1. ASCE 7-05 Seismic Provisions
University of Alaska Anchorage
CE 694R – Loads on Structures
Fall 2007
Dr. T. Bart Quimby, P.E.
Quimby & Associates/UAA School of Engineering

2. Earthquake Protective Design Philosophical Issues
High probability of “Failure”
“Failure” redefined to permit behavior (yielding) that would be considered failure under other loads.
High Uncertainty
Importance of Details
“In dealing with earthquakes we must contend with appreciable probabilities that failure will occur in the near future. Otherwise, all the wealth of the world would prove insufficient… We must also face uncertainty on a large scale… In a way, earthquake engineering is a cartoon… Earthquakes systematically bring out the mistakes made in design and construction, even the minutest mistakes.” Newmark & Rosenblueth

3. Performance Levels Hazard Levels Occasional Incipient Collapse Rare
50% in 50 years
Rare
10% in 50 years
Very Rare
5% in 50 years
Max Considered
2% in 50 years
Incipient Collapse
Life Safety
Immediate Reoccupancy
Fully Operational

4. Design Objective Defined
A specific performance level given a specific earthquake hazard level
Stated basis of current codes:
Life safety (+some damage control) at 10% in 50 year event (nominally)

5. Purpose of the Provisions
FEMA 302 Section 1.1
“The design earthquake ground motion levels specified herein could result in both structural and nonstructural damage. For most structures designed and constructed according to these Provisions, structural damage from the design earthquake ground motion would be repairable although perhaps not economically so. For essential facilities, it is expected that the damage from the design earthquake ground motion would not be so severe as to preclude continued occupancy and function of the facility.”
“For ground motions larger than the design levels, the intent of these Provisions is that there be a low likelihood of structural collapse”

8. This example uses old version of NEHRP
This example uses old version of NEHRP. It is used for illustrative purposes only.

13. Elastic vs. Inelastic Response
The red line shows the force and displacement that would be reached if the structure responded elastically.
The green line shows the actual force vs. displacement response of the structure
The pink line indicates the minimum strength required to hold everything together during inelastic behavior
The blue line is the force level that we design for.
We rely on the ductility of the system to prevent collapse.
From 1997 NEHRP Provisions

14. Historical Development of Seismic Codes
Lisbon: ground shaking waves
San Francisco: Fire, lateral force from wind
Messina, Italy: Static inertial force (10%), First recognition of F=ma
Tokyo: Prediction by seismic gap
Santa Barbara: USCGS instructed to develop strong motion seismographs.
U.B.C.: Inertial forces and soil effects in the U.S. (7.5% or 10% of D+L)
Long Beach: First instrumental records (flawed): reinforcement required for masonry; quality assurance; design review & construction inspection.

15. Historical Development of Seismic Codes
El Centro: Earthquake ground motion record. Makes possible the computation of structural response. Became the most used record.
City of Los Angles Building Code: Dynamic property of building used in addition to mass (Number of stories relates to period and to distribution of force)
San Francisco Joint Committee:
Modal analysis used as a basis for static forces and distribution.
Difference between design force and computed forces not resolved.
Distinction for soils types dropped
Overturning reductions
Torsion
World Conference on Earthquake Engineering
Mexico City: Success with design using dynamic analysis.

16. Historical Development of Seismic Codes
SEAOC blue book
Design accel. Similar to 1943 LA and 1952 SF
Factor for performance of structural systems (K)
Effect of higher modes on vertical distribution
“Design of Multi-Story Reinforced Concrete Buildings for Earthquake Motions”, Blume, Newmark, and Corning
Inelastic response
Ductility in concrete
1964 Alaska Earthquake: Lack of instrumental data. Observations influenced thinking on torsional response, anchorage of cladding, and overall load path concepts.
Niigata, Japan: Liquefaction
Caracas Earthquake: Non structural infill and overturning.

17. Historical Development of Seismic Codes
1974 Applied Technology Council Report ATC 2
Continued to use single design spectrum for buildings
1976 ATC 3
Probabilistic ground accelerations
Realistic response accelerations and explicit factors for inelastic action
Strength design
Ground motion attenuation
Nationwide applicability
Existing buildings
1977 National Earthquake Hazards Reduction Act: Federal support and direction
1979 Building Seismic Safety Council: response to ATC 3 - extensive review and trial designs
BSSC/NEHRP Recommended provisions: Son of ATC 3

18. Historical Development of Seismic Codes
Mexico City Earthquake: Extreme site effects
New SEAOC (1987) and UBC requirements: Allowable stress design and a single map.
1988 Armenia Earthquake: Structural details and site effects
1989 Loma Prieta Earthquake: A performance test for buildings & bridges.
1991 NEHRP Provisions into Model Codes

19. Building Seismic Safety Council http://www.bssconline.org/
Members are organizations (ASCE, ACI, AISC, AIA, ICBO, BOCA, EERI, SEAOC, etc…)
Consensus Process
Private
Voluntary
National Forum
Issues:
Technical
Social
Economical

20. ASCE 7-05 Seismic Provisions

21. Seismic Ground Motion Values
See ASCE
Mapped Acceleration Parameters
Ss = Mapped 5% damped, spectral response acceleration parameter at short periods
S1 = Mapped 5% damped spectral response acceleration parameter at a period of 1 sec.
Can be found online at
You need Java to run the downloadable application.

22. SS
See ASCE
Use Map to find the maximum considered ground motion for short periods.
The contours are irregularly spaced
Values are in % of g

23. S1
See ASCE
Use Map to find the maximum considered ground motion for a period of 1 sec
The contours are irregularly spaced
Values are in % of g

24. Site Classes Site Classes are also labeled A-F
See ASCE , 20
Site Classes are also labeled A-F
A is for hard rock, F for very soft soils
See definitions in ASCE
Choice of site class is based on soil stiffness which is measured in different ways for different types of soil.
See ASCE for procedure
If insufficient data is available, assume Site Class D unless there is a probability of a Site Class F.

25. Compute SMS and SM1 SMS = FaSS SM1= FvS1 Fa from Table 11.4-1
See ASCE
SMS = FaSS
Fa from Table
SM1= FvS1
Fv from Table

26. Spectral Response Accelerations SDS and SD1
See ASCE
SDS = 2*SMS/3 SD1 = 2*SM1/3
SDS is the design, 5% damped, spectral response acceleration for short periods.
SD1 is the design, 5% damped, spectral response acceleration at a period of 1 sec.
SDS and SD1 are used in selecting the Seismic Design Category and in the analysis methods.

27. Design Response Spectrum
See ASCE
Period Limiting Values
T0 = .2 SD1/SDS
TS = SD1/SDS
TL from ASCE
Sa, design spectral response acceleration
Sa is a function of structure period, T
Four regions, four equations.

28. Importance Factor, I See ASCE 7-05 Table 11.5-1
Function of Occupancy Category
Requirement for structures adjacent to occupancy category IV structures where access is needed to get to the category IV structure.

29. Seismic Design Categories
See ASCE
To be determined for every structure
function of:
Occupancy Category
Spectral Response Accelerations SDS and SD1.
Used to determine analysis options, detailed requirements, height limitations, and other limits on usage.
Seismic Design Categories labeled A-F

30. Seismic Design Categories
The most restrictive value controls
SDC E:
OC I, II, III where S1 > 0.75
SDC F:
OC IV where S1 > 0.75

31. Seismic Design Category A
See ASCE
Very limited seismic exposure and risk
Lateral forces taken to equal 1% of structure weight.
A complete load path must be in place.

32. Soil Report Requirements
See ASCE
Limits on where you can place a structure (SDC E or F)
SDC C – F:
specific evaluation of listed hazards.
SDC D-F:
Even more evaluation requirements.

33. Seismic Load Analysis Procedures
See ASCE
Equivalent Lateral Force (ELF)
Static approximation.
May not be used on structures of Seismic Design Categories E or F with particular irregularities. (ASCE 7-05 Table )
Modal Analysis
2D and 3D dynamic analysis
Required for buildings with particular irregularities
Site Specific Response Spectrum
Permitted for all structures

34. Analysis Procedures
Category A: regular and irregular structures designed for a minimum lateral force
Category B & C: regular and irregular structures using any of the three methods
Category D, E, & F: Table with some limits on SDS and SD1
ELF for regular and some irregular
Modal for some irregular
Site specific required in Site Classes E or F

35. Structure Configuration (regular or irregular)
Plan Configuration
ASCE
Vertical Configuration
ASCE

36. Plan Structural Irregularities
1a - Torsional Irregularity
1b - Extreme Torsional Irregularity
2 - Re-entrant Corners
3 - Diaphragm Discontinuity
4 - Out-of-plane Offsets
5 - Nonparallel Systems

37. Type 1: Torsional Irregularities
1a - Torsional Irregularity
larger story drift more than 1.2 times average story drift
1b - Extreme Torsional Irregularity
larger story drift more than 1.4 times average story drift
Not permitted in Design Categories E & F
Design forces for lateral force connections to be increased 25% in Design Categories D, E, & F.

38. Type 2: Re-entrant Corners
Both projections beyond the corner are more than 15% of the plan dimension of the structure in the same direction

39. Type 3: Diaphragm Discontinuities
Diaphragms with abrupt discontinuities or variations in stiffness, including those having cutout or open areas greater than 50% of the gross enclosed diaphragm area, or changes in effective diaphragm stiffness of more than 50% from one story to the next.
Design forces for lateral force connections to be increased 25% in Design Categories D, E, & F.

40. Type 4: Out-of-Plane Offsets
Discontinuities in a lateral force resistance path, such as out-of-plane offsets of the vertical elements.
Design forces for lateral force connections to be increased 25% in Design Categories D, E, & F.

41. Type 5: Nonparallel Systems
The vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral force resisting system.
Analyze for forces applied in the direction that causes the most critical load effect for Design Categories C - F.

42. Vertical Irregularities
1a - Stiffness Irregularity -Soft Story
1b - Stiffness Irregularity - Extreme Soft Story
2 - Weight (Mass) Irregularity
3 - Vertical Geometry Irregularity
4 - In-plane Discontinuity in Vertical Lateral Force Resisting Elements
5 - Discontinuity in Capacity - Weak Story

43. Type 1: Stiffness Irregularities
1a - Soft Story
the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.
1b - Extreme Soft Story
the lateral stiffness is less than 60% of that in the story above or less than 70% of the average stiffness of the three stories above.
Not permitted in Design Categories E & F

44. Type 2: Weight (Mass) Irregularity
Mass irregularity shall be considered to exist where the effective mass of any story is more than 150% of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.

45. Type 3: Vertical Geometry Irregularity
Vertical geometry irregularity shall be considered to exist where the horizontal dimension of the lateral force-resisting system in any story is more than 130% of that in an adjacent story.

46. Type 4: In-Plane Discontinuity in Vertical Lateral Force Resisting Elements
An in-plane offset of the lateral force-resisting elements greater than the length of those elements or a reduction in stiffness in the resisting element in the story below.
Design forces for lateral force connections to be increased 25% in Design Categories D, E, & F.

47. Type 5: Discontinuity in Capacity - Soft Story
A weak story is one in which the story lateral strength is less than 80% of that in the story above. The story strength is the total strength of all seismic-resisting elements sharing the story shear for the direction under consideration.
Do not confuse STIFFNESS with STRENGTH.
Not permitted in Design Categories E & F.

48. Equivalent Force Method (ASCE 7-05 12.8)

49. Base Shear Determination
See ASCE
Base Shear, V = CsW
Where:
Cs = seismic response coefficient
W = the effective seismic weight, including applicable portions of other storage and snow loads (See ASCE )

50. Seismic Weight, W W is to include:
See ASCE
W is to include:
all dead load (all permanent components of the building, including permanent equipment)
25% of any design storage floor live loads except for floor live load in public garages and open parking structures.
If partition loads are considered in floor design, at least 10 psf is to be included.
A portion of the snow load (20% pf minimum) in regions where the flat roof snow load exceeds 30 psf.

51. Seismic Response Coefficient, Cs
See ASCE
Cs = SDS /(R/I)
Cs need not exceed
SD1/(T(R/I)) for T < TL
SD1TL/(T2(R/I)) for T > TL
Cs shall not be taken less than
0.01 for S1 < 0.6g
0.5S1/(R/I) for S1 > 0.6g

52. Response Modification Coefficient, R
See ASCE
The response modification factor, R, accounts for the dynamic characteristics, lateral force resistance, and energy dissipation capacity of the structural system.
Can be different for different directions.

53. Fundamental Period, T May be computed by analytical means
May be computed by approximate means, Ta
Where analysis is used to compute T:
T < Cu Ta
May also use Ta in place of actual T

54. Approximate Fundamental Period, Ta
See ASCE
An approximate means may be used.
Ta = CThnx
Where:
CT = Building period coefficient.
hn = height above the base to the highest level of the building
for moment frames not exceeding 12 stories and having a minimum story height of 10 ft, Ta may be taken as 0.1N, where N = number of stories.
For masonry or concrete shear wall buildings use eq
Ta may be different in each direction.

55. Building Period Coefficient, CT
See ASCE

56. Base Shear Summary V = CsW From Design Spectrum
W = Building Seismic Weight
0.01 or 0.5S1/(R/I) < SDS/(R/I) < SD1/(T(R/I)) or TLSD1/(T2(R/I))
From map
Numerical Analysis or Ta = CThnx or Ta = 0.1N
R from Table based on the Basic Seismic-Force-Resisting system
CT = 0.028, 0.016, 0.030, or 0.020
hn = building height
N = number of storys
I from Table based on Occupancy Category

57. Vertical Distribution of Base Shear
See ASCE
For short period buildings the vertical distribution follows generally follows the first mode of vibration in which the force increases linearly with height for evenly distributed mass.
For long period buildings the force is shifted upwards to account for the whipping action associated with increased flexibility

58. Story Force, Fx Fx = CvxV Where Cvx = Vertical Distribution Factor
Wx = Weight at level x
hx = elevation of level x above the base
k = exponent related to structure period
When T < 0.5 s, k =1, When T > 2.5 s, k =2,
Linearly interpolate when 0.5 < T < 2.5 s

59. Story Shear, Vx
Story shear, Vx, is the shear force at a given story level
Vx is the sum of all the forces above that level.

60. Horizontal Distribution
See ASCE
Being an inertial force, the Story Force, Fx, is distributed in accordance with the distribution of the mass at each level.
The Story Shear, Vx, is distributed to the vertical lateral force resisting elements based on the relative lateral stiffnesses of the vertical resisting elements and the diaphragm.

61. Torsion
See ASCE
The analysis must take into account any torsional effects resulting from the location of the masses relative to the centers of resistance.
In addition to the predicted torsion, accidental torsion must be applied for structures with rigid diaphragms by assuming the center of mass at each level is moved from its actual location a distance equal to 5% the building dimension perpendicular to the direction of motion.
Buildings of Seismic Design Categories C, D, E, and F with torsional irregularities are to have torsional moments magnified.

62. Using the results of the Seismic Analysis
“The effects on the structure and its components due to gravity loads and seismic forces shall be combined in accordance with the factored load combinations as presented in ASCE 7 except that the effect of seismic loads, E, shall be as defined herein.”

63. Overturning The effects of overturning must be considered.
See ASCE
The effects of overturning must be considered.
The overturning moment at any level is the sum of the moments at that level created by the Story Forces at each level above it.

64. ASCE 7 Load Combinations that include Seismic Effects
See ASCE & 2.4
LRFD
5: 1.2D + 1.0E + L + 0.2S
7: 0.9D + 1.0E
ASD
5: D + (W or 0.7E)
6: D (W or 0.7E) L (Lr or S or R)
8: 0.6D + 0.7E

65. Definition of E
See ASCE
When Seismic effects and Dead Load effects are additive:
E = Eh + Ev = DQE + 0.2SDSD
When Seismic effects and Dead Load effects counteract:
E = Eh - Ev = DQE - 0.2SDSD
QE = Effect of horizontal seismic forces
D = the reliability factor

66. The Reliability Factor, D
See ASCE
The reliability factor is intended to account for redundancy in the structure.
The factor, D, may be taken as 1.0 for eight cases listed in ASCE , including Seismic Design Categories A-C.
For structures of Seismic Design Categories D-F:
D = 1.3
With listed exceptions (ASCE )