Slip Critical Connections (Friction Type Connections) (1)

Steel Connections Steel Connections A lecture prepared  p p By

A Prof. Assis. P Dr D Ehab E Boghdadi B M Matar

Acknowledgement • I acknowledge photos found in this lecture to the  k l d h f d i hi l h scientific teaching aids found in different sources,  especially  i ll • AISC digital library • ESDEP lecture notes • AISC AISC‐ connection teaching toolkit connection teaching toolkit • Personal photos taken in Germany, Holland,  Austria and Egypt Austria and Egypt. Ehab matar Assis. Prof. of steel structural

Objectives Through the following 3 lectures, we shall study  together the steel connections. Our main  objectives will be: 1. Identify the different types of steel connections Identify the different types of steel connections 2. Understanding the force transfer through steel  connections ti 3. Practicing the design of bolted and welded  connections through neat self explained  g details. calculations and full dwgs

Pre‐tensioned High strength bolted  connections (Slip‐Critical connections (Slip Critical Joints) (AISC) Joints) (AISC)

• In a slip‐critical joint the bolts must be fully pretensioned to cause a clamping force between the connected elements • This force develops frictional resistance between the connected elements • The frictional resistance allows the joint to withstand loading without slipping into bearingg against g the bodyy of the bolt,, although g the bolts must still be designed g for bearing • 4 The faying surfaces in slip‐critical joints require special preparation

(AISC)

When to Use Slip‐Critical Joints

Per the RCSC Specification (2000), Slip‐critical joints are only required in the following applications involving shear or combined shear and tension: 1. Joints that are subject to fatigue load with reversal of the loading direction applicable to wind bracing)

(not

2 Joints 2. J i that h utilize ili oversized i d holes h l 3. Joints that utilize slotted holes, except those with applied load approximately perpendicular p p to the direction of the longg dimension of the slot 4. Joints in which slip at the faying surfaces would be detrimental to the performance of 5 the structure

Snug‐tight Installation

Snug‐tight is the tightness attained with a few hits of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the connected plies into firm contact 6 (RCSC 2000)

Turn‐of‐Nut Installation

•

Installation beyond snug‐tight is called pretensioning

•

Turn‐of‐nut p pretensioningg involves several steps: p 1. The bolt is snug‐tightened 2. Matchmarks are placed on each nut, bolt, and steel surface in a straight line 3. The part not turned by the wrench is prevented from turning 4. The bolt is tightened with a prescribed rotation past the snug‐tight condition

•

The specified rotation varies by diameter and length (between 1/3 and 1 turn) 7

(RCSC 2000, AISC)

Calibrated Wrench Installation

•

Calibrated Wrench pretensioning uses an impact wrench (above left) to tighten the b l to a specified bolt ifi d tension i

•

A Skidmore‐Wilhelm calibration device (above right) is used to calibrate the impact q level which will achieve the specified p tension wrench to the torque

•

A sample of bolts representative of those to be used in the connections are tested to verify that the correct tension will be achieved (RCSC 2000, AISC)

8

ASTM F1852 Installation (AISC)

• F1852 bolts are twist‐off‐type tension‐control bolts • These bolts must be pretensioned with a twist‐ off‐type tension‐control bolt installation wrench that has two coaxial chucks • The inner chuck engages the splined end of the bolt • The Th outer t chuck h k engages the th nutt • The two chucks turn opposite to one another to tighten the bolt • The splined end of the F1852 bolt shears off at 9 a specified tension (AISC 2003)

ASTM F959 Direct Tension Indicators DTI s DTI’s

Feeler Gages • Another way to try to ensure proper pretensioning of a bolt is through the use of di t tension direct t i indicators i di t (DTIs) (DTI ) • These washers have protrusions that must bear against the unturned element • As the bolt is tightened the clamping force flattens the protrusions and reduces the gap • The gap is measured with a feeler gage • When the gap reaches the specified size the bolt is properly pretensioned 10

(AISC & NISD 2000)

Installation of DTIs

(Adapted from Figure C 8 1 RCSC 2000) (Adapted from Figure C‐8.1 RCSC 2000)

It is essential that direct tension indicators be properly oriented in the assembly a) The bolt head is stationary while the nut is turned – DTI under bolt head b) The bolt head is stationary while the nut is turned – DTI under nut (washer required) c) The nut is stationary while the bolt head is turned – DTI under bolt head (washer required) d) The nut is stationary while the bolt head is turned – DTI under nut 11 2000) (RCSC

Pre tensioning force T Torque Ma Pre‐tensioning force T, Torque M

T  0.7 xf yb xAs

Where:

M a  0.2.d .T

fyb= yield point stress of the bolt material = 6.4t/cm2 for bolts grade 8.8 and = 9t/cm2 for bolts grade 10.9 As= stress area in the table given later later. d= bolt diameter

The design shear strength of a single  bolt in slip critical connections Ps  m

T 

Where : m  no. of shear plans T = pre - tensioning force in the bolt

 = friction coefficient  = safety factor with regard to slip = 1.25 for case I loading in ordinary steel structure = 1.05for case II loading in ordinary steel structure = 1.6 for case I loading in bridges and cranes = 1.35 for case II loading in bridges and cranes

Coefficient of friction  Coefficient of friction  •

• • • • • • • •

The friction coefficient is of prime importance for friction type connection.  It depends on the condition and preparation of the surfaces to be joined.  Surface treatments are classified into three main classes A, B and C as  follows: ) Class A  ((=0.5): Surfaces are blasted with shot or grit with any loose rust removed, no  painting. Surfaces are blasted with shot or grit and spray metalized with aluminum. f bl d h h d l d h l Surfaces are blasted with shot or grit and spray metalized with a zinc  based coating. g Class B (=0.4) : Surfaces are blasted with shot or grit and painted with alkali‐zinc silicate  painting to produce a coating thickness of 50‐80m. d h k f Class C (=0.3): Surfaces are cleaned by wire brushing or flame cleaning with any loose Surfaces are cleaned by wire brushing or flame cleaning with any loose  rust removed. 

Table for the allowable shear  force/bolt (grade 10.9) Note for grade 8.8, 70% of these values is to be used

Bolts Subjected to shear Bolts Subjected to shear

• Q /b≤ Ps=mT/  /

Bolts subjected to Tension Bolts subjected to Tension

• T/b≤0.6*T 0 6*

Bolts subjected to shear and tension Bolts subjected to shear and tension Acting forces in each bolt Text  Fd . sin  & Q  Fd . cos  Text Text / b  n Q Q/ b  n S f t conditions Safety diti Text / b  0.8T  Text / b  Q/ b  mPs 1   T  

Bolts subjected to Shear and Moment Bolts subjected to Shear and Moment Acting forces in each bolt M T C  h T Text / b  nt Q n Safety conditions Text / b  0.8T Q/ b 

Q/ b  mPs n t = number of bolts subjected to tension only due to moment n = total number of bolts

h

Bolts subjected to shear, tension and  bending moment Acting forces in each bolt M T C  h T Text / bM  nt Text Text / b  n Q Q/ b  n S f t conditions Safety diti Text / b  Text / bM  P  0.8T  T  Q/ b  mPs 1  ext / b  in friction type connection T  

h

Prying forces (P) Prying forces (P) • . In the bending of the  I h b di f h flanges of the Tee, the  bolts act as a pivot point bolts act as a pivot point  so that there is a  compressive reaction (Q)  between the outer edges  of the flanges, which is  defined as the Prying defined as the Prying  Force. The tension  induced in the bolts, for  , equilibrium, is thus Fb = F  + Q.

Value of Prying forces y g

4   W .t p 1    2 30.a.b 2 . As  .T P ext ,b , M orText ,b 4   3a  a  W . t  p     1   2   4  4b  30.a.b . As  a, b = Bolt outer overhang and inner bolt dimension w.r.t. the stem tee stub in cm. W = Flange Tee stub breadth w.r.t. one column of bolts.

As

= Bolt stress area

Text,b, = Applied external tension force on one bolt column due to either Text,b,M

an external tension force = Text or due to the replacement of the applied moment M by two equal and opposite couples Tb = C b = M/d b