building code requirements for structural concrete (aci 318m-11) overview of aci 318m design of...
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Building Code Requirements for Structural Concrete (ACI 318M-11)
Overview of ACI 318MDesign of Prestressed Concrete
Evaluation of Existing Structures
David Darwin
Vietnam Institute for Building Science and Technology (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)
This afternoon
Analysis and design of
Flexure
Shear
Torsion
Axial load
Tomorrow morning
Design of slender columns
Design of wall structures
High-strength concrete
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).
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
Chapters
3. MATERIALS
Cementitious Materials, Water, Aggregates, Admixtures, Reinforcing Materials
4. DURABILITY REQUIREMENTSFreezing and Thawing, Sulfates, Permeability, Corrosion
5. CONCRETE QUALITY, MIXING, AND PLACING
6. FORMWORK, EMBEDMENTS, AND CONSTRUCTION JOINTS
7. DETAILS OF REINFORCEMENTHooks and Bends, Surface Condition, Tolerances, Spacing, Concrete Cover, Columns, Flexural Members, Shrinkage and Temperature Steel, Structural Integrity
8. ANALYSIS AND DESIGN — GENERAL CONSIDERATIONSDesign 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 REQUIREMENTSLoad Combinations, Strength Reduction Factors, Deflection Control
10. FLEXURE AND AXIAL LOADS
Beams and One-way Slabs, Columns, Deep Beams, Bearing
11. SHEAR AND TORSION
12. DEVELOPMENT
AND SPLICES OF REINFORCEMENT
13. TWO-WAY SLAB SYSTEMS
14. WALLS
15. FOOTINGS
16. PRECAST
CONCRETE
17. COMPOSITE CONCRETE FLEXURAL MEMBERS
18. PRESTRESSED CONCRETE
19. SHELLS AND FOLDED PLATE MEMBERS
20. STRENGTH EVALUATION OF EXISTING STRUCTURES
21. EARTHQUAKE-
RESISTANT
STRUCTURES
22. STRUCTURAL PLAIN CONCRETE
AppendicesA. STRUT-AND-TIE MODELS*
B. ALTERNATIVE PROVISIONS FOR REINFORCED AND PRESTRESSED CONCRETE FLEXURAL AND COMPRESSION MEMBERS
C. ALTERNATIVE LOAD AND STRENGTH REDUCTION 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
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
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 load
Earthquake loads (E) full load
Rain loads (R)*
Snow loads (S)*
* Service-level loads
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(Lr or S or R)
U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W)
U = 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R)
U = 1.2D + 1.0E + 1.0L + 0.2S
U = 0.9D + 1.0W
U = 0.9D + 1.0E
Load cases and load factors by ASCE 7 and ACI 318M
If W based on service-level forces, use 1.6W place of 1.0W
If E based on service-level forces, use 1.4E in place of 1.0E
Details of other cases covered in the Code
Load factors by ACI 318M
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
T-beam
dh
b
hf
bw
As
dt
Strain through depth of beam
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
Load distributions and modeling requirements
Structure may be analyzed as elastic
using properties of gross sections
Ig = moment of inertia of gross (uncracked) cross section
Beams: Ib = ½ Ig Iweb =
Columns: Ic = Ig =
wb h3
12
bh3
12
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
(a)
(b)
(c)
Moment and shear envelopes
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 integrally with 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
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 l 2factorln
Composite
Max –ve right
Max –ve leftMax +ve
Allowable adjustment in maximum moments for t 0.0075
Design of prestressed concrete(Chapter 18)
Behavior of reinforced concrete
Reinforced concrete under service loads
Theory of prestressed concrete
Stresses
57
Methods of prestressing concrete members
• Post-Tensioning
• Pretensioning
Prestressing steels
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 strength
fpy = 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
Strength of prestressing steels available in U.S.
High-strength steel bars: fpu 1035 MPa
fpy 85% (for plain bars) and 80% (for deformed bars) of fpu
fpy based on either 0.2% offset or 0.7% strain
Maximum permissible stresses in prestressing steel
Due to prestressing steel jacking force:0.94fpy
0.80fpu
manufacturers recommendation
Post-tensioning tendons, at anchorage devices and couplers, immediately after force transfer:0.70fpu
Prestressed concrete members are designed based on both
Elastic flexural analysis
Strength
Elastic flexural analysis
Considers stresses under both the
Initial prestress force Pi and the
Effective prestress force Pe
Note: = concrete compressive strength
= initial concrete compressive strength (value at prestress transfer)
cfcif
Classes of membersU – 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
Bending moments
Mo = self-weight moment
Md = superimposed dead load moment
Ml = live load moment
Concrete stresses under Pi
Concrete stresses under Pi + Mo
Concrete stresses under Pe + Mo + Md + Ml
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
. cif0 25
. cif050
Maximum permissible compressive stresses in concrete at service loadsClass U and T members
(a) Extreme fiber stress in compression due to prestress plus sustained load
(b) Extreme fiber stress in compression due to prestress plus total load
. 0 45 cf
. 0 60 cf
Flexural strength
ApsT = 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
Stress in prestressing steel at ultimateMembers 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 p
c p
f df f
f d
1
1
and y c y cf / f f / f
and refer to compression reinforcement, sA
shall be taken pup p
c p
f d. , d . d
f d
017 015
Members with unbonded tendons with span/depth ratios 35:
but not greater than fpy or greater than fpe + 420 MPa
fpe = stress in Aps at Pe = e
ps
P
A
Members with unbonded tendons with span/depth ratios > 35:
but not greater than fpy or greater than fpe + 210 MPa
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
Minimum bonded reinforcement As 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
Distribute As 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, use As =
Nc = resultant tensile force acting on portion of concrete cross section in tension under effective prestress and service loads
Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber
c. f0 17
c
y
N
. f0 5
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
Distribute As 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)
Banded tendon distribution
Photo courtesy of Portland Cement Association
Development of prestressing strand
development length
= transfer length
ese pe
ps
Pf f
A
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
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
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 the structure is to remain in service
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
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-strength test
2.The average of the three cores 3.No individual core has a strength <
. cf085
. cf075
Steel
Reinforcing and prestressing steel may be evaluated based on representative material
If analysis is used, values of may be increased
Tension-controlled 0.90 1.0
Compression controlled 0.75 and 0.65 0.90 and 0.80
Shear and torsion 0.75 0.80
Bearing 0.65 0.80
Load test procedure
Load arrangement:Select number and arrangement of spans or panels loaded to maximize the deflection and stresses in the critical regions
Use more than one arrangement if needed (deflection, rotation, stress)
Load intensityTotal test load = larger of
(a) 1.15D + 1.5L + 0.4(Lr or S or R)
(b) 1.15D + 0.9L + 1.5(Lr or S or R)
(c) 1.3D
In (b), load factor for L may be reduced to 0.45, except for garages, places of assembly, and where L > 4.8 kN/m2
L may be reduced as permitted by general building code
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
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 hours after 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
Acceptance criteria
Measured deflections
At maximum load:
24 hours after load removed:
,
2
1 20 000t
h
1
4r
MIN(distance between supports, clear span + )
2 x span for cantilevert 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
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?
Safety shoring
Deflection
measurement
devices
Load through
window
Moving lead ingots through the window
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)
Summary
Overview
Prestressed concrete
Strength evaluation of existing structures
118
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 University of
Kansas
David Darwin, Ph.D., P.E.Deane E. Ackers Distinguished Professor Director, Structural Engineering & Materials Laboratory
Dept. of Civil, Environmental & Architectural Engineering2142 Learned HallLawrence, Kansas, 66045-7609(785) 864-3827 Fax: (785) 864-5631