High-Strength Bolts

Transcription

High-Strength Bolts

High Strength Bolts - Geoff Kulak
April 14, 2011
American Institute of Steel Construction
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High-Strength Bolts: The Basics
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Presented by
Geoff Kulak, Ph.D.
Professor Emeritus at the University of Alberta
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1
April 14, 2011
High-Strength Bolts: The Basics
• Fundamentals and Behavior
• Specification Requirements (AISC 2010)
Role of the Structural
Engineer…
•
Selection of suitable bolt
types and grades
•
•
•
Design of the fasteners
Responsibility for installation
Responsibility for inspection
5
ASTM A307 Bolts
ASTM A325 Bolts
• Type 1 or Type 3 (weathering steel)
• often a good choice when loads are
static
• ASTM Spec.
• strength level inferior to high-
RCSC Spec.
• Minimum tensile strength: 120 ksi
strength bolts (60 ksi tensile ult.)
• Pretension can be induced if desired
• pretension indeterminate
7
6
8
2
April 14, 2011
Comparison of Bolts: Direct Tension
ASTM A490 Bolts
•
•
80
Types 1 or Type 3 (weathering steel)
Minimum tensile strength: 150 ksi,
bolt
tension
kips
(maximum 170 ksi)
RCSC Spec.
• ASTM Spec.
• Pretension can be induced if desired
7/8 in. dia. A490 bolt
60
40
20
0.05
0.10
0.15
0.20
elongation (inches)
9
…comments cont’d
Comments…
• Nuts: ASTM A563
• Washers: if needed, ASTM F436
• Note: we quote the ultimate tensile
strength of the bolt
– benchmark for strength statements (e.g.
shear strength is some fraction of
ultimate tensile strength)
• Bolt – nut – washer sets implied so far,
but other configurations available
• Bolt notation: Group A (A325, F1852)
• What about yield strength?
• What is “proof load”
and Group B (A490, F2280)
11
10
12
3
April 14, 2011
Loading of Bolts
Shear Loading
• Shear
– load transfer by shear in bolt and
bearing in connected material OR
– load transfer by friction (followed by
shear and bearing)
• Tension
• Combined Tension and Shear
Truss Joint
13
14
Bolts in Tension – prying
Bolts Loaded in Tension
Prying force
Bolt force
bolts in tension
Applied force
these
bolts in
shear
HighHigh-strength bolts in tension can
be a source of problems
15
16
4
April 14, 2011
Bolts in combined tension and shear
bolts in
combined
shear and
tension
bolts in shear
Consider a simple joint —
P
P
17
18
Finally...
P
P{
and associated
shear stress
τ=
d
P
A
P/2
Free body
of bolt
P { a bearing force
P
19
t
P
P/2
this force is equal and
opposite to the bearing force
shown previously
20
5
April 14, 2011
In the example, we identified…
AISC Standard 2010
• Parallel LRFD and ASD rules
• force in the bolt (a shear force)
• LRFD uses a resistance factor, Ø
• force that the bolt imposed on the plate (a
bearing force)
• ASD uses a safety factor, Ω
• Loads as appropriate:
• force in the plate itself (a tensile force)
– factored loads for LRFD
– non-factored loads for ASD
• force transfer could also be by friction:
friction:
not included in this illustration
21
Installation —
AISC Specification cont’d
LRFD:
req’d strength LRFD ≤ φ R n
ASD:
req’d strength ASD ≤ R n / Ω
22
• SnugSnug-tight only
• Pretensioned
– Calibrated wrench
– TurnTurn-ofof-nut
– Other means:
 Tension control bolts
 LoadLoad-indicator washers
23
24
6
April 14, 2011
Behavior of a large joint (shear splice) —
Bolts in Shear: Issues
• Shear strength of bolt (single shear
or double shear, threads in shear
plane?)
average
bolt
shear
• Bearing capacity of bolt (never
MPa
governs)
• Bearing capacity of plate
• Tensile (comp.) capacity of plate
deformation over α , mm
25
26
Bolts in shear-type connections:
Slip in bolted joints…
• Can be as much as two hole
Specifications include information for:
clearances
• Some bolts will already be in bearing
at start of loading
– bearing type connections
– slipslip-critical connections
• Both laboratory tests and field
measurements indicate that slip is
more like 1/2 hole clearance
27
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7
April 14, 2011
Bearing-type connections:
Bolts in bearing-type connections…
• Issues
Region of bearingbearingtype behavior
load
– bolt shear strength
– bearing capacity of connected material
– member strength
• Shear strength of bolts is not dependent
on presence or absence of pretension.
pretension
(How come?)
deformation
29
30
Individual bolt in shear
Bolt Shear Strength
• Bolt shear strength ≈ 62% of bolt ultimate
tensile strength (theory
(theory + tests)
tests)
– Design rule takes 90% of this value
– Threads in shear plane?
– Long joint effect: another discount
applied.
31
32
8
April 14, 2011
Physical test —
Uneven loading
of bolts –
(End four bolts of 13)
33
34
Back to bolt in shear —
Bolt Pretension v. Shear
• The bolt pretension is attained as a result of small
axial elongations introduced as nut is turned on
• These small elongations are relieved as shear
deformations and shear yielding take place
Shear strength
of single bolt
(tests) —
τ = 0.62 σ u bolt
• Confirmed by both bolt tension measurements
and shear strength tests
• So, bolt shear strength NOT dependent on
pretension in the bolt.
Shear deformation
35
36
9
April 14, 2011
Bolts in Shear — AISC
nominal shear strength …
φ R n = φ Fnv A b
Fnv = 90% (0.625 × Fu ) = 0.563 Fu
φ R n = design shear strength
Fnv = nominal shear strength, ksi
e.g. A325 bolt, no threads in shear plane ,
Group A : see tabulated value in Table J3.2
(0.563 ksi Fu = 0.563 x 120 ksi = 68 ksi)
37
38
Comments…
and…
• The discount for length (use of 90%) is
For threads included, the tabulated
values are 80% of the above.
• If joint length > 38 in., a further reduction,
conservative
to 83%
• The ø – value used for this case (0.75) is
conservative
39
40
10
April 14, 2011
Let’s return now to slip-critical
connections…
Slip-Critical Connection
Clamping force from bolts (bolt pretension)
Load at which slip takes place
will be a function of …?
42
Bolts in slip-critical connections…
Slip-critical joints specified when…
• Load is repetitive and changes
from tension to compression
(fatigue by fretting could
occur.)
load
• Change in geometry of
structure would affect its
performance.
region of slipslip-critical
joint behavior
critical joints should be the
exception, not the rule (but,
see also seismic rules)
deformation
43
• Certain other cases.
• Comment:
Comment: for buildings, slipslip-
44
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April 14, 2011
First principles, slip resistance is —
Design slip resistance, AISC
R n = μ D u h f Tb n s
P = ks n ΣTi
ks = slip coefficient (µ)
n = number of slip planes (usually 1 or 2)
Ti = clamping force (i.e., bolt pretension)
slip coefficient
no. slip planes
clamping force
…terms hf and Du need to be defined
and a value inserted for Ø
45
and the modifiers …
46
A note for advanced readers!!
h f = modifier re fills : either 1.0 or 0.85
φ = 1.0 for std. holes and for short slots ⊥
Du is a statistical parameter that
results in a probability of slip of 5% at
the service load level when the joint is
designed using factored loads.
= 0.85 for oversize and short slots parallel
= 0.70 for long slotted holes
D u = 1.13, ratio of installed bolt
The resistance factor reflects the
consequence of exceeding the “slip
limit state.” As the consequence of
slip gets more severe, the resistance
factor is decreased.
tension to specified minimum bolt tension
μ = 0.30 clean mill scale, hot − dipped galvanized
and roughened, etc. (Class A surfaces)
μ = 0.50 unpa int ed and blast − cleaned, etc.
(Class B surfaces)
47
48
12
April 14, 2011
Bolts in Tension
•
Bolts in Tension – some comments
Capacity of a bolt in tension: product of
the ultimate tensile strength of the bolt
and the tensile stress area of the bolt
(i.e. Fu Ast )
•
Specifications directly reflect this
calculated capacity (…
(…to come)
•
Force in bolt must reflect any prying
action effect
• Preference: avoid joints that put bolts
into tension, especially if fatigue is an
issue
• Use A325 bolts rather than A490 bolts
• Minimize the prying action
49
50
Bolt tension + external tension
Question…
1. Pretension the bolt → tension in
the bolt, compression in the plates
• pretensioned bolt in a connection
2. Add external tension force on
connection →
• apply external tension force to the
connection
• Bolt tension increases
• do the bolt pretension and the
• Compression between plates decreases
external tension add?
Examine equilibrium and compatibility…
compatibility…
51
52
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April 14, 2011
And the result is…
AISC rule, bolts in tension—
• The bolt force does increase, but not
by very much (≅ 7%)
• This increase is accommodated
φ R n = φ Fnt A b
bolt area for nominal
diameter
nominal tensile strength
within the design rule.
φ R n = design tensile strength
53
What is nominal tensile strength, Fnt ?
Adjusted area
Call this Fnt
55
where Fnt = 0.75 Fu as tabulated
in the Specification
As we now know, the 0.75 really
has nothing to do with Fu
{
Pult = 0.75 Fu A b
So, the AISC rule for bolts in tension…
φ R n = φ Fn t A b
Pult = Fu A st = Fu (0.75A b )
or,
54
56
14
April 14, 2011
Returning to shear splice joints,
we still have to deal with the
bearing capacity of the connected
material.
Bearing capacity (of
connected material)
d
ShearShear-out of a
block of material
or yielding
t
P/2
P
P/2
57
Bearing stresses at bolt holes…
s Le
58
Shear-out rule…
Shear - out is 2 ( τ ult × Lc × t )
Lc
t1
t2
Needed:
1. shearshear-out rule
2. yield rule
(deformation)
or, R n = 2 ( 0.75 σu × Lc × t )
and AISC rule is: R n = 1.5 Fu Lc t
d
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60
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April 14, 2011
Plate bearing…
from tests:
σb
σ pl
u
=
Plate bearing…
Le
d
Making the substitution and using
Fu ≡ σ pl
u
L 
..after some arithmetic R n = σ b d t = σ upl  e  d t
 d 
R n = 3 d t Fu
valid for L e ≥ 3 d
61
Further note re bearing…
Finally, the AISC rule for
plate bearing capacity is …
When deformation a consideration,
use
R n = 1.5 Fu Lc t ≤ 3.0 d t Fu
R n = 1.2 Fu L c t ≤ 2.4 d t Fu
(with a φ-value still to be inserted)
Why this difference, and when do we
use the latter? (value of φ still to be
applied)
63
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64
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April 14, 2011
Failure (ult. load) is
by tensile fracture at
location shown,
regardless of
geometric
proportions.
Block shear
rupture
Shear yield along vertical
planes.
Failure is controlled by
ductility – not strength.
65
Basics…
An example of
shear + tension
failure in a
coped beam…
Tr + Vr = φ A nt Fu + 0 .60 φ A gv Fy
where A nt = net area in tension
and
66
A gv = gross area in shear
tension fracture
shear yield
…and some other requirements, including
specific case of coped beams, limit on shear
67
68
17
April 14, 2011
Back to installation…
Bearing-Type Connections—
Installation of Bolts
• Bolts can be installed to “snugsnug-tight
condition — ordinary effort of worker using
a spud wrench. (Pretension unknown, but
usually small)
69
70
1. Calibrated Wrench
Installation
Installation —
• Reliable relationship between torque
and resultant bolt tension?
NO ! (and forbidden by RCSC)
• Establish relationship by calibration
of the installing wrench.
– bring parts together, continue turning nut,
bolt elongates, tension develops in bolt, and
clamped parts compress
71
72
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April 14, 2011
Calibrated wrench, cont’d
Hydraulic calibrator –
• Adjust wrench to stall or cut out at
•
•
•
•
desired level of bolt pretension
Target value of pretension (RCSC) is
1.05 times specified min. value
Calibrate using at least three bolts
Calibration is unique to bolt lot,
length, diameter, grade of bolt
Washers must be used
73
2. Turn-of-Nut Installation
74
Does this
definition of
snug-tight
seem a little
vague?
• Run nut down, bring parts into close
contact
• Work from stiffer regions to edges
• Establish “snugsnug-tight”
tight” condition (first
impact of impact wrench or full effort of
worker using a spud wrench)
• Apply additional oneone-half turn (or other
value, depending on bolt length)
How influential is “snug-tight?”
75
76
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April 14, 2011
60
Bolt Tension by Turning the Nut
bolt
tension 40
(kips)
60
Bolt Tension by Turning the Nut
bolt
tension 40
(kips)
specified minimum
tension
20
specified minimum
tension
20
0.02
0.04
0.06
0.02
0.08
0.04
bolt elongation (in.)
bolt elongation (in.)
range of bolt
elongations
at snug
bolt elongation at one-half turn
bolt elongation at one-half turn
77
Inspection of Installation
78
Inspection of Installation
• Principles:
• Is bolt tension required? — if not, why
– Determination of the bolt pretension
after installation is not practical
inspect for it !
• Know what calibration process is required
– Understand the requirements e.g., are
pretensioned bolts required?
and monitor it on the job site
• Observe the work in progress on a regular
– Monitor the installation on the site
basis
– Proper storage of bolts is required
79
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April 14, 2011
Inspection of installation:
Snug tight only req’d….
Consider the following AISC cases —
• Bearing-type connections
1. Bolts need be snugsnug-tight only
2. Bolts are pretensioned (but not a slipslipcritical joint)
• Bolts in tension (A325 only)
– only when no fatigue or vibration (bolt
could loosen)
3. SlipSlip-critical joint
81
82
Inspection: if pretensioned bolts required…
required…
Inspection – snug tight
• Bolts, nuts, and washers (if any) must meet
the requirements of the specifications
• All of requirements for snugsnug-tight case
• Observe the prepre-installation verification process
• Hole types (e.g., slotted, oversize) must
– turn of nut, or;
• Contact surfaces are reasonably clean
• Parts are in close contact after bolts
– other (direct tension washers, tensiontension-control
bolts)
meet specified requirements
– calibrated wrench, or;
• Calibration process done minimum once per day
snugged
• All material within bolt grip must be steel
• Calibration process done any time conditions
change
83
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April 14, 2011
An inspected joint (turn-of-nut)
Inspection: for slip-critical joints
• All of the above, plus
• Condition of faying surfaces, holes, etc.
• In addition to observing the calibration
process, the inspection must ensure that
the same process is applied to the field
joints
85
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87
88
and some other comments…
• Pretension values greater than
those specified are not cause
for rejection.
• Rotation tests are useful for
short-grip bolts or coated
fasteners (requirement is in
ASTM A325 spec. and is for
galvanized bolts)
22
April 14, 2011
Actual pretensions, cont’d
Actual pretensions, cont’d
• For A325 bolts, turn-of-nut:
– Average tensile strength exceeds spec.
min. tensile by about 1.18
• A325, ½ turn-of-nut: 35% increase
• A490, ½ turn-of-nut: 26% increase
• A325 and A490, calibrated wrench: 13%
increase
etc. for other cases
– Average pretension force is 80% of
actual tensile
•
– Result is that actual bolt tension is
about 35% greater than specified bolt
tension
Note: these increased pretensions are
embodied in the specification rules
89
Some other options for bolts —
90
Tension Control Bolts
region of
constant torque
ASTM F1852,
F2280
groove at which shear
will take place
91
92
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April 14, 2011
Tension-Control Bolts
Tension control bolts….
• NOTE: evidence that tips have
• Advantages
sheared off is not in itself evidence
that desired pretension is present
– Installation is from one side
– Electric wrench is used
– Installation is quiet
• Consider limits:
– Friction conditions are very high…
high…
• Disadvantages
– Friction conditions are very low…
low…
– More expensive
– Pre-installation calibration required
• Hence, calibration is essential!
93
94
Direct Tension Indicators
Direct tension indicators—
• Protrusions formed in
special washer
• Protrusions compress
as force in bolt is
developed
• Use feeler gage to
measure gap (or refusal)
• User must verify the process
(like calibrated wrench)
95
ASTM 959
96
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April 14, 2011
Reliability of these...
•
•
•
•
Some additional topics …
Calibration required
• Details, other topics
Reliability same as calibrated wrench
– washers (but not today!)
TensionTension-control bolt is torquetorque-dependent
– slotted or oversize holes (but not today!)
LoadLoad-indicating washer is elongationelongationdependent
– seismic design
97
Seismic design of connections
98
Pre-qualified bolted connections
• Analyze structure in order to
compute the forces
– Use FEMA 350 and/or AISC Seismic
Design Spec.
• With forces now known, design
connectors
• Advisable to use pre-qualified
configurations
Note: some details not shown,
e.g., continuity plates
99
100
25
April 14, 2011
…bolted joints, seismic design
• All bolts pretensioned
• Faying surfaces as per slip-critical
• Use bearing values for bolts
AllAll-bolted connection
– moderate quakes: no slip
– major quakes: slip will occur and bolts go into bearing
• Normal holes or short slotted only (perpendicular)
• No bolts + welds in same faying surface
101
Seismic design, cont’d
102
It all started with rivets….
• Non-ductile limit state in either member or
connection must not govern.
• Calculate bolt shear strength as per
bearing type but use 2.4 d t Fu bearing rule
• Must use expected yield and ultimate
strengths, not the specified values
e.g. A36 plate : use 1.3 σ y spec.
103
104
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April 14, 2011
Determine ultimate load for this gusset
plate (which is one that was tested)
Design
example:
gusset plate
connection
14.76
2
30°
Fy = 39.9 ksi
Fu = 69.0 ksi
15.75
[email protected]=8.27
7/8 A325 bolts
(holes 15/16 in.)
2.68
t = 0.26 in.
7.22
Pu test =164 kips
(compression)
19.69
105
106
Set out the issues…
Continuing…
• Brace force in tension–
• Brace force in compression
– slip load of bolts (no slip at service load)
– slip capacity of bolts (already checked for load
in tension)
– shear load of bolts
– shear capacity of bolts (already checked for
load in tension)
– bearing capacity of plate
– bearing capacity of plate (already checked)
– block shear
– block shear (doesn’
(doesn’t apply)
– capacity of gusset plate in compression (New)
107
108
27
April 14, 2011
Slip load (calculate at factored load level)
R n = μ D u h f Tb n s (per bolt )
μ = 0.30 (clean mill scale)
h f = 1.0 (no fills))
Slip load calculation cont’d.
R n = μ D u h f Tb N s (per bolt )
= 0.30 × 1.13 × 1.0 × 37.88 kip × 2 slip planes (std.holes) :
= 25.68 kips / bolt
A b = π d 2 / 4 = 0.60 in.2 (7/8 in.dia.)
Fu = 120 ksi (A325 bolts)
n s = 2 slip planes
or, for 8 bolts, (φ = 1.0); R n = 205 kips
Tb = spec. min. bolt pretension = (0.75 × A b )(Fu )70%
= 0.75 × 0.60 in.2 ×120 ksi × 70% = 37.88 kips
109
Shear resistance of bolts
110
Bearing resistance (use φ = 1.0)
φ R n = φ Fv A b
R n = 1.5 Fu L c t ≤ 3.0 d t Fu
Use ø =1.0 so that we can compare this
load with the test load, assume threads in
shear plane, no joint length effect
3 d t Fu =
Fv = 90% [0.62 × 120 ksi] = 68 ksi
3 × 7 / 8 in. × 0.26 in. × 69.0 ksi = 47.1 k/bolt
φ R n = 1.0 × 68 ksi × 0.60 in.2 = 41.0 kips (per bolt )
1.5 Lc t Fu =
or, for 8 bolts, 2 shear planes, threads in shear plane
= (41.0 × 8 × 2)kips × 0.80 = 525 kips
1.5 × 1.53 in. × 0.26 in. × 69.0 ksi = 41.2 k
111
112
28
April 14, 2011
Block shear
Bearing resistance…
2.00
…the governing value is 41.2 kips/bolt
and, for 8 bolts—
[email protected]=8.27
Bearing resistance is 330 kips
2.68
A nt = (0.26)( 2.68 − 15 / 16) = 0.45 in.2
A gv = (8.27 + 2.00)2 × 0.26 = 5.34 in.2
Tr + Vr = φ A nt Fu + 0.60 φ A gv Fy
113
114
Brace force in compression:
Block shear, cont’d
Tr = 0.45 in.2 × 69.0 ksi = 31.0 kips
Vr = 0.60 × 5.34 in.2 × 39.9 ksi = 127.8 kips
and the total block shear resistance
(unfactored) is (31 +128) =159 kips
issue is sway
buckling in
this region
115
116
29
April 14, 2011
Whitmore
method….
Checking the buckling…
30°
• Use beam formulae to
• Whitmore method (checks yield)
• Thornton method (checks buckling)
• Modified Thornton method (checks
•
buckling)
•
check perceived
critical sections
Use 30o , as shown to
check yielding at
location shown.
Does not predict
ultimate capacity very
well, usually
conservative but
sometimes nonnonconservative
117
118
Thornton method, modified
Thornton method…
• Use longest (or
average) of L1, L2, L3
to compute a
buckling load on a
unit width column,
then apply this to the
total width.
• Use k = 0.65 in the
column formulae
As per Thornton
method but
spread load out
at 45o
30°
L1
45o
L2
L2
L3
L3
119
L1
120
30
April 14, 2011
Calculations for buckling capacity:
Yam & Cheng gusset plate tests
(U of A, 13 tests)
Pu
PW
Pu
PT
Pu
PT '
mean
1.33
1.67
1.06
std. dev.
0.26
0.12
0.08
Using scale dwg.
L2 = 9.65 in.
L1
Width of the 45o
base is 19.2 in.
L2
L3
φc Pn = φc A g Fcr (use φc = 1.0)
we’ll use this method
Fcr = (0.658
Fy / Fe
) Fy
use k = 0.65
121
Consider a 1 in. wide
strip that is 9.65 in. long
length =9.65
And applying this to the total width…
Pu = (6.91 k/in.) (19.2 in.) = 132 kips
width = 1
and the test ultimate load on this particular
specimen was 164 kips
t = 0.26
r=
I
=
A
1
× 1 × 0.263
12
= 0.0751 in.
0.26 × 1
and then completing the calculations,
Pn = 6.91 kips (on a 1 in. wide strip)
123
122
so, Pu / PT’ = 1.23
(The corresponding ratios for Whitmore
and Thornton for this specimen were 1.31
and 1.80)
124
31
April 14, 2011
Some references —
Summary of our calculations
Brace
Force
Tension
Compress.
slip bolt
plate block buckling test
load shear bearing shear
load
205
—
525
—
330
—
159
—
—
132
—
164
Load and Resistance Factor Design
Specification for Structural Joints
Using ASTM A325 or A490 Bolts,
Research Council on Structural
Connections, 2004 (RCSC)
(free download available at
boltcouncil.org)
boltcouncil.org)
125
126
References, cont’d.
• G.L. Kulak, J.W. Fisher, and J.A.H. Struik,
Struik, Guide to Design
Criteria for Bolted and Riveted Joints,
Joints, Second Edition, John
Wiley, New York, 1987 (free download at RCSC website)
• Bickford, John H., "An Introduction to the Design and
Behavior of Bolted Joints," Second Edition, Marcel Dekker
Inc., New York, 1990
Thank You!
• G.L. Kulak, A Bolting Primer for Structural Engineers,
Engineers, AISC
Design Guide 17, Chicago, 2002
• Larry Kloiber and Larry Muir, “The 2010 AISC Specification:
Changes in Design of Connections,”
Connections,” Modern Steel
Construction, Sept. 2010
127
Please give us your feedback!
www.aisc.org/cesurvey
128
32
April 14, 2011
AISC Seminars
AISC Seminars
2011 Spring Schedule – 6 seminar topics coming to
26 cities
Upcoming Cities for April:
Detroit – Houston – Denver
Upcoming Cities for May:
St. Louis – Boston - Miami
Philadelphia – Sacramento
2nd Thursday of the month
• No webinar in May
• June 9, 2011: Extended Shear Plate Connections –
•
Larry Muir
July 14, 2011: Design For Stability – Lou
Geschwindner
www.aisc.org/seminars
www.aisc.org/webinars
AISC eLearning
AISC SteelCamp
Over 60 hours of presentations available
anytime, online.
2 day, 4 topics, 15 hours of Continuing
Education,
One low price.
CEUs/PDHs are available.
New York City – June 16-17
San Francisco – July 21-22
www.aisc.org/elearning
www.aisc.org/steelcamp
33

High-Strength Bolts

Transcription

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