Design of Steel Tension Members


Tension Member Connections Three methods of connecting steel members: • Rivets • Bolts • Welds

Advantages of Welding • Materials saving (often no connecting members necessary) • Quieter than impact wrenches normally used for bolts • More forgiving when dimensions don’t match • Forms rigid joints • Simpler detailing

Advantages of Bolts • Does not require as highly skilled workers • Requires less equipment • Reduced risk of fire • Better quality control

Types of Connections by Bolt Action

Bolts Available Bolts • A307 – Non-high strength bolts • A325 - High strength bolts • A490 Strengths given in Table J3.2 (p. 16.1-61)

Bolt Holes (Sizes in Table J3.3) p. 16.1-62 • Standard • Oversized • Short-slotted • Long-slotted Connection types • Bearing • Slip-critical

Spacing, Edge, and End Distances Minimums • Center to center ≥ 2 2/3 x bolt diameter, preferably 3 x bolt diameter • Edge distance – Table J3.4

For maximum bearing Strength (pitch): (J3.10) • Center to center ≥ 3 x bolt diameter • Edge (end) distance – p. 7-35, Table 7-13

Spacing, Edge and End Distances Maximums • Center to Center (pitch) ≤ 24 x thickness of thinner part ≤ 12” • Edge distance ≤ 12 x thickness of part ≤ 6”

Bolt Shear Strength • J3.6→ФF n A b – Ф – Table J3.2

– F n – A b – Table J3.2

– Area of bolt • Tabulated on page 7-33, Table 7-10

Bearing Strength at Bolt Holes • J3.10 → ФR n – Ф – 0.75

– R n given by equations J3-2 • Tabulated on page 7-34,35

Analysis of Tension Connection Example (p. 15 notes) Two A36 steel plate tension members have been connected with a lap splice using ¾” diameter A307 bolts, arranged as shown.

Find the allowable service live load. Also find required distances




. Assume service live load is two times dead load.

Plate edges are likely to be sheared.

P P Analysis of Tension Connection Example (p. 15 notes) 3/8 in.

P 3/8 in.

a 3 in.

a b b 10 in.


Boted Tension Connection Design Procedure p. 15 -16 notes

Bolt Design Example p. 16 notes Design a pair of splice plates to connect the two parts of the channel tension member shown.

The forces in the member are 110 kips live load and 69.7 kips dead load. The bolts in the surrounding joints are 1 ” diameter A325N. All steel is A36. Slip is not critical.

Bolt Design Example p. 16 notes

Block Shear Rupture J4.3 – Equations J4-3a and J4-3b

Welded Tension Connections Two primary types of welds:

LRFD Specification for Welds p. 16.1-52 Effective throat thickness = 0.707 x l Where l = weld size or leg size

Limitations on Fillet Welds • Minimum weld size – Table J2.4

• Maximum weld size: – along edge of material < ¼” thick = thickness of material – along edge of material > ¼” thick = material thickness 1/16” • Minimum length – Greater than 4 x weld size and, – If longitudinal filet welds are used alone in end connections of flat bar tension members length ≥ perpendicular distance between them (p. 16.1-234)

Limitations on Fillet Welds • Intermittent fillet weld segments must be at least 4 times the weld size and ≥ 1 ½” • Lap joints require a lap of at least 5 times the thickness of the thinner part joined, but not less than 1 in. (p. 16.1-235) • Returns are required when practicable at the ends of fillet welds. The must be at least 2 times the weld size and usually not more than 4 times the weld size.

Design Strength of Welds Table J2.5 p. 16.1-57 Fillet welds (shear on effective area): 

F w A w

 ( 0 .

75 )( 0 .




A w

) Where: A w = Area of weld = throat thickness x length F EXX = Weld metal designation (F E70 = 70 ksi) Welded connection design procedure p. 22 notes

Weld Design Example p. 22 notes An L 6 x 4x ½ is stressed in tension by a 50 kip live load and a 40 kip dead load. At the truss panel point the angle member is attached to a 5/8 ” thick gusset plate. The 4 ” leg of the angle is outstanding. Check the angle for tension strength and design the welded connection of the angle to the plate.

Assume all material is A36 steel and that the loading will be static.

Weld Design Example p. 22 notes