ACI VN 2011 Topic 1 (David Darwin)
Transcript of ACI VN 2011 Topic 1 (David Darwin)
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
1/60
Building Code Requirements forStructural Concrete (ACI 318M-11)
Overview of ACI 318MDesign of Prestressed ConcreteEvaluation of Existing Structures
David Darwin
Vietnam Institute for Building Science andTechnology (IBST)
Hanoi and Ho Chi Minh City
December 12-16, 2011
This morning
Overview of ACI 318M-11
Design of Prestressed Concrete(Chapter 18)
Strength Evaluation of Existing
Structures (Chapter 20)
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
2/60
This afternoon
Analysis and design of
Flexure
Shear
Torsion
Axial load
Tomorrow morning
Design of slender columns
Design of wall structures
High-strength concrete
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
3/60
Overview of ACI 318M-11
Legal standing
Scope
Approach to Design
Loads and Load Cases
Strength Reduction Factors
Legal standing
Serves as the legal structural concrete
building code in the U.S. because it is
adopted by the general building code (IBC).
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
4/60
Scope
ACI 318M consists of 22 chapters and 6
appendices that cover all aspects of building
design
Chapters
1. GENERAL REQUIREMENTS
Scope, Contract Documents, Inspection,
Approval of Special Systems
2. NOTATION AND DEFINITIONS
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
5/60
Chapters
3. MATERIALS
Cementitious Materials, Water, Aggregates,
Admixtures, Reinforcing Materials
4. DURABILITY REQUIREMENTS
Freezing and Thawing, Sulfates, Permeability,
Corrosion
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
6/60
5. CONCRETE QUALITY, MIXING, AND PLACING
6. FORMWORK, EMBEDMENTS,
AND CONSTRUCTION JOINTS
7. DETAILS OF REINFORCEMENT
Hooks and Bends, Surface Condition, Tolerances,
Spacing, Concrete Cover, Columns, Flexural Members,
Shrinkage and Temperature Steel, Structural Integrity
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
7/60
8. ANALYSIS AND DESIGN GENERAL
CONSIDERATIONS
Design Methods; Loading, including Arrangement of
Load; Methods of Analysis; Redistribution of Moments;
Selected Concrete Properties; Requirements for
Modeling Structures (Spans, T-beams, Joists...)
9. STRENGTH AND SERVICEABILITY
REQUIREMENTS
Load Combinations, Strength Reduction Factors,Deflection Control
10. FLEXURE AND AXIAL LOADS
Beams and One-way Slabs, Columns, Deep Beams,
Bearing
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
8/60
11. SHEAR AND TORSION
12. DEVELOPMENT
AND SPLICES OF REINFORCEMENT
13. TWO-WAY SLAB SYSTEMS
14. WALLS
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
9/60
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
10/60
19. SHELLS AND FOLDED PLATE MEMBERS
20. STRENGTH EVALUATION OF EXISTING
STRUCTURES
21. EARTHQUAKE-
RESISTANTSTRUCTURES
22. STRUCTURAL PLAIN CONCRETE
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
11/60
Appendices
A. STRUT-AND-TIE MODELS*
B. ALTERNATIVE
PROVISIONS FOR REINFORCED AND
PRESTRESSED CONCRETE FLEXURAL AND
COMPRESSION MEMBERS
C. ALTERNATIVE LOAD AND STRENGTHREDUCTION FACTORS
D. ANCHORING TO CONCRETE*
E. STEEL REINFORCEMENT INFORMATION
F. EQUIVALENCE BETWEEN SI-METRIC, MKS-
METRIC, AND U.S. CUSTOMARY UNITS OF
NONHOMOGENOUS EQUATIONS IN THE CODE
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
12/60
Approach to design
Qd= design loads
Sn = nominal strength
Sd= design strength
M = safety margin
Design Strength Required Strength
Sd= Sn Qd
Sd = design strength = Sn
= strength reduction factor
= load factors
Qd = design loads
and in Chapter 9 of ACI 318M
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
13/60
Loads Qd
specified in ASCE 7, Minimum Design Loads
for Buildings and Other Structures
American Society of Civil Engineers (ASCE)
Reston, Virginia, USA
Loads
Dead loads (D)*
Live loads (L)*
Roof live loads (Lr)*
Wind loads (W)
full loadEarthquake loads (E) full load
Rain loads (R)*
Snow loads (S)*
* Service-level loads
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
14/60
Loads
Impact include in L
Self-straining effects (temperature, creep,
shrinkage, differential settlement, and
shrinkage compensating concrete) (T)
Fluid loads (F)
Lateral soil pressure (H)
Factored Load = U= Qd
Load cases and load factors
by ASCE 7 and ACI 318M
U= 1.4D
U= 1.2D + 1.6L + + 0.5(Lror S or R)
U= 1.2D + 1.6(Lror S or R) + (1.0L or 0.5W)
U= 1.2D + 1.0W+ 1.0L + 0.5(Lror S or R)
U= 1.2D + 1.0E+ 1.0L + 0.2S
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
15/60
U= 0.9D + 1.0W
U= 0.9D + 1.0E
Load cases and load factorsby ASCE 7 and ACI 318M
If Wbased on service-level forces, use 1.6Wplace of
1.0W
If Ebased on service-level forces, use 1.4Ein placeof 1.0E
Details of other cases covered in the Code
Load factors by ACI 318M
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
16/60
Strength reduction () factors
Tension-controlled sections 0.90
Compression-controlled sections
Members with spiral reinforcement 0.75
Other members 0.65
Shear and torsion 0.75
Bearing
0.65
Post-tensioning anchorages
0.85
Other cases 0.60 0.90
Tension-controlled and compression-
controlled sections
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
17/60
T-beam
d
h
b
hf
bw
As
dt
Strain through depth of beam
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
18/60
Design Strength (x nominal strength) must
exceed the Required Strength (factored load)
Bending Mn Mu
Axial load Pn Pu
Shear Vn Vu
Torsion Tn Tu
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
19/60
Load distributions and modeling
requirements
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
20/60
Structure may be analyzed as elastic
using properties of gross sections
Ig= moment of inertia of gross (uncracked)
cross section
Beams: Ib = Ig Iweb =
Columns: Ic= Ig=
wb h3
12
bh3
12
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
21/60
Analysis by subframes
1. The live load applied only to the floor or roof
under consideration, and the far ends of
columns built integrally with the structure
considered fixed
2. The arrangement of load may be limited to
combinations of
(a)factored dead load on all spans with full
factored live load on alternate spans, and(b)factored dead load on all spans with full
factored live load on two adjacent spans
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
22/60
(a)
(b)
(c)
Moment and shear envelopes
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
23/60
Columns designed to resist
(a) axial forces from factored loads on all floors
or roof and maximum moment from factored
live loads on a single adjacent span of the
floor or roof under consideration
(b) loading condition giving maximum ratio of
moment to axial load
More on columns
For frames or continuous construction, consider
effect of unbalanced floor or roof loads on both
exterior and interior columns and of eccentric
loading due to other causes
For gravity load, far ends of columns built integrallywith the structure may be considered fixed
At any floor or roof level, distribute the moment
between columns immediately above and below
that floor in proportion to the relative column
stiffness
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
24/60
Simplified loading criteria
Beams, two
or more spans
Beams, two
spans only
Slabs,
spans 3 m
Beams, col stiffnesses 8 beam stiffnesses
u nM w l2factor
ln
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
25/60
Composite
Max ve right
Max ve leftMax +ve
Allowable adjustment in maximum
moments for t 0.0075
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
26/60
Design of prestressed concrete
(Chapter 18)
Behavior of reinforced concrete
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
27/60
Reinforced concrete under service loads
Theory of prestressed concrete
Stresses
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
28/60
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
29/60
57
Methods of prestressing concrete members
Post-Tensioning
Pretensioning
Prestressing steels
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
30/60
Strength of prestressing steels available in
U.S.Seven-wire strand: fpu 1725, 1860 MPa
fpy(stress at 1% extension) 85% (for stress-relieved strand) or 90% (for low-relaxation
strand) of fpu
fpu = ultimate strengthfpy= yield strength
Strength of prestressing steels available in
U.S.
Prestressing wire: fpu 1620 to 1725 MPa(function of size)
fpy(at 1% extension) 85% of fpu
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
31/60
Strength of prestressing steels available in
U.S.
High-strength steel bars: fpu 1035 MPa
fpy 85% (for plain bars) and 80% (for deformedbars) of fpu
fpybased on either 0.2% offset or 0.7% strain
Maximum permissible stresses in
prestressing steel
Due to prestressing steel jacking force:
0.94fpy0.80fpumanufacturers recommendation
Post-tensioning tendons, at anchorage devices
and couplers, immediately after force transfer:
0.70fpu
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
32/60
Prestressed concrete members are
designed based on both
Elastic flexural analysis
Strength
Elastic flexural analysis
Considers stresses under both the
Initial prestress force Piand the
Effective prestress force Pe
Note: = concrete compressive strength
= initial concrete compressive
strength (value at prestress transfer)
cf
cif
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
33/60
Classes of members
U uncracked calculated tensile stress in
precompressed tensile zone at service
loads = ft
T transition between uncracked and
cracked < ft
C cracked ft>
. cf0 62
. cf0 62 . cf10
.
cf10
cf in MPa
Concrete section properties
e = tendon eccentricity
k1= upper kern point
k2= lower kern point
Ic
= moment of inertia
Ac= area
radius of gyration:
r2 = Ic/Ac
section moduli:
S1 = Ic/c1
S2 = Ic/c2
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
34/60
Bending moments
Mo = self-weight moment
Md= superimposed dead load moment
Ml= live load moment
Concrete stresses under Pi
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
35/60
Concrete stresses under Pi + Mo
Concrete stresses under Pe + Mo + Md+ Ml
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
36/60
Maximum permissible stresses in concrete at
transfer(a) Extreme fiber stress in compression, except as in
(b),
(b) Extreme fiber stress in compression at ends of
simply supported members
(c) Extreme fiber stress in tension at ends of simply
supported members *
(d) Extreme fiber stress in tension at other locations
*
* Add tensile reinforcement if exceeded
. 0 60 cif
. 070 cif
. cif025
. cif050
Maximum permissible compressive
stresses in concrete at service loads
Class U and T members
(a) Extreme fiber stress in compression due toprestress plus sustained load
(b) Extreme fiber stress in compression due to
prestress plus total load
. 0 45 cf
. 0 60 cf
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
37/60
Flexural strength
Aps T = Apsfps
ps
Stress-block parameter 1
1
1
1
0.85 for 17 MPa 28 MPa
For between 28 and 56 MPa,decreases by 0.05 for each 7 MPa
increase in
0.65 for 56 MPa
c
c
c
c
f
f
f
f
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
38/60
Stress in prestressing steel at ultimate
Members with bonded tendons:
p =Aps/bdp = reinforcement ratio
b = width of compression face
dp= d(effective depth) of prestressing steel
Members with bonded tendons and non-prestressed bars:
p pups pu pc p
f df f
f d
11
andc y cf / f f / f
and refer to compression reinforcement, sA
shall be takenpup pc p
f d. , d . d
f d
017 015
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
39/60
Members with unbonded tendons with span/depth
ratios 35:
but not greater thanfpy or greater thanfpe + 420 MPa
fpe = stress inAps atPe=e
ps
P
Members with unbonded tendons with span/depth
ratios > 35:
but not greater thanfpy or greater thanfpe + 210 MPa
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
40/60
Loss of prestress
(a) Prestessing steel seating at transfer
(b) Elastic shortening of concrete
(c) Creep of concrete
(d) Shrinkage of concrete
(e) Relaxation of prestressing steel
(f) Friction loss due to intended or
unintended curvature of post-tensioning
tendons
Limits on reinforcement in flexural
members
Classify as tension-controlled, transition, or
compression-controlled to determine
Total amount of prestressed and nonprestressed
reinforcement in members with bonded
reinforcement must be able to carry 1.2 cracking load
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
41/60
Minimum bonded reinforcementAs in
members with unbonded tendons
Except in two-way slabs,As = 0.004Act
Act= area of that part of cross section
between the flexural tension face and
center of gravity of gross section
DistributeAs uniformly over precompressed
tension zone as close as possible to
extreme tensile fiber
Two-way slabs:
Positive moment regions:
Bonded reinforcement not required where tensile
stress ft
Otherwise, useAs =
Nc= resultant tensile force acting on portion of
concrete cross section in tension under effective
prestress and service loads
DistributeAs uniformly over precompressed
tension zone as close as possible to extreme
tensile fiber
c. f0 17
c
y
N
. f0 5
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
42/60
Two-way slabs:
Negative moment areas at column supports:
As = 0.00075Acf
Acf= larger gross cross-sectional area of slab-
beam strips in two orthogonal equivalent
frames intersecting at the columns
DistributeAs between lines 1.5h on outside
opposite edges of the column support
Code includes spacing and length requirements
Two-way slabsUse Equivalent Frame Design Method
(Section 13.7)
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
43/60
Banded tendon distribution
Photo courtesy of Portland Cement Association
Development of prestressing strand
development length
= transfer length
ese peps
Pf f
A
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
44/60
Shear for prestressed concrete members is
similar to that for reinforced concrete
members, but it takes advantage of
presence of prestressing force
Post-tensioned tendon anchorage zone
design
Load factor = 1.2 Ppu = 1.2Pj
Pj
= maximum jacking force
= 0.85
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
45/60
Strength evaluation of existing structures
(Chapter 20)
Strength evaluation of existing structures
(Chapter 20)
When it is required
When we use analysis and when perform a load test
When core testing is sufficient
Load testing
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
46/60
A strength evaluation is required
when there is a doubt if a part or all of a structure
meets safety requirements of the Code
If the effect of the strength deficiency is well
understood and if it is feasible to measure the
dimensions and material properties required for
analysis, analytical evaluations of strength
based on those measurements can be used
If the effect of the strength deficiency is not well
understood or if it is not feasible to establish the
required dimensions and material properties by
measurement, a load test is required if thestructure is to remain in service
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
47/60
Establishing dimensions and material
properties
1. Dimensions established at critical sections
2. Reinforcement locations established by
measurement (can use drawings if spot
checks confirm information in drawings)
3. Use cylinder and core tests to estimate cf
Core testing
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
48/60
If the deficiency involves only the
compressive strength of the concrete
based on cylinder tests
Strength is considered satisfactory if:
1.Three cores are taken for each low-strengthtest
2.The average of the three cores
3.No individual core has a strength 4.8 kN/m2
L may be reduced as permitted by general
building code
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
51/60
Age at time of loading 56 days
Loading criteria
Obtain initial measurements (deflection,
rotation, strain, slip, crack widths) not more
than 1 hour before application of the first
load increment
Take readings where maximum response is
expected
Use at least four load increments
Ensure uniform load is uniform no arching
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
52/60
Take measurements after each load
increment and after the total load has been
applied for at least 24 hours
Remove total test load immediately after all
response measurements are made
Take a set of final measurements 24 hoursafter the test load is removed
Acceptance criteria
No signs of failure no crushing or spalling
of concrete
No cracks indicating a shear failure is
imminent
In regions without transverse reinforcement,
evaluate any inclined cracks with horizontal
projection > depth of member
Evaluate cracks along the line of
reinforcement in regions of anchorage and
lap splices
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
53/60
Acceptance criteria
Measured deflections
At maximum load:
24 hours after load removed:
,
2
120 000
t
h
1
4r
MIN(distance between supports, clear span + )
2 x span for cantilever
t h
Acceptance criteria
If deflection criteria not met, may repeat the
test (at least 72 hours after first test)
Satisfactory if:
2
5r
2 maximum deflection of second test relative to
postion of structure at beginning of second test
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
54/60
Provision for lower loading
If the structure does not satisfy conditions or
criteria based on analysis, deflection, or shear,
it may be permitted for use at a lower load
rating based on the results of the load test or
analysis, if approved by the building official
Case study
1905 building
Chicago, Illinois
USA
Cinder concrete
floors
Load capacity OK for use
as an office building?
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
55/60
Safety shoring
Deflection
measurement
devices
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
56/60
Load through
window
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
57/60
Moving lead ingots through the window
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
58/60
Load stage 14
Findings
Floor could carry uniform load of
2.4 kN/m2
Building satisfactory for both apartments (1.9
kN/m2) and offices (2.4 kN/m2)
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
59/60
Summary
Overview
Prestressed concrete
Strength evaluation of existing structures
118
-
7/25/2019 ACI VN 2011 Topic 1 (David Darwin)
60/60
Figures copyright 2010 by
McGraw-Hill Companies, Inc.
1221 Avenue of the America
New York, NY 10020 USA
Figures copyright 2011 by
American Concrete Institute
38800 Country Club Drive
Farmington Hills, MI 48331 USA
Duplication authorized or use with this presentation only.
The Un iversity of Kansas
David Darwin Ph.D. P.E.
Deane E. Ackers Distinguished Professor
Director, Structural Engineering & Materials Laboratory
Dept. of Civil, Environmental & Architectural Engineering
2142 Learned Hall
Lawrence, Kansas, 66045-7609
(785) 864-3827 Fax: (785) 864-5631