AISC - seain.org · AISC FAQ Steel Talks 31 The design procedure for single-plate connections in...

AISC FAQ Steel Talks 1

AISC

Frequently

Asked

Questions

Jennifer Traut-Todaro, SE

AISC Steel Solutions Center Advisor

March 3, 2016

AISC publications:

14th Ed. Manual

2010 Specification

Today’s Presentation

Frequently Asked Questions on these topics:

Steel Solutions Center


FAQ: What came before the AISC Steel

Construction Manual?


Bessemer Process (1854)

(1876) Carnegie Publishes Handbook

Additional mills begin publishing

similar handbooks (1884)

(1887) Use of Bessemer in U.S.

Carnegie handbook w/ wrought

iron and steel shapes (1889)

(Early 1900s) First Grey rolling mill

installed to make wide-flange structural

shapes

Bessemer steel had brittle

characteristics

Cities enacting code regulations

on the use of steel.


First AISC

Specification: 1923


FAQ: Are User Notes part of the

Specification?


User Notes are NOT part of the Specification.

They give direct guidance and recommendations to the user.


FAQ: How are ASD & LRFD related?


Limit States define Nominal Strength:

(Mn, Pn , Vn , Rn ) independent of Design Approach


To get Available Strength,

• In ASD divide Nominal Strength by

• In LRFD multiply Nominal Strength by


Nominal Strength – Strength of a structure or component without the resistance factor () or safety factor () applied.

Available Strength – Design Strength (LRFD) or Allowable

Strength (ASD), with the or factor applied.

Design Strength (LRFD) – Resistance Factor multiplied by the Nominal Strength, Rn

Allowable Strength (ASD) – Nominal Strength divided by the safety factor, Rn/


For ASD, design shall be performed in accordance with:

Ra Rn/

where

Ra = required strength (ASD)

Rn = nominal strength specified in Chapters C through K

= safety factor specified in Chapters C through K

Rn/ = allowable strength (available strength)

Design Basis: ASD


For LRFD, design shall be performed in accordance with:

Ru Rn where

Ru = required strength (LRFD)

Rn = nominal strength specified in Chapters C through K

= resistance factor specified in Chapters C through K

Rn = design strength (available strength)

Design Basis: LRFD


FAQ: How is the ASD/LRFD correlation

equilibrated?


Limit States are equilibrated at a

Live Load / Dead Load of 3 / 1

Load Combinations

(ASD): Total Load = Dead + Live

Total Load = 1 + 3 = 4

(LRFD): Total Load = 1.2Dead + 1.6Live

Total Load = 1.2(1) + 1.6(3) = 6

Load Ratio: LRFD / ASD = 6 / 4 = 1.5

Direct Relationship!


=1.5 / or = 1.5 /

This relationship is carried throughout the Specification Limit States.


Hanger Design: Case #1

DL = 100 kip LL = 300 kip (3 to 1 ratio)

ASD

Pa = 100k + 300k = 400k

LRFD

Pu = 1.2(100)+1.6(300)= 600k

A W14x61 Hanger

Is required for both

Design procedures.


W14x61

436k>400k (ASD)

653k>600k (LRFD)


Hanger Design: Case #2

DL = 200 kip LL = 200 kip (1 to 1 ratio)

ASD

Pa = 200k + 200k = 400k

LRFD

Pu = 1.2(200)+1.6(200)= 560k

A W14x61 hanger

is still required using ASD,

But a W14x53 will work

Using LRFD.


W14x61

436k>400k (ASD)

W14x53

570k>560k (LRFD)


LL

DL

1/1

2/1

3/1

4/1

5/1

SAME

LRFD

ASD


FAQ: Why is the minimum size of fillet welds

now based on the thinner part in the 2010

Specification?


TABLE J2.4

Thickness of

Thinner Part

Minimum Size of

Fillet Weld

t ≤ 1/4 in. 1/8 in.

1/4 in. < t ≤ 1/2 in. 3/16 in.

1/2 in. < t ≤ 3/4 in. 1/4 in.

t > 3/4 in. 5/16 in.


Temperature

distribution

Answer:

1.) Preheat

2.) Low Hydrogen Filler Metals


FAQ: Do HSS have to be designed for 0.93t?

Where did the factor come from?


Yes.

It comes from HSS production

tolerances.

ASTM A500

11.2 Wall Thickness- The minimum wall thickness at any point of measurement

On the tubing shall be not more than 10% less than the specified wall thickness.

The maximum wall thickness, excluding the weld seam of welded tubing, shall be

Not more than 10% greater than the specified wall thickness. For square and

Rectangular tubing, the wall thickness requirements shall apply only to the

Centers of the flats.


New HSS Specification: ASTM A1085


FAQ: For single-plate connections, why

is the fillet weld size required to

be 5/8 of the plate thickness.

The plate thickness is also limited

to a maximum of 1/16” more or

less than the bolt size?


The design procedure for single-plate connections in the AISC 14th

Edition Manual (10-102 thru 10-105) requires….

1 16"2

bPL

dt

w = 5/8 tPL

To ensure that a ductile

limit-state will control ww

Plate yielding before

weld rupture failure.

Bolt bearing before bolt

shear failure.

db

tPL

What does this accomplish?


Section B3.6a Simple Connections

-A simple connection transmits a negligible moment.

-In the analysis of the structure, simple connections may be assumed to allow unrestrained rotation between the framing elements being connected.

-A simple connection shall have sufficient rotation capacity to accommodate the required rotation determined by the analysis of the structure.


There is a lot of rigidity and stiffness in a shear tab connection.

Simply supported beam model.

Moments on a fixed end beam.


How does this connection allow for the rotation of the beam end?

1 16"2

bPL

dt

w = 5/8 tPL

ww

Plate yielding before

weld rupture failure.

Bolt bearing before bolt

shear failure.

db

tPL


How does this connection allow for the rotation of the beam end?

1 16"2

bPL

dt

w = 5/8 tPL

Single Plate


FAQ: What is shear lag and when must it

be considered?


Check Tension Rupture: Ae = An x U = B x tPL x U

B

Tensile stress

along bar


Net reduction on gross

Shear lag reduction on net


FAQ: Shear Lag


For Tension Rupture check

no tension over this area.

A

A

Section A-A

For Tension Rupture check

no tension over this area.

B

B

Section B-B


For tension rupture, no tensiondistributed to this area.


Tension Rupture Check

1 18" Ø A490-SC A Bolts

w/ OVS Holes

ØRn = 22.9 kips/bolt

(TYP.)

A

A

183.

2 k

366.

4 k

549.

6 k

732.

8 k

916

k1099

.2 k

1282

.4 k

Tension Rupture Design

Strength = 1175 kip

(w/ 8 Bolt Holes Removed)

At Section A:

1282.4 kip > 1175 kip (NG)

Pu = 1282.4 kips


Tension Rupture Check

Tension Rupture Design

Strength = 1305 kip

(w/ 4 Bolt Holes Removed)

At Section B:

1282.4 kip < 1305 kip (OK)

1 18" Ø A490-SC A Bolts

w/ OVS Holes

ØRn = 22.9 kips/bolt

(TYP.)

B

B

C

C

183.

2 k

366.

4 k

549.

6 k

732.

8 k

916

k

1099

.2 k12

82.4

k

1190

.8 k

At Section C:

1099.2 kip < 1175 kip (OK)

Pu = 1282.4 kips


FAQ: In many design examples in the 14th Edition Manual of Steel Construction, yielding and buckling in a

gusset plate or similar fitting are checked on a Whitmore section.

What is a Whitmore section?


lw lw

30° 30° 30° 30°

Bolted Joint Welded Joint

Gusset or other

Connection elements


f a1

f a2

Whitmore Section along gusset platestrains resulting in a higher axial stressin the beam web, f .a2

Whitmore Section across differing material strengths.


Section B3.6: Design of

Connections

“Self-limiting inelastic

deformations of the

connections are

permitted.”


(a) Correct Whitmore length. (b) Incorrect Whitmore length.

Be careful not to overestimate the Whitmore section.


Unbraced length determination.

L1

L2

L3

ForkLr

L = L2

or

L = L1 + L2 + L3

3


Unbraced length determination.

L1

L2

L3

ForkLr

L = L2

or

L = L1 + L2 - L3

3


FAQ: Why is use of the least possible

fillet weld size desirable?


1/4

1/2

Cost of welding is essentially

proportional to the volume of weld

metal.

½” Fillet vs. ¼” Fillet Weld

Doubles capacity but quadruples

weld volume.


Larger Welds = More Distortion


FAQ: Why are fillet welds preferred over

groove welds?


Generally, fillet welds and groove

welds require approximately the

same amount of weld material.

However, fillet welds typically do

not require beveling and similar

base metal preparation.

Afillet = 0.0626 in2

APJP = 0.0624 in2

0.354"

3/8 (1/4)38"

14"

3 8"


FAQ: When a weld is placed between

plates forming an angle that is less than

60 degrees, why is a Z-loss factor applied

to determine the effective throat?


The Z-Loss dimension accounts for the probability of incomplete fusion at the

root of the joint (on the acute side of the connection) due to restricted welding

access.


FAQ: When an extended end-plate moment connection is specified as slip-critical, must the slip resistance of the

bolts at the tension flange be reduced for the tension present?


F Ff n Friction Force,

R D h T Nn u sc b s Eq: J3-4


FAQ: Why is the design strength of a bolt

calculated in the AISC Specification on

the basis of the nominal cross-sectional

area rather than the net tensile area that

remains after threading?

Net area

Gross area


A

A

net

gross

0.75 to 0.79

Agross is easy to calculate.

To account for Anet, the nominal

tensile strength per unit area given

in Table J3.2 is equal to the tensile

strength of the bolt material times

0.75.


0.334 in2/ 0.442 in2 = 0.75


For A325, Fu = 120 ksi

For A490, Fu = 150 ksi

Table 2-6 from AISC 14th

Edition Manual


From Table 2-6

• A325 Bolts Fu = 120 ksi

– Fnt = 0.75 x 120 ksi = 90 ksi

• A490 Bolts Fu = 150 ksi

– Fnt = 0.75 x 150 ksi = 113 ksi

R F An nt b Eq. (J3-1)

Nominal unthreaded

body area of bolt.


FAQ: In a built-up I-shaped cross-section,

how are welds connecting the plates

designed?


• Shear Flow

fVQ

I

VAy

Ihoriz


P1

P2


fVQ

I

VAy

Ihoriz


• Example:


Example: V = 189 kips

fVQ

I

VAy

I

k in in

inhoriz

189 27 5 24 375

36 768

2

4

. .

,

I = 36,768 in4

w

w

y =

24.3

75"

27.5 in2

f kip inhoriz 345. /

Try 5/16” Fillet Weld

Req’d = 3.45 kip/in / 2 welds = 1.73 kip/in

0.928 x 5 = 4.64 kip/in

% Continuous Weld = (1.73/4.64) x 100 = 37.3%


For a 5/16” Fillet Weld

% Continuous Weld = 37.3%

Per Table 10 (Blodgett)

5/16” Fillet Weld, 4” at 10” Spacing

5/16

5/16

4@10

4@10


4

3307.5 12 24.37526.3

36,768b midspan

k ft inf ksi

in

2

lg 26.3 27.5 723.5f plateP ksi in kips

f kipsft

kip inhoriz

7235

35

212 345. / . /


FAQ: Bolt Bearing Deformation

What is the philosophical difference between these two options?


1 4"

MA

X.

When deformation is a

design consideration

Designs driven by blast or progressive collapse loading could take

advantage of higher capacity.


FAQ: As indicated in the 2010 AISC

Specification Table J3.2, when the pattern of

fasteners in a bolted joint exceeds 38 in. in

length, tabulated design strengths should be

reduced to 83.3% of the tabulated values. Why?


End bolts will continue to

deform until next row of

bolts slip into bearing.

End bolts will slip

into bearing first.

This behavior continues as

load increases. End bolts

continue to deform.


If connection is long enough,

end bolts will deform enough

that they will fail. This results

in the sequential failure of

fasteners…”unbuttoning.”


FAQ: Effective Width of Stiffened Plate

• When a stiffening element is attached to a plate,

what width of plate is effective in resisting load with

the stiffening element?

B


General Rule of Thumb used in

American Petroleum Institute and

American Iron and Steel Institute

standards for liquid storage tanks.

SP

AN

S1 S2

Bt

PL

B = 16 x tPL < MIN

Projecting Width

(S1+S2)/2

Span/4

Width/Thickness (Table

B4.1 per AISC Spec.)

www.aisc.org/faq

www.aisc.org/epubs


Basic

Requirements

ANSI/AISC 360

Additional Seismic

Requirements

ANSI/AISC 341

Additional Nuclear

Requirements

ANSI/AISC N690

Code of Standard

Practice

AISC 303

freePubs : AISC Standards

Prequalified

Connections

ANSI/AISC 358

Engineering Journal

AISC Design Guides

Modern Steel Construction

SSC Staff

Joe Dardis Katherine Quigg

Carlo Lini Tabitha Stine

Jennie Traut-Todaro

Larry Muir


• Parking Garage

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Reference Material


• Parking

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Project Prototypes

& Similar Studies

Conceptual Solutions

AISC Regional Engineers

For more information or to contact your regional engineer, log on to www.aisc.org/myregion


A second option for the location of the

“branding title.” A second option for the location of the

“branding title.”

There’s always a solution in steel.

Thank You

American Institute of Steel Construction

One East Wacker Drive, Suite 700

Chicago, IL 60601

Jennie Traut-Todaro

[email protected]

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