Ies TechBrief - AWWA D100-05 Seismic Parameters

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AWWA D100-05 Seismic

Specification Requirements for FBT’s

ies – AWWA D100-05 Seismic Parameters INNOVUM © 2006-2011 Pg. 1 of 13

This brief is provided as a convenient summary of the seismic design parameters required for seismic design of flat-bottom, ground-

supported water storage tanks in accordance with AWWA D100-05. This summary is not all inclusive. Consult AWWA D100-05 for

complete requirements. This document may not be reproduced in any form with the prior written consent of innovum.

E N G I N E E R I N G S E R V I C E S

AWWA D100-05 Seismic Design Specifications

Design Feature Options

Tank Geometry

Diameter, ft. __________

Height, ft. __________

TCL, ft. __________

MOL, ft. __________

Anchorage Type � Self-anchored

� Mechanically anchored

� Mechanically anchored only if required by design

Seismic Use Group � I

(Sec. 13.2.1) � II

(see summary below) � III

Site Class � A

(Sec. 13.2.4) � B

(from Geotech Report) � C

(see discussion below) � D

� E

� F

Seismic Design Method � General Procedure

(Sec. 13.2.7 or 13.2.8) � Site-Specific Procedure

Site Location

City, State ___________________________

Latitude ( °N) ____________

Longitude (- °W) ____________

Seismic design of roof � Yes � No

If yes, then

Vertical acceleration _________ %g

Live load included _________ psf

Column lateral wave load _________ psf

Column horizontal acceleration _________ %g









REQUIRED DATA NEEDED FOR THE GEOTECHNICAL INVESTIGATION

The proposed tank structure will impose both static and dynamic loads on the foundation and the

supporting soil. In order to develop proper recommendations for the foundation requirements and

to verify suitability of the site for the proposed structure, the following information should be

provided to the geotechnical engineer:

1. Tank Diameter, ft.

2. Tank Height, ft.

3. Maximum Liquid Level, ft.

4. Specific Gravity of stored liquid

5. Static tank shell load, lbs/ft, dead plus live (includes shell-supported roof weight)

6. Static roof support column loads, dead plus live

7. Maximum overturning shell compression loading, lbs/ft, dead load plus seismic (includes

shell weight and shell-supported roof weight)

8. Maximum overturning shell compression loading, lbs/ft, dead load plus wind (includes

shell weight and shell-supported roof weight)

REQUIRED CONTENT OF THE GEOTECHNICAL INVESTIGATION (excerpts from AWWA D100-05, © AWWA)

The types of soil present and their engineering properties shall be established by a geotechnical

investigation. A geotechnical investigation shall be performed to determine the following:

1. The presence or absence of rock, old excavation, or fill.

2. Whether the site is suitable for the structure to be built thereon and what remediation, if

any, is necessary to make it suitable.

3. The classification of soil strata after appropriate sampling.

4. The type of foundation that will be required at the site.

5. The elevation of groundwater and whether dewatering is required.

6. The bearing capacity of the soil and depth at which foundation must be founded.

7. Whether a deep foundation will be required and the type, capacity, and required length of

piles, caissons, piers, etc.

8. The elevations of the existing grade and other topographical features that may affect the

foundation design or construction.

9. The homogeneity and compressibility of the soils across the tank site and estimated

magnitude of uniform and differential settlement.

10. For standpipes and reservoirs, the minimum allowable foundation width for continuous

and isolated footings, if applicable.

11. Site Class in accordance with Sec. 13.2.4 and Table 25

12. When the Site-Specific Procedure of Sec. 13.2.8 is specified or when the site has been

determined to be Site Class F, a Site-Specific Design Response Spectra shall be prepared

in accordance with the requirements of 13.2.8.1 (Sec. 3.4 of FEMA 450, see page 9 of

this TechBrief).

13. The shear wave velocity (feet per second), at small strains, of the top 100 feet of the site

14. The average shear modulus, at small strains, of the soil beneath the foundation

15. The average unit weight of the soils

16. For Site Class F sites, the Site Coefficients Fa and Fv









SEISMIC USE GROUPS (excerpt from AWWA D100-05, © AWWA)

13.2.1 Seismic use group. The Seismic Use Group is a classification assigned to the tank

based on its intended use and expected performance. The following Seismic Use Group

definitions shall be used. For tanks serving multiple facilities, the facility having the

highest Seismic Use Group shall be used. Seismic Use Group III shall be used unless

otherwise specified.

13.2.1.1 Seismic use group III.

Seismic Use Group III shall be used for tanks that provide direct service to facilities that

are deemed essential for post earthquake recovery and essential to the life, health, and

safety of the public, including post-earthquake fire suppression.

13.2.1.2 Seismic use group II.

Seismic Use Group II shall be used for tanks that provide direct service to facilities that

are deemed important to the welfare of the public.

13.2.1.3 Seismic use group I.

Seismic Use Group I shall be used for tanks not assigned to Seismic Use Group III or II.

SITE CLASS (excerpt from AWWA D100-05, © AWWA)

Site Class shall be determined in accordance with Sec. 13.2.4 and Table 25

13.2.4 Site Class. Site Class accounts for the effect of local soil conditions on the ground

motion and shall be based on the types of soil present and their engineering properties.

The types of soil present and their engineering properties shall be established by a

geotechnical investigation. The site shall be specified as one of the site classes in Table

25. Site Class D shall be used when the soil properties are not known in sufficient detail

to determine the Site Class.

13.2.4.1 Site classification for seismic design. The parameters used to define the Site

Class are based on the upper 100 ft (30 m) of the site profile. Profiles containing

distinctly different soil and rock layers shall be subdivided into those layers designated

by a number that ranges from 1 to n at the bottom where there are a total of n distinct

layers in the upper 100 ft (30 m). Where some of the n layers are cohesive and others are

not, k is the number of cohesive layers and m is the number of cohesionless layers. The

symbol i refers to any one of the layers between 1 and n. The following parameters shall

be used to classify the site:









13.2.4.1.1 Average shear wave velocity, sv . The average shear wave velocity sv shall

be determined using the following equation where ∑=

n

i

id1

is equal to 100 ft (30 m):

∑

∑

=

==ν

n

i si

i

n

i

i

s

v

d

d

1

1 (Eq 13-1)

Where:

sv = average shear wave velocity in the top 100 ft (30 m) in feet per second

di = thickness of layer 'i' in feet (m)

vsi = shear wave velocity of layer 'i' in feet per second

13.2.4.1.2 Average standard penetration resistance N or chN . The average standard

penetration resistance N for cohesionless soil, cohesive soil, and rock layers shall be

determined using the equation

∑

∑

=

==

n

i i

i

n

i

i

N

d

d

N

1

1 (Eq 13-2)

The average standard penetration resistance chN for cohesionless soil layers only shall be

determined using the following equation where ∑=

=m

i

si dd1

:

∑=

=m

i i

i

sch

N

d

dN

1

(Eq 13-3)

Where:

N or chN = average standard penetration in the top 100 ft (30 m) in blows per foot

Ni = standard penetration resistance of layer 'i' in blows per foot. Ni shall be

determined in accordance with ASTM D1586 and measured directly in the









field without corrections. Ni shall not be taken greater than 100 blows/ft (328

blows/m). Where refusal is met for a rock layer, Ni shall be taken as 100

blows/ft (328 blows/m).

ds = total thickness of cohesionless soil layers in the top 100 ft (30 m) in feet

The other symbols have been previously defined in this section.

13.2.4.1.3 Average undrained shear strength us . The average undrained shear

strength us shall be determined using the following equation where ∑=

=k

i

ci dd1

:

∑=

=k

i ui

i

cu

s

d

ds

1

(Eq 13-4)

Where:

us = average undrained shear strength in the top 100 ft (30 m) in pounds per

square foot

dc = total thickness of cohesive soil layers in the top 100 ft (30 m) in feet

sui = undrained shear strength of layer 'i' in pounds per square foot. The undrained

shear strength shall be determined in accordance with ASTM D2166 or

D2850, and shall not be taken greater than 5,000 psf (250 kPa).

The other symbols have been previously defined in this section.

13.2.4.2 Procedure for classifying a site. The following procedure shall be used when

classifying a site:

13.2.4.2.1 Check for the four characteristics of Site Class F (Table 25) requiring site-

specific evaluation. If the site has any of these characteristics, classify the site as Site

Class F and conduct a site-specific evaluation (Sec. 13.2.8.1).









13.2.4.2.2 Check for the existence of a total thickness of soft clay greater than 10 ft (3 m).

If the layer has all three of the characteristics of soft clay (su < 500, w ≥ 40 percent, and

PI > 20), classify the site as Site Class E.

13.2.4.2.3 Classify the site as Site Class E, D, or C based on one of the following

parameters and Table 25:

1. Average shear wave velocity sv in the top 100 ft (30 m)

2. Average standard penetration resistance N in the top 100 ft (30 m)

3. Average standard penetration resistance chN for cohesionless soil layers (PI ≤ 20)

in the top 100 ft (30 m) and average undrained shear strength us for cohesive soil

layers (PI > 20) in the top 100 ft (30 m). If the average undrained shear strength

us is used and the chN and us criteria differ, select the category with the softer

soils.

13.2.4.2.4 Assignment of Site Class B shall be based on the shear wave velocity for rock.

For competent rock with moderate fracturing and weathering, estimation of this shear

wave velocity shall be permitted. For more highly fractured and weathered rock, the

shear wave velocity shall be directly measured or the site shall be assigned to Site Class

C. Site Class B shall not be used where there is more than 10 ft (3 m) of soil between the

rock surface and the bottom of the spread footing or mat foundation.

13.2.4.2.5 Assignment of Site Class A shall be supported by either shear wave velocity

measurements on site or shear wave velocity measurements on profiles of the same rock

type in the same formation with an equal or greater degree of weathering and fracturing.

Where hard rock condition are known to be continuous to a depth of 100 ft (30 m),

surficial shear wave velocity measurements may be extrapolated to assess sv . Site Class

A shall not be used where there is more than 10 ft (3 m) of soil between the rock surface

and the bottom of the spread footing or mat foundation.









SITE CLASS DEFINITIONS (excerpt from AWWA D100-05, © AWWA)

Table 25 - Site Class definitions

Site

Class

Soil

Profile

Name

Average Properties in Top 100 ft

Shear

Wave

Velocity

sν (ft/s)

Standard

Penetration

Resistance

N or chN

Undrained

Shear

Strength

us (psf)

A Hard rock sν > 5,000 Not applicable Not applicable

B Rock 2,500 < sν ≤ 5,000 Not applicable Not applicable

C Very dense soil

and soft rock 1,200 < sν ≤ 2,500 N or chN > 50 us > 2,000

D Stiff soil

profile 600 ≤ sν ≤ 1,200 15 ≤ N or chN ≤ 50 1,000 ≤ us ≤ 2,000

Soft soil

profile sν < 600 N or chN < 15 us < 1,000

-- or --

E Any profile with more than 10 ft of soil having all of the following characteristics:

1. Plasticity index PI > 20,

2. Moisture content w ≥ 40 percent, and

3. Undrained shear strength us < 500 psf

F* Any profile containing soils having one or more of the following

characteristics:

1. Soils vulnerable to potential failure or collapse under seismic

loading such as liquefiable soils, quick and highly sensitive

clays, and collapsible weakly cemented soils

2. Peats and/or highly organic clays (more than 10 ft of peat and/or

highly organic clay)

3. Very high plasticity clays (more than 25 ft of soil thickness with

plasticity index PI > 75)

4. Very thick soft/medium stiff clays (more than 120 ft of soil

thickness)

* Site-specific evaluation and procedure (Sec. 13.2.8) are required.









SITE-SPECIFIC RESPONSE SPECTRA (excerpt from AWWA D100-05, Appendix A – Commentary © AWWA

A13.2.8 Design Response Spectra – Site-Specific Procedure. The site-specific

procedure is used to develop ground motions that are determined with higher confidence

for the local seismic and site conditions than can be determined by using the general

procedure of Sec. 13.2.7, and is required for tanks located on Site Class F soils.

A.13.2.8.6 Design Response Spectrum. Special care must be exercised when

generating a design response spectrum from a site-specific spectrum with humps and

jagged variations. FEMA 450 requires that the parameter SDS be taken as the spectral

acceleration from the site-specific spectrum at a 0.2-second period, except that it shall not

be taken less than 90 percent of the peak spectral acceleration at any period larger than

0.2 seconds. Similarly, the parameter SD1 shall be taken as the greater of the spectral

acceleration at 1-second period or two times the spectral acceleration at 0.2-second

period. The parameters SMS and SM1 shall be taken as 1.5 times SDS and SD1, respectively.

The values so obtained shall not be taken as less than 80 percent of the values obtained

from the general procedure of Sec. 13.2.7. The resulting site-specific design spectrum

should be generated in accordance with Sec. 13.2.7.3.1 and should be smoothed to

eliminate extreme humps and jagged variations.









SITE-SPECIFIC PROCEDURE (from FEMA 450-03, © FEMA)

3.4 SITE-SPECIFIC PROCEDURE A site-specific study shall account for the regional tectonic setting, geology, and seismicity, the

expected recurrence rates and maximum magnitudes of earthquakes on known faults and source

zones, the characteristics of ground motion attenuation, near-fault effects if any on ground

motions, and the effects of subsurface site conditions on ground motions. The study shall

incorporate current scientific interpretations, including uncertainties, for models and parameter

values for seismic sources and ground motions. The study shall be documented in a report.

3.4.1 Probabilistic maximum considered earthquake. Where site-specific procedures are

utilized, the probabilistic maximum considered earthquake ground motion shall be taken as that

motion represented by a 5-percent-damped acceleration response spectrum having a 2 percent

probability of exceedance in a 50 year period.

3.4.2 Deterministic maximum considered earthquake. The deterministic maximum considered

earthquake spectral response acceleration at each period shall be taken as 150 percent of the

largest median 5-percent-damped spectral response acceleration computed at that period for

characteristic earthquakes on all known active faults within the region. For the purposes of these

Provisions, the ordinates of the deterministic maximum considered earthquake ground motion

response spectrum shall not be taken lower than the corresponding ordinates of the response

spectrum determined in accordance with Figure 3.4-1, where Fa and Fv are determined using

Tables 3.3-1 and 3.3-2, with the value of SS taken as 1.5 and the value of S1 taken as 0.6.









Figure 3.4-1 Deterministic Lower Limit on Maximum Considered Earthquake

3.4.3 Site-specific maximum considered earthquake. The site-specific maximum considered

earthquake spectral response acceleration at any period, SaM, shall be taken as the lesser of the

spectral response accelerations from the probabilistic maximum considered earthquake ground

motion of Sec. 3.4.1 and the deterministic maximum considered earthquake ground motion of

Sec. 3.4.2.

3.4.4 Design response spectrum. Where site-specific procedures are used to determine the

maximum considered earthquake ground motion, the design spectral response acceleration at any

period shall be determined from Eq. 3.4-1:

Sa = (2/3) SaM (3.4-1)

and shall be greater than or equal to 80 percent of Sa determined in accordance with Sec. 3.3.4.

For sites classified as Site Class F requiring site-specific evaluations (Note b to Tables 3.3-1 and

3.3-2 and Sec. 3.5.1), the design spectral response acceleration at any period shall be greater than

or equal to 80 percent of Sa determined for Site Class E in accordance with Sec. 3.3.4.

3.4.5 Design acceleration parameters. Where the site-specific procedure is used to determine

the design response spectrum in accordance with Section 3.4.4, the parameter SDS shall be taken

as the spectral acceleration, Sa, obtained from the site-specific spectrum at a period of 0.2 second,

except that it shall not be taken as less than 90 percent of the peak spectral acceleration, Sa , at any

period larger than 0.2 second. The parameter SD1 shall be taken as the greater of the spectral

acceleration, Sa , at a period of 1 second or two times the spectral acceleration, Sa , at a period 2

seconds. The parameters SMS and SM1 shall be taken as 1.5 times SDS and SD1, respectively. The

values so obtained shall not be taken as less than 80 percent of the values obtained from the

general procedure of Section 3.3.









SITE-SPECIFIC PROCEDURE COMMENTARY (from FEMA 450-03 Commentary, © NIBS/BSSC/FEMA)

C3.4 SITE-SPECIFIC PROCEDURE The objective in conducting a site-specific ground motion analysis is to develop ground motions

that are determined with higher confidence for the local seismic and site conditions than can be

determined from national ground motion maps and the general procedure of Sec. 3.3.

Accordingly, such studies must be comprehensive and incorporate current scientific

interpretations. Because there is typically more than one scientifically credible alternative for

models and parameter values used to characterize seismic sources and ground motions, it is

important to formally incorporate these uncertainties in a site-specific probabilistic analysis. For

example, uncertainties may exist in seismic source location, extent and geometry; maximum

earthquake magnitude; earthquake recurrence rate; choices for ground motion attenuation

relationships; and local site conditions including soil layering and dynamic soil properties as well

as possible two- or three-dimensional wave propagation effects. The use of peer review for a site-

specific ground motion analysis is encouraged.

Near-fault effects on horizontal response spectra include (1) directivity effects that increase

ground motions for periods of vibration greater than approximately 0.5 second for fault rupture

propagating toward the site; and (2) directionality effects that increase ground motions for periods

greater than approximately 0.5 second in the direction normal (perpendicular) to the strike of the

fault. Further discussion of these effects is contained in Somerville et al. (1997) and Abrahamson

(2000).

Conducting site-specific geotechnical investigations and dynamic site response analyses. Provisions. Tables 3.3-1 and 3.3-2 and Sec. 3.5.1 require that site-specific geotechnical

investigations and dynamic site response analysis be performed for sites having Site Class F soils.

Guidelines are provided below for conducting site-specific investigations and site response

analyses for these soils. These guidelines are also applicable if it is desired to conduct dynamic

site response analyses for other site classes.

Site-specific geotechnical investigation:

For purposes of obtaining data to conduct a site response analysis, site-specific geotechnical

investigations should include borings with sampling, standard penetration tests (SPTs) for sandy

soils, cone penetrometer tests (CPTs), and/or other subsurface investigative techniques and

laboratory soil testing to establish the soil types, properties, and layering and the depth to rock or

rock-like material. For very deep soil sites, the depth of investigation need not necessarily extend

to bedrock but to a depth that may serve as the location of input motion for a dynamic site

response analysis (see below). It is desirable to measure shear wave velocities in all soil layers.

Alternatively, shear wave velocities may be estimated based on shear wave velocity data

available for similar soils in the local area or through correlations with soil types and properties.

A number of such correlations are summarized by Kramer (1996).









Dynamic site response analysis:

Components of a dynamic site response analysis include the following steps:

1. Modeling the soil profile: Typically, a one-dimensional soil column extending from the

ground surface to bedrock is adequate to capture first-order site response characteristics. For

very deep soils, the model of the soil columns may extend to very stiff or very dense soils at

depth in the column. Two- or three-dimensional models should be considered for critical

projects when two or three-dimensional wave propagation effects should be significant (e.g.,

in basins). The soil layers in a one-dimensional model are characterized by their total unit

weights and shear wave velocities from which low-strain (maximum) shear moduli may be

obtained, and by relationships defining the nonlinear shear stress-strain relationships of the

soils. The required relationships for analysis are often in the form of curves that describe the

variation of soil shear modulus with shear strain (modulus reduction curves) and by curves

that describe the variation of soil damping with shear strain (damping curves). In a two- or

three-dimensional model, compression wave velocities or moduli or Poisson ratios also are

required. In an analysis to estimate the effects of liquefaction on soil site response, the

nonlinear soil model also must incorporate the buildup of soil pore water pressures and the

consequent effects on reducing soil stiffness and strength. Typically, modulus reduction

curves and damping curves are selected on the basis of published relationships for similar

soils (e.g., Seed and Idriss, 1970; Seed et al., 1986; Sun et al., 1988; Vucetic and Dobry,

1991; Electric Power Research Institute, 1993; Kramer, 1996). Site-specific laboratory

dynamic tests on soil samples to establish nonlinear soil characteristics can be considered

where published relationships are judged to be inadequate for the types of soils present at the

site. Shear and compression wave velocities and associated maximum moduli should be

selected on the basis of field tests to determine these parameters or published relationships

and experience for similar soils in the local area. The uncertainty in soil properties should be

estimated, especially the uncertainty in the selected maximum shear moduli and modulus

reduction and damping curves.

2. Selecting input rock motions: Acceleration time histories that are representative of horizontal

rock motions at the site are required as input to the soil model. Unless a site-specific analysis

is carried out to develop the rock response spectrum at the site, the maximum considered

earthquake (MCE) rock spectrum for Site Class B rock can be defined using the general

procedure described in Sec. 3.3. For hard rock (Site Class A), the spectrum may be adjusted

using the site factors in Tables 3.3-1 and 3.3-2. For profiles having great depths of soil above

Site Class A or B rock, consideration can be given to defining the base of the soil profile and

the input rock motions at a depth at which soft rock or very stiff soil of Site Class C is

encountered. In such cases, the MCE rock response spectrum may be taken as the spectrum

for Site Class C defined using the site factors in Tables 3.3-1 and 3.3-2. Several acceleration

time histories of rock motions, typically at least four, should be selected for site response

analysis. These time histories should be selected after evaluating the types of earthquake

sources, magnitudes, and distances that predominantly contribute to the seismic hazard at the

site. Preferably, the time histories selected for analysis should have been recorded on

geologic materials similar to the site class of materials at the base of the site soil profile

during earthquakes of similar types (e.g. with respect to tectonic environment and type of

faulting), magnitudes, and distances as those predominantly contributing to the site seismic









hazard. The U.S. Geological Survey national seismic hazard mapping project website

(http://geohazards.cr.usgs.gov/eq/) includes hazard deaggregation options and can be used to

evaluate the predominant types of earthquake sources, magnitudes, and distances contributing

to the hazard. Sources of recorded acceleration time histories include the data bases of the

Consortium of Organizations for Strong Motion Observation Systems (COSMOS) Virtual

Data Center web site (db.cosmos-eq.org) and the Pacific Earthquake Engineering Research

Center (PEER) Strong Motion Data Base website (http://peer.berkeley.edu/smcat/). Prior to

analysis, each time history should be scaled so that its spectrum is at the approximate level of

the MCE rock response spectrum in the period range of interest. It is desirable that the

average of the response spectra of the suite of scaled input time histories be approximately at

the level of the MCE rock response spectrum in the period range of interest. Because rock

response spectra are defined at the ground surface rather than at depth below a soil deposit,

the rock time histories should be input in the analysis as outcropping rock motions rather than

at the soil-rock interface.

3. Site response analysis and results interpretation: Analytical methods may be equivalent

linear or nonlinear. Frequently used computer programs for one-dimensional analysis

include the equivalent linear program SHAKE (Schnabel et al., 1972; Idriss and Sun,

1992) and the nonlinear programs DESRA-2 (Lee and Finn, 1978), MARDES (Chang et

al., 1991), SUMDES (Li et al., 1992), D-MOD (Matasovic, 1993), TESS (Pyke, 1992),

and DESRAMUSC (Qiu, 1998). If the soil response is highly nonlinear (e.g., high

acceleration levels and soft soils), nonlinear programs may be preferable to equivalent

linear programs. For analysis of liquefaction effects on site response, computer programs

incorporating pore water pressure development (effective stress analyses) must be used

(e.g., DESRA-2, SUMDES, D-MOD, TESS, and DESRAMUSC). Response spectra of

output motions at the ground surface should be calculated and the ratios of response

spectra of ground surface motions to input outcropping rock motions should be

calculated. Typically, an average of the response spectral ratio curves is obtained and

multiplied by the MCE rock response spectrum to obtain a soil response spectrum.

Sensitivity analyses to evaluate effects of soil property uncertainties should be conducted

and considered in developing the design response spectrum.

C3.4.2 Deterministic maximum considered earthquake.

It is required that ground motions for the deterministic maximum considered earthquake be based

on characteristic earthquakes on all known active faults in a region. As defined in Sec. 3.1.3, the

magnitude of a characteristic earthquake on a given fault should be a best-estimate of the

maximum magnitude capable for that fault but not less than the largest magnitude that has

occurred historically on the fault. The maximum magnitude should be estimated considering all

seismic-geologic evidence for the fault, including fault length and paleoseismic observations. For

faults characterized as having more than a single segment, the potential for rupture of multiple

segments in a single earthquake should be considered in assessing the characteristic maximum

magnitude for the fault.

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Ies TechBrief - AWWA D100-05 Seismic Parameters