Selected New Provisions of ASCE 7-05
Selected New Provisions of ASCE 7-05
Selected New Provisions of ASCE 7-05
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s
L D D DS
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R
T
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2007 Structures Congress Seismic Design Under ASCE 7-2005 53
Adjustments to Base Shear
Soil-Structure Interaction per Section 19 is
permitted
Low rise buildings in high ground motion
areas:
If stories < 6 and T < 0.5 seconds
Can use S
S
= 1.5 max
2007 Structures Congress Seismic Design Under ASCE 7-2005 54
12.8.2 Period of Vibration
Follow modeling
criteria in 12.7 to
compute T
Upper bound for forces:
a u
T C T s
S
D1
C
u
> 0.4 1.4
0.3 1.4
0.2 1.5
0.15 1.6
<0.1 1.7
2007 Structures Congress Seismic Design Under ASCE 7-2005 55
Approximate Period
x
n t a
h C T =
Structure C
t
x
100% Moment Frames:
Steel
Concrete
0.028
0.016
0.8
0.9
Eccentrically Braced 0.03 0.75
All others 0.02 0.75
2007 Structures Congress Seismic Design Under ASCE 7-2005 56
What is h
n
? Concept of Base
h
n
h
n
Base
RC frame
RC wall
Masonry
wall
RC wall
Base
2007 Structures Congress Seismic Design Under ASCE 7-2005 57
Alternate Estimates for T
a
100% moment frames up to 12 stories with
story heights at least 10 feet: , T
a
= 0.1N
Shear walls of concrete or masonry:
n
w
a
h
C
T
0019 . 0
=
=
(
(
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+
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=
x
i
i
i
i
i
n
B
w
D
h
A
h
h
A
C
1
2
2
83 . 0 1
100
2007 Structures Congress Seismic Design Under ASCE 7-2005 58
Vertical Distribution of Force
Equivalent static force at level x:
V C F
vx x
=
=
=
n
i
k
i i
k
x x
vx
h w
h w
C
1
where
w
i
, w
x
: Portion of W assigned to level i or x
h
i
, h
x
: Height of level i or x above base
k sets the shape of distribution and depends on T
Story Shear:
=
=
n
x i
i x
F V
2007 Structures Congress Seismic Design Under ASCE 7-2005 59
Shape of Vertical Distribution
1 k 2: Varies with T
For T 0.5, k = 1 (linear distribution)
For T 2.5, k = 2 (parabolic distribution;
impact of higher modes)
For 0.5 < T < 2.5, k = 2 or
k = 0.75 + T/2
(interpolation)
2007 Structures Congress Seismic Design Under ASCE 7-2005 60
ELF - Story Shears
F V
=
=
n
x i
i x
F V
Sum the story forces from the top down
Distribution of story forces intended to give proper envelope
of maximum story shears for a regular building. It does not
give envelope of maximum story forces.
2007 Structures Congress Seismic Design Under ASCE 7-2005 61
ELF - Horizontal Distribution
Distribute story shear to vertical elements
per relative stiffness of vertical elements and
diaphragm
Account for computed (inherent) torsion - eccentricity
between mass and resistance
Add accidental torsion, except for flexible diaphragms
Amplify torsion if torsionally irregular
2007 Structures Congress Seismic Design Under ASCE 7-2005 62
Torsional Irregularity
For S.D. Category C, D, E or F accidental
eccentricity must be multiplied by A
x
0 . 3
2 . 1
2
max
s
|
|
.
|
\
|
=
avg
x
A where
o
o
avg
o max
o
min
o
)
`
>
extreme
irregular
avg
4 . 1
2 . 1
max
o
o
2007 Structures Congress Seismic Design Under ASCE 7-2005 63
ELF - Overturning Moments
.
Story force times height
to level under consideration.
Overestimate where higher modes are significant
Prior codes allowed up to a 20% reduction in tall buildings
Now require modal analysis for such structures, thus this
provision is now deleted
Moment can be reduced 25% at foundation, permitting
some rocking
F V M
( )
x i
n
x i
i x
h h F M =
=
2007 Structures Congress Seismic Design Under ASCE 7-2005 64
Definition of Drift
I
C
xe d
x
o
o
=
Structural displacement,
where,
=
xe
o Elastic deflection calculated
from design forces
=
d
C
Deflection amplification factor
Importance factor
= I
No reduction for ASD,
but, can ignore limit on T
2007 Structures Congress Seismic Design Under ASCE 7-2005 65
Stability: P- Effects
P
Deflection introduces P- moment
which increases deflection, which
increases moment ..
Structure must be designed to
prevent collapse due to P-
effects
2007 Structures Congress Seismic Design Under ASCE 7-2005 66
Stability: P- Effects
Determine stability coefficient, , for each story
d sx x
x
C h V
P A
= u
If > 0.10 at any level, then all design forces and
moments must be increased by factor 1+a
d
u
u
=
1
d
a where
Check
25 . 0
5 . 0
max
s = <
d
C |
u u
2007 Structures Congress Seismic Design Under ASCE 7-2005 67
Derivation of Stability Factor
f
h
V
P
k
V
P
V
Undeformed
A
Deformed
B
Response
C
Equilibrium at B: 0 = + h k P Vh
f f
o o
Define: 0 , = + =
o
f
f o
h V
P Vh then V k
o
o
o o
Rearranging terms:
u
o
o
o
o
=
1
1
o
o
o
f
Vh
P
2007 Structures Congress Seismic Design Under ASCE 7-2005 68
P-Delta
What if your analysis program includes
P-Delta and you dont want to make a
second set of output?
u
max
must still be checked
Compute u* from displacements that
include P-Delta, then
max
* 1
*
u
u
u
u s
+
=
2007 Structures Congress Seismic Design Under ASCE 7-2005 69
12.9 Modal Response Spectrum
Analysis Method
1. Minimum Number of Modes
2. Adjustment of Response Parameters by
R/I (forces) and C
d
/I (displacements)
3. Combining Modes for Total Response
4. Scaling of Design Values
5. Horizontal Shear Distribution, Torsion
6. P-Delta
7. Soil Structure Interaction
2007 Structures Congress Seismic Design Under ASCE 7-2005 70
M.R.S. Analysis
Include enough modes to obtain a combined
modal mass participation of at least 90% of
the actual mass in each of the orthogonal
directions of response
Short period branch of spectrum is usable
Divide spectrum by (R/I ) to obtain force
responses
Multiply each displacement by (C
d
/I )
2007 Structures Congress Seismic Design Under ASCE 7-2005 71
Combining Modal Results
Basic rule is Square Root of Sum of
Squares (SRSS)
Complete Quadratic Combination (CQC)
always permitted (see ASCE 4)
CQC required where modal periods are
closely spaced or where translational and
torsional modes are cross correlated
2007 Structures Congress Seismic Design Under ASCE 7-2005 72
Design Response Spectrum
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 1 2 3 4 5 6 7
T
S
T
0
Period, seconds
S
p
e
c
t
r
a
l
A
c
c
e
l
e
r
a
t
i
o
n
,
g
0.4S
DS
S
a
= S
D1
/ T
S
a
= S
DS
(0.4 + 0.6 T/T
0
)
S
a
= S
D1
T
L
/ T
2
Drawn for
S
S
= 1.0, F
a
= 1.0
S
1
= 0.4, F
v
= 1.5
T
L
= 4
2007 Structures Congress Seismic Design Under ASCE 7-2005 73
Scaling Modal Results
Compute limiting base shear, V, by ELF; if T
exceeds C
u
T
a
, then use T = C
u
T
a
Compare 85% of this force with combined
modal base shear, V
t
If V
t
< 0.85V then multiply all combined
response quantities from modal analysis by
0.85V / V
t
2007 Structures Congress Seismic Design Under ASCE 7-2005 74
M.R.S. Analysis
Critical direction of load applies (orthogonal
combinations)
Inherent torsion automatically included
Accidental torsion: two choices:
Offset mass to achieve accidental eccentricity
- Include static torsion as a load case
P-Delta applies as for ELF
Soil Structure Interaction analysis permitted
2007 Structures Congress Seismic Design Under ASCE 7-2005 75
12.12 Drift and Deformation
1. Story Drift
Satisfy limits per table; occupancy is factor
If irregular, include torsion effect in SD Cat C+
Divide allowable by for MF in SD Cat D+
2. Diaphragm Deflection
3. Building Separation
4. Compatibility for SD Category D+
2007 Structures Congress Seismic Design Under ASCE 7-2005 76
Drift Ratio Limits
Structure Occupancy Category UBC
I or II III IV
4 stories, no masonry 0.025 0.020 0.015 0.025*
Masonry cantilever 0.010 0.010 0.010
Other masonry 0.007 0.007 0.007
All other 0.020 0.015 0.010 0.020*
2007 Structures Congress Seismic Design Under ASCE 7-2005 77
Deformation Compatibility
Applies to
SD Category D+
All structural components not in SFRS
Check capacity for gravity load combined
with effects induced from design drift;
rational analysis of restraint required
ACI 318 Chap 21 acceptable alternate
2007 Structures Congress Seismic Design Under ASCE 7-2005 78
Overview
Reorganization
New maps; Long period map
New systems, revised R factors and limitations
Diaphragm assumptions
Redundancy factor
Dynamic analysis triggers
Near fault spectral shape applicability
Modal response spectrum analysis
Simplified design method
2007 Structures Congress Seismic Design Under ASCE 7-2005 79
12.14 Simplified Alternate
1. General (Limitations/Eligibility)
2. Design Basis (& Load Combinations)
3. SFRS Identification (& Combinations)
4. Diaphragm Flexibility
5. Direction of Loading
6. Design & Detailing: Load path connections,
collectors, wall anchorage
7. ELF Analysis
2007 Structures Congress Seismic Design Under ASCE 7-2005 80
Limitations
Occupancy Category I or II
Site Class A, B, C, or D
1, 2, or 3 stories
Bearing Wall or Building Frame System
Braced frames or shear walls
No unbraced (moment) frames
Regular
2007 Structures Congress Seismic Design Under ASCE 7-2005 81
Torsional Regularity
Flexible diaphragms:
Overhang (cantilever) < depth / 5
Controls displacement at edge
Controls torsion in non-flexible
Non-flexible diaphragm
Eccentricity < 15% width of diaphragm
Minimum torsional stiffness
2007 Structures Congress Seismic Design Under ASCE 7-2005 82
Torsion
= = =
+ > +
m
i
i
n
j
j j
m
i
i i
k b
b
e
d k d k
1
1
2
1
1
1
1
2
2 2
1
2
1 1
) 05 . 0 ( 5 . 2
k
1i
- lateral stiffness, wall i parallel to 1
k
2j
- lateral stiffness, wall j parallel to 2
d
1i,
- the distance from the wall i to the
center of rigidity, perpendicular to axis 1
d
2j
is the distance from the wall j to the
center of rigidity, perpendicular to axis 2
e
1
is the distance perpendicular to axis 1
between the center of rigidity and the
center of mass
b
1
is the width of the diaphragm
perpendicular to axis 1
m is the number of walls in direction 1
n is the number of walls in direction 2
2007 Structures Congress Seismic Design Under ASCE 7-2005 83
Regularity
Limit skewed alignments to 15 degrees
Use simplified method for design in both
horizontal directions
No in-plane or out-of-plane offsets
Exception: shear walls in 2 story light frame
Must use
0
( = 2.5 for all structures)
No weak stories (80% rule)
2007 Structures Congress Seismic Design Under ASCE 7-2005 84
Seismic Design Category
Limited to Occupancy groups I and II
Only use S
DS
, therefore
S
DS
< 0.33 gives Category B
S
DS
< 0.50 gives Category C
S
DS
> 0.50 gives Category D
Can have Category E if S
1
is high
2007 Structures Congress Seismic Design Under ASCE 7-2005 85
Combined Effects
Vertical Seismic Load =
Combine positive vertical seismic load
where gravity and effect of horizontal
seismic add; combine negative vertical
seismic load where gravity offsets effect of
horizontal seismic
Orthogonal combinations not required
- 02 . S W
DS
2007 Structures Congress Seismic Design Under ASCE 7-2005 86
R Factor Table
Includes wood shear walls, all concrete and
masonry walls and all steel bracing systems
special, ordinary, intermediate, plain,
detailed, etc and composite steel/concrete
walls
Includes specific citations to reference
standards for detailing the systems
No C
d
or
0
factors here
2007 Structures Congress Seismic Design Under ASCE 7-2005 87
Detailed Rules
Connections: 0.2S
DS
w
i
or 0.05w
i
Openings and corners in shear panels
Collectors:
0
= 2.5 (except light frame)
Diaphragms:
use the story force
provide continuous ties
Anchor concrete/masonry walls (flexible)
2007 Structures Congress Seismic Design Under ASCE 7-2005 88
Detailed Rules
No orthogonal combinations
Redundancy factor = 1.0
Bearing walls: out of plane = 0.4S
DS
w
c
Nonstructural components same as any
other building
2007 Structures Congress Seismic Design Under ASCE 7-2005 89
Simplified ESF Analysis
Base Shear
Story Force
Story factor F =
1.0 for one story
1.1 for two stories
1.2 for three stories
Same acceleration at all
levels of building (very
simple!)
No I factor, No period T
V
F S
R
W
DS
=
-
F
F S
R
w
i
DS
i
=
-
2007 Structures Congress Seismic Design Under ASCE 7-2005 90
Simplified Response
Acceleration
Use only S
DS
; dont use S
1S
Equation
Site response amplifier F
a
=
1.0 for rock
1.4 for soil
S F S
DS a S
=
|
\
|
.
| -
2
3
2007 Structures Congress Seismic Design Under ASCE 7-2005 91
Drift and Separation
Do not have to check drift
Use 1% drift for purposes of building
separation, nonstructural component
compatibility, etc, unless actually computed
2007 Structures Congress Seismic Design Under ASCE 7-2005 92
Overview
Introduction to Earthquake Engineering
Basic Criteria in ASCE 7 2005
Ground Motions
Response Spectrum
Occupancy and Seismic Design Categories
Seismic Design for Buildings
Basic Requirements
System Requirements
Analysis
Diaphragms, Walls, Foundations
Simplified Method
2007 Structures Congress Seismic Design Under ASCE 7-2005 93
Errata
Go to the SEI website
www.seinstitute.org
Go to the Publications in the bar at the top
Click on the Errata tab
Download pdf files for ASCE 7-05 (and any
other structural standards you may need)