1. Building Systems (Seismic)
PCI 6th Edition
Building Systems
(Seismic)

2. Presentation Outline Building System Loads Structural Integrity
Seismic
Structural Integrity
LFRS – Walls
LFRS – Frames
Diaphragms

3. Seismic Changes Based on new changes to ASCE 7 and ACI 318
Based current seismic research and observations

4. Seismic Changes Some of these changes are:
Recognition of jointed panel construction
Recognition of strong and ductile connections in precast frames
Recognition and requirements for connections in precast walls
Recognition of jointed panel construction as an alternative to emulation of monolithic construction.
Achieving ductile structural behavior by using “strong” connections that remain elastic while nonlinear action (plastic hinging) occurs in the member away from the connection.
Modification of drift computation and limiting drift.
Deformation compatibility of structural elements and attached non-structural elements.
Additional soil type classifications.
Special considerations for building sites located near seismic faults.
Special considerations for structures possessing redundancy.

5. Seismic Changes Additional changes are:
Modification of drift computation and limiting drift
Deformation compatibility of elements
Additional soil type classifications
Special considerations locations near seismic faults
Consideration of redundancy and reliability in strength design requirements
Recognition of jointed panel construction as an alternative to emulation of monolithic construction.
Achieving ductile structural behavior by using “strong” connections that remain elastic while nonlinear action (plastic hinging) occurs in the member away from the connection.
Modification of drift computation and limiting drift.
Deformation compatibility of structural elements and attached non-structural elements.
Additional soil type classifications.
Special considerations for building sites located near seismic faults.
Special considerations for structures possessing redundancy.

6. Seismic Changes Design Forces are Based on Risk
Previous codes based on 10% chance of exceedance in 50 years
IBC 2000, 2003 codes based on 2% chance of exceedance in 50 years

7. Seismic Risk Soil factors
Other regions of high seismic risk - not just west coast anymore

8. Practically every precast, prestressed concrete structure designed under IBC 2000 will require some consideration of seismic effects.

9. Seismic Performance Objectives
Current design - minor damage for moderate earthquakes
Accepts major damage for severe earthquakes
Collapse is prevented of severe events

10. Seismic Performance Objectives
In order to achieve the design objectives, the current code approach requires details capable of undergoing large inelastic deformations for energy dissipation.

11. Seismic Design Approach
Emulation
No special requirements for low seismic risk
Chapter 21 requirements for moderate and high seismic risk
Non-emulative design
PRESSS
Acceptance criteria for frames

12. Earthquake Loads – Equivalent Lateral Force Method
Base Shear, V
V= Cs·W
Where:
Cs - Seismic Response Coefficient
W - Total Weight

13. Equivalent Lateral Force Method Limitations
This method may not apply to buildings with irregularities in Seismic Design Categories D, E, or F

14. Earthquake Loads – Total Weight, W
Dead Load of structure plus:
25% of reduced floor live load in storage areas
live load in parking structures not included
Partition load if included in gravity dead
Total weight of permanent equipment
20% of flat roof snow load, pf
where pf > 30 psf

15. Seismic Response Coefficient, Cs
Function of
Spectral response acceleration
Site soil factors
Building Period
Response modification factors
Importance factor

16. Seismic Response Coefficient, Cs
Step 1 - Determine SS and S1
Step 2 - Determine site Soil Classification
Step 3 - Calculate Response Accelerations
Step 4 - Calculate the 5% Damped Design Spectral Response Accelerations
Step 5 - Determine the Seismic Design Category
Step 6 - Determine the Fundamental Period
Step 7 - Calculate Seismic Response Coefficient

17. Step 1 – Determine SS and S1
From IBC Map
From local building codes
IBC 2003 CD-ROM
Based on
Longitude / Latitude
Zip Code

19. Step 2 – Determine Site Soil Classification
If site soils are not known use Site Class D
Figure (a) (page 3-111)
From soil reports

20. Step 3 – Calculate Response Accelerations
SMS = Fa·SS
SM1 = Fv·S1
Where:
Fa and Fv are site coefficients from Figure (b) and (c) (page 3-111)
SS spectral accelerations for short periods
S1 spectral accelerations for 1-second period
All values based on IBC 2003

21. Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations
SDS = (2/3)SMS
SD1 = (2/3)SM1

22. Step 5 – Determine the Seismic Design Category
Table
Sometimes this restricts the type of Seismic Force Resisting System (SFRS) used (see Figure ) (page 3-112)

23. Step 6 – (Approximate Period) Determine the Buildings Fundamental Period
Where:
Ct = for moment resisting frame systems of reinforced concrete
0.020 for other concrete structural systems
x = 0.9 for concrete moment resisting frames
0.75 for other concrete structural systems
hn = distance from base to highest level (in feet)
Ct = for moment resisting frame systems of reinforced concrete in which the frames resist 100% of the required seismic forces and are not enclosed or adjoined by more rigid components that prevent that frame from deflecting when subjected to seismic forces .

24. Step 6 – (Exact Period) Determine the Buildings Fundamental Period
Rayleigh’s formula
Where:
wi = dead load weight at Floor i
δi = elastic displacement at Floor i
Fi = lateral force at Floor i
g = acceleration of gravity
n = total number of floors

25. Step 7 – Determine Seismic Response Coefficient, Cs
Lesser of
Where:
R = Response Modification Factor
Figure (page 3-112)
Ι = Seismic Importance Factor

26. Step 7 – Determine Cs Cs = 0.044·SDS·Ι Minimum Value of Cs
Special Cases In Seismic Design Categories E and F
Cs = 0.044·SDS·Ι

27. Vertical Distribution of Lateral Force
Where:
Fx = Force per floor
Cvx = Vertical distribution factor
V = Base shear
k = 1 - buildings with a period ≤ 0.5 sec
= 2 - buildings with a period > 2.5 sec
hi and hx = height from base to Level i or x
wi and wx = Level i or x portion of total gravity load
For buildings with a period between 0.5 and 2.5, determine k by linear interpolation.

28. Location of Force in Plane
Accidental Torsion
calculated by assuming that the center of mass is located a distance of 5% of the plan dimension perpendicular to the applied load on either side of the actual center of mass
Total torsion = sum of the actual torsion plus the accidental torsion

29. Seismic Drift Requirements
Elastic Displacement Amplification Factor, dx
Stability Coefficient Limits, q
P-D Effects

30. Drift Limits
Figure (page 3-113)

31. Drift Amplification Factor, dx
Where:
δx = Amplified deflection of Level x
δxe = Deflection of Level x determined from elastic analysis, includes consideration of cracking
Cd = Deflection amplification factor
(Figure )
Ι = Seismic Importance Factor
The Cd factor represents an approximation of the post-yield or Non-linear displacement.

32. Stability Coefficient, θ
Where:
Px = Total vertical unfactored load including and above Level x
∆ = Difference of deflections between levels x and x-1
Vx = Seismic shear force acting between levels x and x-1
hsx = Story height below Level x
Cd = Deflection amplification factor

33. Stability Coefficient, θ
The stability coefficient is limited to:
Where:
β = ratio of shear demand to shear capacity between Levels x and x-1

34. P-D Effects
To account for P-∆ effects, the design story drift is increased by
(1− θ)-1
If θ < 0.10, P-∆ effects may be neglected

35. Reliability Factor, ri
Required in High Seismic Design Categories D, E, and F
The Earthquake Force is increase by a Reliability Factor, ri
1.5 Maximum Required Value
ri = 1.0 for structures in Seismic Design Categories A, B and C

36. Reliability Factor, ri For Moment Frames
Where, for each level:
Ai = floor area
rmaxi = For moment frames, the maximum of the sum of the shears in any two adjacent columns divided by the story shear. For columns common to two bays with moment-resisting connections on opposite sides, 70% of the shear in that column may be used in the column shear summary.

37. Reliability Factor, ri For Shear Walls
Where, for each level:
Ai = floor area
rmaxi = For shear walls, the maximum value of the product of the shear in the wall and 10/lw divided by the story shear.

38. Load Combinations U = 1.4(D+F) U = 1.2(D+F+T) + 1.6(L+H)
U = 1.2D +1.6(Lr or S or R) + (1.0L or 0.8W)
U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R)
U = 1.2D + 1.0E + f1L + 0.2S
U = 0.9D + 1.6W + 1.6H
U = 0.9D + 1.0E + 1.6H
f1 = 1.0 Parking garages
= 1.0 Live load ≥ 100 psf on public assembly floors
= 0.5 All others
f1 = 1.0 for floors in places of public assembly, for live loads in excess of 100 psf, and for parking garages, otherwise, f1 = 0.5
D = Dead load
F = Pressure of fluids of known density and controlled depths
T = Effects of temperature, creep and shrinkage
L = Live load
H = Soil load
Lr = Roof live load
S = Snow load
R = Rain load
W = Wind load
E = Seismic load

39. Modification for Vertical Acceleration
E = ρ·QE ± 0.2·SDS·D
Seismic Load Combinations Become
U = ( ·SDS)D + ρ·QE + f1L + 0.2S
U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H
Notice Building weight increase as Ground move Up
Where
QE = Horizontal Seismic Force

40. Modification for Vertical Acceleration
E = ρ·QE ± 0.2·SDS·D
Seismic Load Combinations Become
U = ( ·SDS)D + ρ·QE + f1L + 0.2S
U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H
Notice Building weight decreases as Ground move Down

41. Overstrength Factor, Wo
Components within the Diaphragm
Chord ties
Shear Steel
Connectors
Ωo = Seismic Design Categories C, D, E and F
Ωo = Seismic Design Categories A and B

42. Special Load Combinations
U = 1.2D + fi·L + Em
U = 0.9D + E
Where:
Em = Wo·QE + 0.2·SDS·D
and
Wo = Overstrength Factor

43. Overstrength Factor, Wo
Connections from Diaphragms to Seismic Force Resisting System (SFRS)
Ωo = Seismic Design Categories C and higher
Figure (page 3-112)

44. Structural Integrity Requirements
All members must be connected to the Lateral Force Resisting System (LFRS)
Tension ties must be provided in all directions
The LFRS is continuous to the foundation
A diaphragm must be provided with
Connections between diaphragm elements
Tension ties around its perimeter
Perimeter ties provided
Nominal strength of at least 16 kips
Within 4 ft of the edge
Column splices and column base connections must have a nominal tensile strength not less than 200Ag in pounds
All members must be connected to the lateral force resisting system and their supporting members. Tension ties must be provided in the transverse, longitudinal, and vertical directions and around the perimeter of the structure.
The lateral force resisting system must be continuous to the foundation.
A diaphragm must be provided with connections between diaphragm elements, with tension ties around its perimeter and around openings that significantly interrupt diaphragm action. Section of ACI requires perimeter ties to provide a nominal strength of at least 16 kips and to be within 4 ft of the edge.
Column splices and column base connections must have a nominal tensile strength not less than 200Ag in lbs, where Ag is the gross area of the column in sq in. For a compression member with a larger cross section than required by consideration of loading, a reduced effective area, Ag, not less than one-half the total area, may be used.

45. Structural Integrity Requirements
Precast vertical panels connected by a minimum of two connections
Each connection is to have a nominal strength of 10 kips
Precast diaphragm connections to members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ft
Connection details allow volume change strains
Connection details that rely solely on friction caused by gravity loads are not to be used
Precast walls, other than cladding panels, must be connected across horizontal joints by a minimum of two connections per panel. Each connection is to have a nominal tensile strength of not less than 10 kips. When design forces result in no tension at the base, these connections are permitted to be anchored into an appropriately reinforced slab on grade. If panels are too narrow to accommodate two connections, a single connection is satisfactory, as long as it is connected to adjacent panels.
Where precast elements form roof or floor diaphragms, the connections between the diaphragm and those members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ft.
To accommodate volume change strains (temperature and shrinkage) in supported beams, tie connections are typically located at the top of the member, with elastomeric pads used at the bottom-bearing surface. Such ties can be accomplished by welding, bolting, reinforcing steel in grout joints or bonded topping, or by doweling.
Connection details that rely solely on friction caused by gravity loads are not to be used. Exceptions may be permitted for heavy modular unit structures where resistance to overturning or sliding has a large factor of safety.

46. Lateral Force Resisting Systems (LFRS)
Rigid frames and shear walls exhibit different responses to lateral loads

47. Influential Factors The supporting soil and footings
The stiffness of the diaphragm
The stiffness LFRS elements and connections
Lateral load eccentricity with respect to center of rigidity of the shear walls or frames
The relative flexural and shear stiffnesses of the shear walls, and of connections.

48. Shear Wall Systems Most common lateral force resisting systems
Design typically follows principles used for cast-in-place structures

49. International Building Code (IBC) Requirements
Two categories of shear walls
Ordinary
Special

50. ACI Requirements
Created an additional intermediate category, but has assigned no distinct R, Ωo and Cd

51. ACI Wall Definitions
Defines all shear walls as “structural walls”
Three levels of definition
Ordinary structural (shear) wall
Intermediate precast structural (shear) wall
Special precast structural (shear) wall

52. Ordinary Structural (Shear) Wall
Wall complying with the requirements of Chapters 1 through 18
No special seismic detailing

53. Intermediate Precast Structural (Shear) Wall
Wall complying with all applicable requirements of Chapters 1 through 18
Added requirements of Section 21.13
Ductile connections with steel yielding
1.5 factor for non-yielding elements
IBC imposes restriction that yielding be in the reinforcing

54. Special Precast Structural (Shear) Wall
Precast wall complying with the requirements of 21.8.
Meeting the requirements for ordinary structural walls and the requirements of 21.2
Requires precast walls to be designed and detailed like cast-in-place walls, “emulative” design
Meet the connection requirements of Section

55. Design Guidelines for Shear Wall Structures
Evaluation of building function and applicable precast frame
Preliminary development of shear wall system
Determination of vertical and lateral loads
In a warehouse type structure, it is common to include the exterior walls as part of the lateral load resisting system.
In parking structures, shear walls can be located at stair and elevator towers, at the perimeter or ramped bays, at selected locations on the perimeter of the structure, or any in combination of the above locations
a. Provide at least three non-collinear walls to ensure torsional as well as direct lateral resistance.
b. Overturning will often be the governing criterion. Thus, the first choice is to use shear walls that also function as bearing walls.
c. Arrange shear walls so that they minimize restraint due to volume changes.
d. Consider whether the shear walls could be individual full height walls (vertical joints only).
e. Consider the practicality of shipping and erection when selecting the size of wall panels.
f. Balance the design requirements of the shear walls with the design requirements of the associated diaphragms.
Determine the vertical gravity loads that are applicable to each of the shear walls.
Use the applicable seismic design criteria to determine the magnitude of lateral load at each floor, and compare with wind loading. Choose the critical condition for design.

56. Design Guidelines for Shear Wall Structures
Preliminary load analysis
Selection of shear walls
Final load analysis
Final shear wall design
Diaphragm design
Determine the overturning moment, the lateral in plane shear and the axial load at the base of each of the shear walls.
a. Review the preliminary choice of shear wall size and location.
b. Modify the number, location, and dimensions of shear walls as necessary to satisfy the requirements at the base of each. It is economically preferable that
foundations not be subject to uplift.
Based on the final location and dimensions of shear walls, perform the final lateral load and vertical load analysis to determine the design load for each of the shear walls. Consider shear stiffness as well as flexural stiffness when distributing lateral loads to the shear walls.
Design the shear wall reinforcement and the connections to the associated diaphragms.
Where there is insufficient length of shear wall available to accommodate the necessary number of shear connectors, consider using an element in the plane of the diaphragm (drag strut) as an extension of the shear wall to pick up additional connectors to the diaphragm.
Consider the additional requirements necessary to satisfy the structural integrity provisions of the code (see Section 3.3).
a. Design the diaphragms to respond elastically to applied lateral loads in order to prevent formation of plastic regions in any diaphragm. See Section 3.8 for a
more detailed discussion of diaphragm design.
b. Design the diaphragms as beams, provide the necessary tensile reinforcement for each chord, and choose shear connectors using design procedures of Chapter 6, or shear reinforcement using shear-friction methods.
c. Consider the additional requirements necessary to satisfy the structural integrity provisions of the code (see Section 3.3).

57. Moment Frame Classifications
Three Classifications
Ordinary Moment Frame
Intermediate Moment Frames
Special Moment Frames
Based on Detailing
Seismic Design Categories
The same frame with different rebar details can be Ordinary, Intermediate or Special.

58. Ordinary Moment Frames
Seismic Performance Categories A & B
ACI 318 Chapters 1 to 18
Response modification factor, R = 3

59. Intermediate Moment Frames
Seismic Performance Category C
ACI 318 only defines intermediate as cast-in-place
Response modification factor, R = 5

60. Special Moment Frames Seismic Performance Categories D, E, and F
Yielding will be concentrated in the beam, Strong column -weak beam behavior
Special Moment frames
ACI 318 Sections 21.2 through 21.6
Response modification factor, R = 8

61. Diaphragms
A diaphragm is classified as rigid if it can distribute the horizontal forces to the vertical lateral load resisting elements in proportion to their relative stiffness
Long-span applications suggest that many precast diaphragms may in fact be flexible

62. Diaphragm Design
The distinction between rigid and flexible diaphragms is important not just for diaphragm design, but also for the design of the entire lateral force resisting system.

63. Diaphragm Classification
Flexible diaphragm
Lateral deflection twice average story drift
Rigid diaphragm
Not flexible
Implies capability to distribute load based on relative stiffness of LFRS elements

64. Steps in the Design Method
Step 1 - Calculate and compare distribution and diaphragm forces
Based on rigid diaphragm action
Based on flexible diaphragm action
Step 2 - Check of diaphragm deformation with respect to drift limits
Step 3 - Check attached element drift limits
Step 4 - Adjustments in vertical element stiffness and placement to limit drift

65. Diaphragm Design Forces
Based on Wind and Seismic Events
Wind
Combined windward and leeward wind pressures
Act as uniform load on building perimeter
Distributed to the LFRS based on diaphragm behavior

66. Seismic Diaphragm Design Forces
Separate calculations from the design of the LFRS
Diaphragm Design force, FP
Seismic Design Categories B or C
Fp = 0.2·IE·SDS·Wp + Vpx
Where
Vpx – represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.
Vpx – represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

67. Seismic Diaphragm Design Forces
Seismic Design Category D
0.2·IE·SDS·wpx< Fp < 0.4·IE·SDS·wpx
Vpx – represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

68. Diaphragm Detailing
Wind and Low Seismic Hazards
Moderate Seismic Hazards
Seismic Design Category D - Topped Systems
High Seismic Hazards - Untopped Systems

69. Wind and Low Seismic Hazard
Seismic Design Category A
Strength requirements imposed by the applied forces, No Amplification
Seismic Design Category B
Requires the design of collector elements
Does not require forces to be increased by over strength factor, Ωo (Revised from IBC 2000)

70. Moderate Seismic Hazard
Topped and Pretopped Systems
Seismic Design Category C
Concrete wall systems have special requirements IBC 2003
Diaphragm must include
special continuous struts or ties between diaphragm chords for wall anchorage.
use of Sub-Diaphragms, the aspect ratio of is limited to 2½ to 1

71. Moderate Seismic Hazard
Walls classified as Intermediate Precast Walls
Collector elements, their connections based on special load combinations
Need to include overstrength factor
Ductile connections with wall interface
The body of the connection must have sufficient strength to permit development of 1.5fy in the reinforcing steel
the interface with the wall must be with ductile connections including the reinforcing steel that would yield prior to crushing of the concrete. The body or mechanical parts of the connection (e.g., plates, welds, etc.) must have sufficient strength to permit development of 1.5fy in the reinforcing steel.

72. Seismic Design Category (SDC) D
Topped Systems
Untopped Systems
Not implicitly recognized in ACI
Section
permits a system to be used if it is shown by experimental evidence and analysis to be equivalent in strength and toughness to comparable monolithic cast-in-place systems
The provisions for precast diaphragms that are included in ACI are for topped composite and topped non-composite diaphragms, so untopped diaphragms are implicitly not recognized. Section of ACI ,

73. SDC D – Topped Systems High strain demand across the joints
Reinforcing steel needs to be compatible with this demand
Use of larger wire spacing or bars may be needed
Mesh in the topping must take the entire shear across the joint.
Correct lapping to maintain diaphragm integrity
These provisions include systems with the topping composite with the precast components and systems with the topping noncomposite part acting alone.
The composite system requirements recognize that connections may be part of the design proportioned and detailed to transfer forces. They require that the interface be “clean, free of laitance, and intentionally roughened.”

74. SDC D – Topped Systems Specific provisions in ACI 318-02
Chord steel determined from flexural analysis
Shear strength based entirely on reinforcement crossing the joint:
Vn = Acv·rn·fy
Where
Acv = thickness of the topping slab
ρn = steel ratio of the reinforcement
These provisions include systems with the topping composite with the precast components and systems with the topping noncomposite part acting alone.
The composite system requirements recognize that connections may be part of the design proportioned and detailed to transfer forces. They require that the interface be “clean, free of laitance, and intentionally roughened.”

75. SDC D – Topped Systems ACI 318-02 minimum spacing requirement of 10 in
Diaphragm f -factor ≤ vertical element fshear -factor
May result in f = 0.6, based on ACI Section 9.3.4
Diaphragm phi-factor must be no greater than that used in the shear design of the supporting vertical elements (columns or walls).

76. Questions?