STRC18 Lateral Forces Seismic Part1 0716 R1

S t r u c t u ra lE n g i n e e r i n gE xa mR e v i e wCo u rs e

L a t e r a lFo r c e s :S e i s m i cL o a d s(Pa r t1 )

Seismic Loads (Part 1) Structural Engineering Review Course

STRC ©2015 Professional Publications, Inc.

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Seismic Loads (Part 1)

Lesson Overview Lateral Forces Seismic Design •

Equivalent Lateral Force Procedure

•

Vertical Distribution of Seismic Forces

Lateral-Force Resisting Systems •

Structural Systems



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Learning Objectives You will learn •

•

•

•

simple approximations for the fundamental period of vibration of typical structures how to calculate seismic loads how to distribute seismic loads to typical building structures

•

guidance on choosing variables

•

use of tables and figures

•

application of minimum load limits

•

interpretation of important text

tips on how to navigate ASCE/SEI7 2010 and the IBC 2012 design codes for seismic loads



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Learning Objectives You should already be familiar with •

layout of the referenced design codes

•

load application by tributary areas

•

linear interpolation

•

calculating weighted averages

•

common terms for seismic loading

•

•

•

typical building components (braces, beams, trusses, etc.) theory of earthquake forces (inertial forced induced by ground shaking)

typical LFRS (braced frames, moment frames, shear walls, etc.)



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Referenced Codes and Standards •

Minimum Design Loads for Buildings and Other Structures (ASCE/SEI7, 2010)

•

International Building Code (IBC, 2012)

•

Specification for Structural Steel Buildings (AISC 360, 2010)

•

Seismic Provisions for Structural Steel Buildings (AISC 341, 2010)



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Equivalent Lateral Force Procedure: Factors Depends on several factors including •

ground motion parameters

•

classification of the structural system

•

site classification

•

response modification coefficient

•

site coefficient

•

deflection amplification factor

•

adjusted response acceleration

•

overstrength factor

•

design spectral response acceleration

•

effective seismic weight

•

risk category and importance factor

•

fundamental period of vibration

•

seismic design category

•

seismic response coefficient



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Ground Motion Parameters •

maximum considered ground acceleration that may be experienced at a specific

•

location [defined in ASCE/SEI7 Sec. 11.4.1] most severe earthquake effects considered by the code [ASCE/SEI7 Sec. 11.2]

•

•

parameters risk-adjusted to provide uniform risk with a 1% probability of collapse in 50 years two values of ground acceleration (SS and S1) required



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Ground Motion Parameters SS = 5% damped maximum considered earthquake (MCE R) spectral response acceleration for a period of 0.2 sec for structures founded on rock (site classification B); applicable to short period structures

S1 = 5% damped MCER spectral response acceleration for a period of 1 sec for structures founded on rock; applicable to structures with longer periods •

Refer to ASCE/SEI7 Fig. 22-1 through Fig. 22-17 for these parameters.

•

given as a percentage of the acceleration due to gravity (g’s)



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Site Classification Table 7.2 Site Classification Definitions

reduced seismic effects

amplified seismic effects •

defined in ASCE/SEI7 Sec. 11.4.2 and ASCE/SEI7 Table 20.3-1

•

average shear wave velocity in top 100 ft of material may be used to define site class

•

for C, D, or E classification, may be made by measuring standard penetration resistance or undrained shear strength of the material STRC ©2015 Professional Publications, Inc.


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Example: So il Profile Type SEIS Example 6.1


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Site Classification soil classification type •

A: “hard rock” and reduces ground response by 20%

•

E: “soft soil” and amplifies long period response up to 350%

•

can assume soil classification type D when soil parameters are unknown, unless the building official determines type E or F is likely to be present at site [ASCE/SEI7 Sec. 11.4.2]



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Site Coefficients •

amplification factors applied to the maximum considered ground acceleration

•

function of the site classification

Fa = short-period or acceleration-based amplification factor [ASCE/SEI7 Table 11.4-1] Fv = long-period or velocity-based amplification factor [ASCE/SEI7 Table 11.4-2] •

linear interpolation may be used to obtain intermediate values

•

chosen from tables based on site classification and ground motion parameters



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Site Coefficients SEIS Table 6.4 Seismic Coefficient as a Function of Site Class [IBC Table 1613.3.3(1)]

SEIS Table 6.5 Seismic Coefficient as a Function of Site Class [IBC Table 1613.3.3(2)]

Reproduced from the 2012 edition of the International Building Code®, copyright © 2012, with the permission of the publisher, the International Code Council, iccsafe.org.

Reproduced from the 2012 edition of the International Building Code®, copyright © 2012, with the permission of the publisher, the International Code Council, iccsafe.org.



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Adjusted Response Acceleration •

•

defined in ASCE/SEI7 Sec. 11.4.3 adjusted version of the maximum considered ground accelerations to account for site classification effects (site coefficients) •

modified spectral response acceleration parameter at short periods (0.2 sec) ASCE/SEI7 Eq. 11.4-1

•

modified spectral response acceleration parameter at 1 sec period ASCE/SEI7 Eq. 11.4-2



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Design Response Acceleration Reducing the adjusted response acceleration by 33% yields the design response acceleration. 5% damped spectral response acceleration parameter at short periods (0.2 sec) •

ASCE/SEI7 Eq. 11.4-3 •

5% damped spectral response acceleration parameter at 1 sec period ASCE/SEI7 Eq. 11.4-4



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Poll Question: SDS What is the proper name for the variable SDS? (A) 5% damped spectral response acceleration parameter for short periods (0.2 sec) (B) 5% damped MCER spectral response acceleration for short periods (0.2 sec) (C) modified spectral response acceleration for long periods (1.0 sec) (D) 5% damped MCER spectral response acceleration for long periods (1.0 sec)



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Poll Question: SDS What is the proper name for the variable SDS?

(A) 5% damped spectral response acceleration parameter for short periods (0.2 sec) (B) 5% damped MCER spectral response acceleration for short periods (0.2 sec) (C) modified spectral response acceleration for long periods (1.0 sec) (D) 5% damped MCER spectral response acceleration for long periods (1.0 sec)



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Poll Question: S1 What is the proper name for the variable S1? (A) 5% damped spectral response acceleration parameter for short periods (0.2 sec) (B) 5% damped MCER spectral response acceleration for short periods (0.2 sec) (C) modified spectral response acceleration for long periods (1.0 sec) (D) 5% damped MCER spectral response acceleration for long periods (1.0 sec)



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Poll Question: S1 What is the proper name for the variable S1? (A) 5% damped spectral response acceleration parameter for short periods (0.2 sec) (B) 5% damped MCER spectral response acceleration for short periods (0.2 sec) (C) modified spectral response acceleration for long periods (1.0 sec)

(D) 5% damped MCER spectral response acceleration for long periods (1.0 sec)



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Summary of Design Spectral Response Accelerations For short periods (0.2 sec), combining ASCE/SEI7 Eq. 11.4-1 and Eq. 11.4-3 results in SMS 2  

3

 

For long periods (1.0 sec), combining ASCE/SEI7 Eq. 11.4-2 and Eq. 11.4-4 results in SM1 2  

3

 



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Example: Calculate SDS and SD1 Example 7.8



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Example: Calculate SDS and SD1 Example 7.8



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Example: Calculate SDS and SD1



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Risk C ategory and Importance Factor Each structure is assigned to a risk category depending on the nature of its occupancy [ASCE/SEI7 Table 1.5-1]. The corresponding importance factor is indicated in ASCE/SEI7 Table 1.5-2. Table 7.4 Occupancies and Importance Factors

Ie > 1.0

Adapted with permission from Minimum Design Loads for Buildings and Other Structures, copyright © 2010, by the American Society of Civil Engineers.



increases seismic design forces, so inelasticity will occur at higher forces

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Risk Category and Importance Factors category IV structures(Ie = 1.50) •

•

•

structures housing essential facilities required for post-earthquake recovery structures containing substances that would endanger the safety of the public if released essential facilities defined in IBC Sec. 1604.5 •

hospitals, fire and police stations, emergency response centers, buildings housing utilities and equipment for these facilities, etc.



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Risk Category and Importance Factors category III structures(Ie = 1.25) •

facilities that, if they failed, would become a substantial public hazard because of their high occupant load •

buildings where more than 300 people congregate in one area

•

schools with capacity > 250

•

colleges with capacity > 500

•

•

jails

•

power stations

•



health care with capacity > 50 that do not have emergency treatment

facilities containing explosives or toxic substances in a quantity exceeding the exempt amounts in IBC Table 307.1(1) and Table 307.1(2)

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Risk Category and Importance Factors category II structures(Ie = 1.00) •

standard occupancy structures such as residential, commercial, and office buildings

category I structures •

low-hazard structures such as agricultural facilities, temporary facilities, and minor storage facilities



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Seismic Design Category (SDC) •

defined in ASCE/SEI7 Sec. 11.6 and ASCE/SEI7 Table 11.6-1 and Table 11.6-2

•

establishes the design and detailing requirements necessary in a structure

•

Structures are assigned to a seismic design category (A through F) based on •

•

•

risk category design spectral response coefficients (SDS and SD1)

seismic design category is determined twice (worst case controls)



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Seismic Design Category (SDC) determined as a function of SDS and as a function of SD1; worst case controls SEIS Table 6.7 Seismic Design Category Based on SDS (short-period response) Acceleration Parameter [ASCE/SEI7 Table 11.6-1 (IBC Table 1613.3.5(1))]

SEIS Table 6.8 Seismic Design Category Based on SD1 (1 sec period response) Acceleration Parameter [ASCE/SEI7 Table 11.6-2 (IBC Table 1613.3.5(2))]

Reproduced from the 2010 edition of Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers (ASCE), copyright © 2010. Used with permission from ASCE.

Reproduced from the 2010 edition of Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers (ASCE), copyright © 2010. Used with permission from ASCE.



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Seismic Design Category (SDC) SDC

description of seismic design category

A

structures located where anticipated ground motion is minimal [ASCE/SEI7 Sec. 11.7]

B

structures in risk categories I, II, and III in regions of moderate seismicity

C

risk category IV structures in regions of moderate seismicity and risk category I, II, and III structures in regions of somewhat severe seismicity

D

structures in risk categories I, II, III, and IV in regions of high seismicity, but not located close to an active fault and in risk category IV with less seismicity

E

structures in risk categories I, II, and III located close to a major active fault

F

structures in risk category IV located close to a major active fault



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Seismic Design Category (SDC) limitations on lateral force resisting system (LFRS) •

The LFRS must conform to either •

•

•

•

one of the construction types in ASCE/SEI7 Table 12.2-1 a combination of systems as permitted in ASCE/SEI7 Sec. 12.2.2, Sec. 12.2.3, and Sec. 12.2.4

LFRSs must be designed using specific requirements in applicable reference documents listed in ASCE/SEI7 Table 12.2-1 and additional requirements in ASCE/SEI7 Chap. 14 LFRSs not in ASCE/SEI7 Table 12.2-1 are permitted, provided analytical and test data are submitted and approved (see ASCE/SEI7 Sec. 12.2.1 for more information) STRC ©2015 Professional Publications, Inc.


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Response Modification Factor •

represents the inherent overstrength and global ductility capacity of structural

•

components determined from the type of LFRS [ASCE/SEI7 Table 12.2-1] •

•

reduces predicted elastic response to “true” peak design loads accounting for level of inelastic (plastic) response of the LFRS

R = 1.0 signifies a structure that responds purely elastically to seismic loading (lightly damped brittle structures)

•

R > 1.0 signifies a structure that responds inelastically to seismic loading (higher R indicates higher damping and ductility)



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Seismic System Overstrength Factor overstrength characteristic of structures where the actual strength is greater than the design strength •

•

•

A structure’s response to ground motion is significantly affected by the •

natural period (fundamental period of vibration)

•

type of structural system

In severe earthquakes, structures are expected to deform beyond their elastic loadcarrying capacities. Overloading of nonductile elements of the structure can occur if effects of overstrength are not considered.



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Seismic System Overstrength Factor seismic system overstrength factor, ΩO (a.k.a. seismic force amplification factor) •

assigned based on structural system

•

accounts for expected overstrength of structure in the inelastic range produced by conservative design methods, system redundancy, material overstrength, oversized members, application of load factors, and drift limitations that are controlling design

VY = base shear at formation of collapse mechanism VS = design base shear STRC ©2015 Professional Publications, Inc.


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Deflection Amplification Factor •

•

Cd = ratio between the inelastic and elastic lateral deflection tabulated in ASCE/SEI7 Table 12.2-1 as

x

= inelastic deflection

 xe = •

elastic deflection

ASCE/SEI7 Eq. 12.8-15 gives the value of displacement as

Cd xe = approximated inelastic deflection Ie = seismic importance factor STRC ©2015 Professional Publications, Inc.


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Summary of ΩO and R Factors Inelastic Force-Deformation Curve

Reproduced from "A Brief Guide to Seismic Design Factors,” STRUCTURE , September 2008, SEAOC Seismology Committee.



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Tabulated Values ofR, ΩO and Cd Table 7.6 Seismic Parameters and Building Height (partial table)

Adapted with permission from Minimum Design Loads for Buildings and Other Structures , Table 12.2-1, copyright © 2010, by the American Society of Civil Engineers.



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Classification of the Structural System ASCE/SEI7 Sec. 12.2.1 and ASCE/SEI7 Table 12.2-1 detail eight major categories of building types 1. bearing wall

6. shear wall-frame interactive system with ordinary reinforced concrete moment frames and ordinary reinforced concrete shear walls

2. building frame 3. moment-resisting frame 4. dual system with special momentresisting frames

7. cantilevered column

8. steel systems not specifically detailed for seismic resistance, excluding 5. dual system with intermediate moment cantilever column systems frames (undefined systems) STRC ©2015 Professional Publications, Inc.


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Bearing Wall Systems bearing wall wall that resists vertical loads SEIS Figure 6.2 Bearing Wall System

shear wall wall that resists lateral loads.

bearing wall systems structural systems that rely on the same elements to resist both gravity and lateral loads.



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Bearing Wall Systems bearing wall systems •

•

•

lack redundancy and have minimal inelastic capacity Since the bearing walls constitute the gravity and lateral force resisting systems, failure of the seismic system compromises the ability of the structure to support gravity loads. Typical bearing wall systems are •

light-framed walls with shear panels, concrete or masonry walls

•

light steel-framed bearing walls with tension-only braces

•

braced frames where the bracing carries gravity loads



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Building Frame Systems building frame systems •

•

structures with space frames that carry vertical (gravity) loads and a separate set of non-bearing shear walls or braced frames to resist lateral loads

SEIS Figure 6.3 Building Frame System

Failure of the primary LFRS system does not compromise the ability of the structure to support gravity loads.



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Moment-Resisting Frame Systems moment-resisting frame systems •

•

•

structures resist forces in members and joints primarily by flexure

SEIS Figure 6.4 Moment-Resisting Frame System

rely on the frame to carry both vertical and lateral loads may be constructed from concrete, masonry (limited), or steel



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Dual Systems dual systems •

•

have essentially complete space frames that provide support for gravity loads

SEIS Figure 6.5 Dual System

combine two systems to resist lateral loads (moment-resisting frames and shear wall-braced frames)



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Shear Wall-Frame Interaction Systems shear wall-frame interaction systems •

•

•

systems that primarily use a combination of shear walls and moment frames

SEIS Figure 6.6 Shear Wall-Frame Interaction System

Building frames that are part of the LFRS are required to be concrete frames. These systems are restricted to locations of low seismicity.



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Cantilevered Column Systems cantilevered column building system •

•

•

also referred to as inverted pendulum systems

SEIS Figure 6.7 Cantilevered Column Building System

utilize single cantilever column elements supporting beams or framing typically have most of their mass concentrated at or near the top, and are fixed at their bases



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Non-Seismic Steel Systems Non-seismic steel systems are steel systems not specifically designed for seismic loads. •

designed and detailed according to AISC 360

•

not required to meet design and detailing requirements of AISC 341



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Undefined Systems For undefined systems that do not fit into the previous seven categories, the designer must submit a rational basis for the design force level used.



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Effective Seismic Weight effective seismic weight,W [ASCE/SEI7 Sec. 12.7.2] •

•

total dead load of the structure and the part of the service load that may be expected to be attached to the building consists of the dead load and •

25% of floor live load for storage and warehouse occupancies

•

10 lbf/ft2 for moveable partitions

•

flat roof snow loads exceeding 30 lbf/ft2 reduced by 80%

•

total weight of permanent equipment and fittings



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Example: Effective Seismic Weight Calculate the effective seismic weight of a two-story office building given the following information. •

floor area = 15,000 ft2

•

dead load = 100 lbf/ft2

•

roof dead load = 20 lbf/ft2

•

flat roof snow load = 15 lbf/ft 2

•

floor live load = 50 lbf/ft2



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Example: Effective Seismic Weight Calculate the effective seismic weight of a two-story office building given the following information. •


•


•

The floor dead load is D

2  100 lbf2  15,000 ft  ft    1500 kips

1000

The roof dead load is 2

roof dead load = 20 lbf/ft

DR

lbf   2  20 2  15,000 ft  ft     300 kips 1000

lbf/ft 2

•

flat roof snow load = 15

•


lbf

kip

The snow load is 15



lbf kip

lbf ft 2

 30

lbf ft 2

[neglect snow loads]

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Example: Effective Seismic Weight Calculate the effective seismic weight

Since the building is not a warehouse

of a two-story office building given the following information.

and is not used for storage, floor live load may be ignored.

•


The partition load is

lbf/ft2

•

dead load = 100

•


WP

 lbf  2  10 2  15,000 ft  ft    150 kips 1000

lbf/ft 2

•

flat roof snow load = 15

•




lbf kip

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Example: Effective Seismic Weight Calculate the effective seismic weight

The effective seismic weight is

of a two-story office building given the following information

WD 

•


•


•


•

flat roof snow load = 15 lbf/ft 2

•


D 

P

 1500 kips  300 kips 150 kips =1950 kips



W R 

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Poll Question Which of the following should NOT be included in calculating the seismic weight of a two-story warehouse building with a flat roof? (A) dead load above the first floor (B) 25% live load on the second floor (C) 20% of the 30 lbf/ft 2 roof snow load (D) weight of permanent HVAC equipment on the roof



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Poll Question Which of the following should NOT be included in calculating the seismic weight of a two-story warehouse building with a flat roof? (A) dead load above the first floor (B) 25% live load on the second floor

(C) 20% of the 30 lbf/ft 2 roof snow load (D) weight of permanent HVAC equipment on the roof



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Fundamental Period of Vibration There are two simplified methods for calculating the fundamental period of vibration of a structure, Ta, which are summarized in ASCE/SEI7 Sec. 12.8.2. 1. method 1 uses ASCE/SEI7 Eq. 12.8-8 2. method 2 uses ASCE/SEI7 Eq. 12.8-7 This section also allows for a “properly substantiated analysis” method to determine Ta, such as the 3. Rayleigh procedure (method 3)



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Fundamental Period of Vibration method 1 Ta

 0.10 N

N = number of stories only applies for structures with •

12 stories or less above the base, per ASCE/SEI7 Sec. 11.2

•

MWFRS that consists entirely of concrete or steel moment frames

•

an average story height of at least 10 ft



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Fundamental Period of Vibration method 2

Ta

system type

0.028 .

steel moment-resisting frames

0.016 .

concrete moment-resisting frames

0.030 .

steel eccentrically braced frame

. 0.030 

steel buckling restrained braced frames

0.020 .

all other structural systems

hn = structural height as defined in ASCE/SEI7 Sec. 11.2



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Fundamental Period of Vibration method 3 – Rayleigh procedure

Figure 7.22 Application of the Rayleigh Procedure

Using this procedure, the fundamental period is given by



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Fundamental Period of Vibration ASCE/SEI7 Sec. 12.8.2 specifies a maximum limit on the fundamental period of vibration as Values of Cu are given in ASCE/SEI7 Table 12.8-1 and are shown in Table 7.7.

Table 7.7 Coefficient for Upper Limit on the Calculated Period

This applies when using methods like the Rayleigh procedure, computer analysis, etc., rather than the ASCE/SEI7 equations for method 1 and method 2.



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Example: Fundamental Period of Vibration Example 7.12



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Example: Fundamental Period of Vibration Example 7.12



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Example: Fundamental Period of Vibration



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General Procedure Response Spectrum •

procedure defined in ASCE/SEI7 Figure 7.23 Construction of ASCE/SEI7 Response Spectra

Sec. 11.4.5 and shown in ASCE/S EI7 Fig. 11.4-1 •

2

3

four key points are labeled in Fig. 7.23

1 4



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General Procedure Response Spectrum point 1 Figure 7.23 Construction of ASCE/SEI7 Response Spectra

period:

T=0

spectral acceleration:

0.40SDS

2

3

point 2



period:

T0


SDS

0.2S D1 S DS

linear between points 1 and 2

1 4



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

period:

TS


SDS

S D1 S DS

2

3

constant between points 2 and 3

1 4



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period:



2

3

(see ASCE/SEI7 Fig. 22-12 to Fig. 22-16) spectral acceleration:

S D1 TL

between points 3 and 4 1 4

beyond point 4



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Seismic Response Coefficient •

seismic response coefficient, Cs, is given in ASCE/SEI7 Sec. 12.8.1.1

•

For T ≤ TL refer to ASCE/SEI7 Eq. 12.8-3.

•

For T > TL refer to ASCE/SEI7 Eq. 12.8-4.

•

However, Cs is subject to both upper and lower limits.



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Seismic Response Coefficient •

The maximum value for Cs is given by ASCE/SEI7 Eq. 12.8-2.

•

The minimum value for Cs is given by ASCE/SEI7 Eq. 12.8-5.

•

Where S1 ≥ 0.6g, the minimum value for Cs is given by ASCE/SEI7 Eq. 12.8-6.



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Example: Seismic Response Coefficient Example 7.13



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Example: Seismic Response Coefficient Example 7.13



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Example: Seismic Response Coefficient



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Seismic Base Shear •

•

seismic base shear is specified in ASCE/SEI7 Eq. 12.8-1

Cs is the seismic response coefficient (a.k.a. seismic base shear coefficient).

•

W is the effective seismic weight.

•

Per ASCE/SEI7 Sec. 1.4.3, for seismic design category A,

Fx = 0.01wx •

Therefore,

V = 0.01W



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Example: Seismic Base Shear SEIS Example 6.7



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Example: Seismic Base Shear SEIS Example 6.7



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Example: Seismic Base Shear



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Example: Seismic Base Shear



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Example: Seismic Base Shear Calculate the seismic base shear of a onestory structure given the following information. design spectral accelerations

SDS = 0.15 SD1 = 0.05 effective seismic weight at roof

W = 1000 kips



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Example: Seismic Base Shear Calculate the seismic base shear of a onestory structure given the following information.

The design spectral accelerations are as follows.

SDS ≤ 0.167

design spectral accelerations

SD1 ≤ 0.06

SDS = 0.15 SD1 = 0.05

Both accelerations result in seismic design category A, and since the worst case controls, the seismic design category is A.

effective seismic weight at roof

W = 1000 kips

Per ASCE/SEI7 Sec. 1.4.3, V

 0.01W   0.01  1000k ips  10 kips



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Equivalent Lateral Force Procedure: Limitations building configuration requirements applicable to structures that satisfy the following prescribed conditions of regularity, occupancy, location, and height •

mass, stiffness, and strength uniformly distributed over the height of the structure

•

no irregular features that will produce stress concentrations •

vertical irregularities are defined in ASCE/SEI7 Table 12.3-2

•

horizontal irregularities are defined in ASCE/SEI7 Table 12.3-1



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Equivalent Lateral Force Procedure: Limitations The equivalent lateral force procedure may be used in the design of structures under the following conditions. seismic design category A not required; refer to ASCE/SEI7 Sec. 11.7

seismic design category B or C acceptable for all building types in these categories



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Equivalent Lateral Force Procedure: Limitations The equivalent lateral force procedure may be used in the design of structures under the following conditions (a structure only needs to conform to one of the following). •

•

•

seismic design categories D, E, or F buildings built from light-frame construction buildings in risk category I and II not exceeding two stories in height

•

regular buildings exceeding 160 ft in height with fundamental period

T < 3.5TS •

buildings less than or equal to 160 ft in height that do not have the following irregularities •

•

regular buildings less than or equal to 160 ft in height STRC ©2015 Professional Publications, Inc.


horizontal: (1a) torsional, (1b) extreme torsional vertical: (1a) soft story, (1b) extreme soft story, (2) mass, (3) geometric

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Vertical Distribution of Seismic Forces Using the equivalent lateral force procedure, •

•

Figure 7.24 Vertical Force Distribution

base shear is distributed over the height of the structure per ASCE/SEI7 Sec. 12.8.3 lateral force at level x is given by ASCE/SEI7 Eq. 12.8-11 and Eq. 12.8-12 as combined below



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hi = height above the base to any level i hx = height above the base to level x wi = effective seismic weight at level i k = exponent related to structure period = 1 for structures with period less than or equal to 0.5 sec = 2 for structures with period greater than or equal to 2.5 sec

use linear interpolation for 0.5 sec > Ta > 2.5 sec

∑wihik = summation over the whole structure of the product of wi and hik STRC ©2015 Professional Publications, Inc.


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Example: Vert. Distribution of Seismic Forces Example 7.15



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Example: Vert. Distribution of Seismic Forces Example 7.15



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Example: Vert. Distribution of Seismic Forces



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Learning Objectives You have learned •

•

•

•

simple approximations for the fundamental period of vibration of typical structures how to calculate seismic loads how to distribute seismic loads to typical building structures

•

guidance on choosing variables

•

use of tables and figures

•

application of minimum load limits

•

interpretation of important text

tips on how to navigate ASCE/SEI7 2010 and the IBC 2012 design codes for seismic loads



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90




Lesson Overview Lateral Forces Seismic Design •

Equivalent Lateral Force Procedure

•


Lateral-Force Resisting Systems •

Structural Systems



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STRC18 Lateral Forces Seismic Part1 0716 R1

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