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Design of Tension Members. Structural Elements Subjected to Axial Tensile Forces. Trusses. Bracing for Buildings and Bridges. Cables in Suspension and Cable-Stayed Bridges. LAST TIME. Design of Tension Members Tables for the Design Threaded Rods and Cables. LRFD. max. LRFD. ASD. min.

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design of tension members
Design of Tension Members

Structural Elements Subjected to Axial Tensile Forces

Trusses

Bracing for Buildings and Bridges

Cables in Suspension and Cable-Stayed Bridges

last time
LAST TIME
  • Design of Tension Members
  • Tables for the Design
  • Threaded Rods and Cables
design of tension members last time

LRFD

max

LRFD

ASD

min

min

max

ASD

etc

Design of Tension Members LAST TIME
  • Objective
    • Find a member with adequate gross and net areas
    • Find a member that satisfies L/r<300
      • Does not apply to cables and rods

Available Strength

(Nominal Resistance)

Required Strength

design of tension members last time1
Design of Tension Members LAST TIME

Determine required Area

LRFD

To prevent yielding

To avoid fracture

Yielding controls if

design of tension members last time2
Design of Tension Members LAST TIME
  • Determine required Area

ASD

To prevent yielding

To avoid fracture

Yielding controls if

lrfd example last time
LRFD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 1: Required Strength

Step 2: Required Areas

lrfd example last time1
LRFD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 3: Plate Selection based on Ag

Try thickness t = 1 in

Choose PL 1 X 3-1/2

See Manual pp1-8 for availability of plate products

lrfd example last time2
LRFD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 4: Check Effective Area

OK

lrfd example last time3
LRFD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 4: Check Slenderness

OK

asd example last time
ASD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 1: Required Strength

Step 2: Required Areas

asd example last time1
ASD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 3: Plate Selection based on Ag - Same as LRFD

Try thickness t = 1 in

Choose PL 1 X 3-1/2

See Manual pp1-8 for availability of plate products

asd example last time2
ASD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 4: Check Effective Area

OK

lrfd example last time4
LRFD - Example LAST TIME

Tension member with a length 5’-9” resists D=18 kips and L=52 kips

Select a member with rectangular cross section, A36 steel and one line 7/8” bolts

Step 4: Check Slenderness

OK

angles as tension members last time
Angles as Tension Members LAST TIME
  • Must have enough room for bolts

(if bolted connection)

  • Space is a problem if 2 lines of bolts in a leg
  • Usual fabrication practice – standard hole location

Manual pp 1-46

example last time
Example LAST TIME
  • Select and unequal-leg angle tension member 15 feet long to resist a service dead load of 35 kips and a service live load of 70 kips. Use A36
angle example last time
Angle - Example LAST TIME

Step 1: Required Strength

Step 2: Required Areas

angle example last time1
Angle - Example LAST TIME

Step 3: Angle Selection based on Ag

Two lines of bolts, therefore min. length of one leg = 5 in

see table

Choose L6x4x1/2 A=4.75, rmin=0.864

See Manual pp1-42

angle example last time2
Angle - Example LAST TIME

Step 4: Check Effective Area

Length of connection not known

4 – bolts in direction of load U=0.8

NG

angle example last time3
Angle - Example LAST TIME

Step 3: Angle Selection based on Ag – TRY NEXT LARGER

Two lines of bolts, therefore min. length of one leg = 5 in

see table

Choose L5 x 3-1/2 x 5/8 A=4.92, rmin=0.746

See Manual pp1-42

angle example last time4
Angle - Example LAST TIME

Step 4: Check Effective Area

Length of connection not known

4 – bolts in direction of load U=0.8

NG

angle example last time5
Angle - Example LAST TIME

Step 3: Angle Selection based on Ag – TRY NEXT LARGER

Two lines of bolts, therefore min. length of one leg = 5 in

see table

Choose L8 x 4 x 1/2 A=5.75, rmin=0.863

See Manual pp1-42

angle example last time6
Angle - Example LAST TIME

Step 4: Check Effective Area

Iterative Process

Length of connection not known

4 – bolts in direction of load U=0.8

OK

example
Example
  • Select and unequal-leg angle tension member 15 feet long to resist a service dead load of 35 kips and a service live load of 70 kips. Use A36
example using tables
Example – Using Tables

Step 1: Required Strength

Step 2: Choose L based on Pu

Choose L6x4x1/2

A=4.75, rmin=0.980

See Manual pp 5-15

angle example
Angle - Example

Step 3: Check Effective Area

Length of connection not known

4 – bolts in direction of load U=0.8

NG

angle example1
Angle - Example

Shape did not work because table values are for Ae/Ag=0.75

In this problem Ae/Ag=3.1/4.75 = 0.6526

Enter table with adjusted Pu as

example using tables1
Example – Using Tables

Step 4: Choose L based on ADJUSTED Pu

Choose L8x4x1/2

A=5.75, rmin=0.863

See Manual pp 5-14

angle example2
Angle - Example

Step 5: Check Effective Area

Length of connection not known

4 – bolts in direction of load U=0.8

OK

tension members in roof trusses
Tension Members in Roof Trusses
  • Main supporting elements of roof systems where long spans are required
  • Used when the cost and weight of a beam would be prohibitive
  • Often used in industrial or mill buildings
tension members in roof trussed

Pin

Hinge

Tension Members in Roof Trussed

Supporting walls: reinforced concrete, concrete block, brick or combination

tension members in roof trusses1
Tension Members in Roof Trusses

Sag Rods are designed to provide lateral support to purlins and carry the component of the load parallel to the roof

Located at mid-point, third points, or more frequently

tension members in roof trusses2
Tension Members in Roof Trusses

Bottom Chord in tension

Top Chord in compression

Web members: some in compression some in tension

Wind loads may alternate force in some members

tension members in roof trusses3
Tension Members in Roof Trusses

Chord Members are designed as continuous

Joint rigidity introduces small moments that are usually ignored

Bending caused by loads applied directly on members must be taken into account

tension members in roof trusses4
Tension Members in Roof Trusses

Working Lines Intersect at the Working Point in each joint

  • Bolted Truss: Working Lines are the bolt lines
  • Welded Truss: Working Lines are the centroidal axes of the welds
  • For analysis: Member length from working point to working point
tension members in roof trusses5
Tension Members in Roof Trusses

Bolted trusses

Double Angles for chords

Double Angles for web members

Single Gusset plate

tension members in roof trusses6
Tension Members in Roof Trusses

Welded trusses

Structural Tee shapes are used in chords

Angles are used in web members

Angles are usually welded to the stem of the Tee

tension members in roof trusses7
Tension Members in Roof Trusses

Welded trusses

Structural Tee shapes are used in chords

Angles are used in web members

Angles are usually welded to the stem of the Tee

example1
Example

Select a structural Tee for the bottom chord of the Warren roof truss. Trusses are welded and spaced at 20 feet. Assume bottom chord connection is made with 9-inch long longitudinal welds at the flange. Use A992 steel and the following load data (wind is not considered)

Purlins M8x6.5

Snow 20 psf horizontal projection

Metal Deck 2 psf

Roofing 4 psf

Insulation 3 psf

step 1 load analysis
Step 1 – Load Analysis

DEAD (excluding purlins)

Deck 2 psf

Roof 4 psf

Insulation 3 psf

Total 9 psf

Total Dead Load = 9(20) = 180 lb/ft

20ft

180(2.5)=450 lb

180(5)=900 lb

……

step 1 load analysis1
Step 1 – Load Analysis

PURLINS M8x6.5

Purlin Load = 6.5(20) = 130 lb

20ft

130 lb

130 lb

……

step 1 load analysis2
Step 1 – Load Analysis

SNOW

Snow Load = 20(20) = 400 lb/ft

20ft

400(2.5)=1000 lb

400(5)=2000 lb

……

step 1 load analysis3
Step 1 – Load Analysis

Dead Load of Truss

Assume 10% of all other loads

End Joint 0.1(9(20)(20)+130+1000)=158 lb

Interior Joint 0.1(900+130+2000)=303 lb

158 lb

303 lb

……

step 1 load analysis4
Step 1 – Load Analysis

450+130+158 = 738 lb

900+130+303 = 1333 lb

……

D

1000 lb

2000 lb

S

step 2 required force
Step 2 – Required Force

1.2(0.74) + 1.6(1) =

2.48 kips

1.2(1.33)+1.6(2)=

4.8 kips

……

step 2 required force1
Step 2 – Required Force

Method of Sections

step 4 t selection based on ag
Step 4: T Selection based on Ag

Choose MT5x3.75 A=1.10 in2

See Manual pp1-68

step 6 try next larger
Step 6 TRY NEXT LARGER

Choose MT6X5 A=1.46 in2

See Manual pp1-68

step 8 check slenderness
Step 8 – Check Slenderness

Assume bracing points at panel points

OK