
- Yield and tensile strength of structural steel
- Environmental effects, such as reinforcing steel corrosion, concrete spalling, cracking and chemical attack
- Fire damage
- Past and current loading effects, including overload, fatigue or fracture
- Earthquake damage
- Discontinuous components
- Construction deficiencies
Material properties of existing components, not determined from testing procedures, and of new components, shall be established using the following methodology.
The strength of structural components shall be evaluated based on the following values (Section 5.3 of [7.1] and pp. 3-73 and 3-74 of [7.2]):
Specified material strength shall be used for non ductile components (shear controlled), all mechanical, electrical and mooring equipment (attachments to the deck) and for all non seismic load combinations:
In addition, these values (7-1a, 7-1b and 7-1c) may be used conservatively as alternatives to determine the nominal strength of ductile components (N).
Upper bound estimates of material strength shall be used for the determination of moment-curvature relations, to obtain the feasible maximum demand on capacity protected members:
fy = Specified yield strength of reinforcement or specified minimum yield stress steel
fp = Specified yield strength of prestress strands
"Capacity Design" (Section 5.3 of [7.1]) ensures that the strength at protected components (such as pile caps and decks), joints and actions (such as shear), is greater than the maximum feasible demand (over strength), based on realistic upper bound estimates of plastic hinge flexural strength. An additional series of nonlinear analyses using moment curvature characteristics of pile hinges may be required.
Alternatively, if a moment-curvature analysis is performed that takes into account the strain hardening of the steel, the demands used to evaluate the capacity protected components may be estimated by multiplying the moment-curvature values by 1.25.
Based on a historical review of the building materials used in the twentieth century, guidelines for tensile and yield properties of concrete reinforcing bars and the compressive strength of structural concrete have been established (see Tables 6-1 to 6-3 of FEMA 356 [7.3]. The values shown in these tables can be used as default properties, only if as-built information is not available and testing is not performed. The values in Tables 31F-7-1 and 31F-7-2, are adjusted according to equations (7-1) through (7-3).
Knowledge factor, k, shall be applied on a component basis.
The following information is required, at a minimum, for a component strength assessment:
- Original construction records, including drawings and specifications.
- A set of "as-built" drawings and/or sketches, documenting both gravity and lateral systems (Section 3102F.1.5) and any postconstruction modification data.
- A visual condition survey, for structural components including identification of the size, location and connections of these components.
- In the absence of material properties, values from limited in-situ testing or conservative estimates of material properties (Tables 31F-7-1 and 31F-7-2).
- Assessment of component conditions, from an in-situ evaluation, including any observable deterioration.
- Detailed geotechnical information, based on recent test data, including risk of liquefaction, lateral spreading and slope stability.
The knowledge factor, k, is 1.0 when comprehensive knowledge as specified above is utilized. Otherwise, the knowledge factor shall be 0.75 (see Table 2-1 of FEMA 356 [7.3]).
TENSILE AND YIELD PROPERTIES OF REINFORCING BARS FOR VARIOUS ASTM SPECIFICATIONS AND PERIODS (after Table 6-2 of [7.3])
|ASTM||STEEL TYPE||YEAR RANGE3||GRADE||STRUCTURAL1||INTERMEDIATE1||HARD1|
|Minimum Yield2 (psi)||33,000||40,000||50,000||60,000||70,000||75,000|
|Minimum Tensile2 (psi)||55,000||70,000||80,000||90,000||95,000||100,000|
General Note: An entry "X" indicates that grade was available in those years.
1. The terms structural, intermediate and hard became obsolete in 1968.
2. Actual yield and tensile strengths may exceed minimum values.
3. Untilabout 1920, a variety of proprietary reinforcing steels were used. Yield strengths are likely to be in the range from 33,000 psi to 55,000 psi, but higher values are possible. Plain and twisted square bars were sometimes used between 1900 and 1949.
4. Rail bars should be marked with the letter "R. "
5. ASTM steel is marked with the letter "W."
Stiffness that takes into account the stress and deformation levels experienced by the component shall be used. Nonlinear load-deformation relations shall be used to represent the component load-deformation response. However, in lieu of using nonlinear methods to establish the stiffness and moment curvature relation of structural components, the equations of Table 31F-7-3 may be used to approximate the effective elastic stiffness, EIe, for lateral analyses (see Section 3107F.5 for definition of symbols).
EFFECTIVE ELASTIC STIFFNESS
|Reinforced Pile||0.3 + N/(f'cAg)|
|Pile/Deck Dowel Connection1||0.3 + N/(f'cAg)|
|Prestressed Pile1||0.6 < EIe/EIg < 0.75|
|Concrete w/Steel Casing|
1. The pile/deck connection and prestressed pile may also be approximated as one member with an average stiffness of 0.42 EIe/EIg (Ferritto et al, 1999 [7.2])
The stress-strain characteristics of steel piles shall be based on the actual steel properties. If as-built information is not available, the stress-strain relationship may be obtained per Section 3107F.2.4.2.
Each structural component expected to undergo inelastic deformation shall be defined by its moment-curvature relation. The displacement demand and capacity shall be calculated per Sections 3104F.2 and 3104F.3, as appropriate.
The moment-rotation relationship for concrete components shall be derived from the moment-curvature analysis per Section 3107F.2.5.4 and shall be used to determine lateral displacement limitations of the design. Connection details shall be examined per Section 3107F.2.7.
The stress-strain model and terms for reinforcing and structural steel are shown in Figure 31F-7-2.
The stress-strain model of Blakeley and Park [7.4] may be used for prestressed steel. The model and terms are illustrated in Figure 31F-7-3.
Alternative stress-strain models are acceptable if adequately documented and supported by test results, subject to Division approval.
Different values may apply for plastic hinges forming at in-ground and pile-top locations. These procedures are applicable to circular, octagonal, rectangular and square pile cross sections.
Stability considerations are important to pier-type structures. The moment-axial load interaction shall consider effects of high slenderness ratios (kl/r). An additional bending moment due to axial load eccentricity shall be incorporated unless:
e = eccentricity of axial load
h = width of pile in considered direction
The plastic hinge length is required to convert the moment-curvature relationship into a moment-plastic rotation relationship for the nonlinear pushover analysis.
The pile's plastic hinge length, Lp (above ground) for reinforced concrete piles, when the plastic hinge forms against a supporting member is:
L = distance from the critical section of the plastic hinge to the point of contraflexure
db= diameter of the longitudinal reinforcement or dowel, whichever is used to develop the connection
fye = design yield strength of longitudinal reinforcement or dowel, whichever is used to develop the connection (ksi)
If a large reduction in moment capacity occurs due to spalling, then the plastic hinge length shall be:
When the plastic hinge forms in-ground, the plastic hinge length may be determined using Equation (7-7) [7.5]:
D = pile diameter or least cross-sectional dimension
PLASTIC HINGE LENGTH FOR PRESTRESSED CONCRETE PILES [7.5]
|CONNECTION TYPE||Lp AT DECK (in.)|
|Pile Buildup||0.15fyedb ≤ Lp ≤ 0.30fyedb|
db = diameter of the prestressing strand or dowel, whichever is used to develop the connection (in.)
fye = design yield strength of prestressing strand or dowel, as appropriate (ksi)
D = pile diameter or least cross-sectional dimension
dst = diameter of the prestressing strand (in.)
fpye = design yield strength of prestressing strand (ksi)
The plastic rotation is:
Lp = plastic hinge length
Φp = plastic curvature
Φm = maximum curvature
Φy = yield curvature
Alternatively, the maximum curvature, Φm may be calculated as:
εcm= maximum limiting compression strain for the prescribed performance level (Table 31F-7-5)
cu = neutral-axis depth, at ultimate strength of section
LIMITS OF STRAIN
|COMPONENT STRAIN||LEVEL 1||LEVEL 2|
|MCCS Pile/deck hinge||εc ≤ 0.004||εc ≤ 0.025|
|MCCS In-ground hinge||εc ≤ 0.004||εc ≤ 0.008|
|MRSTS Pile/deck hinge||εs ≤ 0.01||εs ≤ 0.05|
|MRSTS In-ground hinge||εs ≤ 0.01||εs ≤ 0.025|
|MPSTS In-ground hinge||εp ≤ 0.005 (incremental)||εp ≤ 0.025 (total strain)|
MCCS = Maximum Concrete Compression Strain, εc
MRSTS = Maximum Reinforcing Steel Tension Strain, εs
MPSTS = Maximum Prestressing Steel Tension Strain, εp
Either Method A or B may be used for idealization of the moment-curvature curve.
For Method A, the yield curvature, Φy is the curvature at the intersection of the secant stiffness, EIc, through first yield and the nominal strength, (εc = 0.004).
For Method B, the elastic portion of the idealized moment-curvature curve is the same as in Method A (see Section 3107F.126.96.36.199). However, the idealized plastic moment capacity, Mp, and the yield curvature, Φy, is obtained by balancing the areas between the actual and the idealized moment-curvature curves beyond the first yield point (see Figure 31F-7-5). Method B applies to moment-curvature curves that do not experience reduction in section moment capacity.
Strain values computed in the nonlinear push-over analysis shall be compared to the following limits.
The maximum allowable concrete strains may not exceed the ultimate values defined in Section 3107F.2.5.5. The following limiting values (Table 31F-7-5) apply for each performance level for both existing and new structures. The "Level 1 or 2" refer to the seismic performance criteria (see Section 3104F.2.1).
For all non-seismic loading combinations, concrete components shall be designed in accordance with the ACI 318 [7.7] requirements.