Design of Steel Tension Members: A Comprehensive Guide

1. ENCE 455 Design of Steel Structures II. Tension Members C. C. Fu, Ph.D., P.E. Civil and Environmental Engineering Department University of Maryland

2. 2 Tension Members Following subjects are covered: Introduction Design strength Net area Staggered fasteners Block shear Design of tension members Threaded rods, pin-connected members Reading: Chapters 3 of Salmon & Johnson AISC Steel Manual Specifications (Part 16) Chapters B (Design Requirements), D (Tension Members), and J (Connections)

3. 3 Introduction Tension members are structural elements that are subjected to axial tensile forces. Examples include: Members in trusses Cables in cable-stayed and suspension bridges Bracing in frames to resist lateral forces from blast, wind, and earthquake Forth Bridge Queensferry, Scotland Main sections: 5360 ft. Maximum span: 1710(2), 4 spans total Built: 1890

4. 4 Introduction (cont.) Stresses ( f ) in axially loaded members are calculated using the equation where P is the load and A is the cross- sectional area normal to the load. Design of this component involves calculations for Tension member (gross area) Tension member at connection (net area) Gusset plate at connection (net area) Gusset plate at support

5. 5 Design Strength A tension member can fail by Excessive deformation (yielding) - Excessive deformation is prevented by limiting stresses on the gross section to less than the yield stress. For yielding on the gross section, the nominal strength is: T n = F y A g and t =0.90 (3.2.1) Fracture - Fracture is avoided by limiting stresses on the net section to less than the ultimate tensile strength. For fracture on the net section, the nominal strength is: T n = F u A e = F u (UA n ) and t =0.75 (3.2.2) where A e is the effective net area, A n is the net area and U is the reduction coefficient (an efficient factor)

6. 6 Net Area Net Area - The performance of a tension member is often governed by the response of its connections. The AISC Steel Manual introduces a measure of connection performance known as joint efficiency, which is a function of Material properties (ductility) Fastener spacing Stress concentrations Shear lag (Most important of the four and addressed specifically by the AISC Steel Manual)

7. 7 Net Area (cont.) The AISC Steel Manual introduces the concept of effective net area to account for shear lag effects. For bolted connections: A e = UA n (3.5.1) For welded connections: A e = UA g (3.5.3) where (3.5.2) and is the distance from the plane of the connection to the centroid of the connected member and L is the length of the connection in the direction of the load. (Salmon & Johnson Example 3.5.1 for U)

8. 8 Net Area (cont.)/ Salmon & Johnson AISC Steel Manual

9. 9 Net Area (cont.)/L

10. 10 Net Area (cont.)/U For bolted connections , AISC Table D3.1 gives values for U that can be used in lieu of detailed calculation.

11. 11 Net Area (cont.)/U For welded connections , AISC Table D3.1 lists

12. 12 Staggered Fasteners Failure line - When a member has staggered bolt holes, a different approach to finding A e for the fracture limit state is taken. This is because the effective net area is different as the line of fracture changes due to the stagger in the holes. The test for the yielding limit state remains unchanged (the gross area is still the same). For calculation of the effective net area, the Section B2 of the AISC Steel Manual makes use of the product of the plate thickness and the net width. The net width is calculated as

13. 13 Staggered Fasteners (cont.)

14. 14 Staggered Fasteners (cont.) All possible failure patterns should be considered. (Example 3.4.2 for A n )

15. 15 Staggered Fasteners (cont.) Figure 3.8.2 Load distribution in plate A (Example 3.8.1)

16. 16 Block Shear Block shear is an important consideration in the design of steel connections. Consider the figure below that shows the connection of a single-angle tension member. The block is shown shaded .

17. 17 Block Shear (cont.) In this example, the block will fail in shear along ab and tension on bc . The AISC Steel Manual procedure is based on one of the two failure surfaces yielding and the other fracturing. Fracture on the shear surface is accompanied by yielding on the tension surface Fracture on the tension surface is accompanied by yielding on the shear surface Both surfaces contribute to the total resistance.

18. 18 Block Shear (cont.) The nominal strength in tension is F u A nt for fracture and F y A gt for yielding where the second subscript t denotes area on the tension surface ( bc in the figure above). The yield and ultimate stresses in shear are taken as 60% of the values in tension. The AISC Steel Manual considers two failure modes: Shear yield - tension fracture - T n = 0.6 F y A gv + F u A nt (3.6.1 ) Shear fracture - tension yield - T n = 0.6 F u A nv + F u A nt (3.6.2 ) One equation to cover all T n = 0.6 F u A nv + U bs F u A nt 0.6 F y A gv + U bs F u A nt (AISC J4-5) Because the limit state is fracture, the equation with the larger of the two fracture values controls where t =0.75 . (Example 3.9.2 for block shear)

19. 19 Design of Tension Members The design of a tension member involves selecting a member from the AISC Steel Manual with adequate Gross area Net area Slenderness (L/r 300 to prevent vibration, etc; does not apply to cables.) If the member has a bolted connection, the choice of cross section must account for the area lost to the bolt holes. Because the section size is not known in advance, the default values of U are generally used for preliminary design.

20. 20 Design of Tension Members (cont.) Detailing of connections is a critical part of structural steel design. Connections to angles are generally problematic if there are two lines of bolts. Consider the Gages for Angle figure shown earlier that provides some guidance on sizing angles and bolts. Gage distance g 1 applies when there is one line of bolts Gage distances g 2 and g 3 apply when there are two lines

21. 21 Design of Tension Members (cont.)/ Thread Rod Threaded Rod

22. 22 Design of Tension Members (cont.)/ Thread Rod Threaded Rod - Tension on the effective net area T n = A s F u = 0.75A b F u , where A s is the stress area (threaded portion), A b is the nominal (unthreaded area), and 0.75 is a lower bound (conservative) factor relating A s and A b . See Section J3.6 of the AISC Steel Manual Specification for details. The design strength of a threaded rod is calculated as T n = 0.75 T n (Example 3.10.2 for Rod Design)

23. 23 Design of Tension Members (cont.)/ Pinned Connections Pinned connections transmit no moment (ideally) and often utilize components machined to tight tolerances (plus, minus 0.001). The figure shows failure modes for pin-connected members and each failure mode must be checked for design. Specifically, the following limit states must be checked.

24. 24 Design of Tension Members (cont.)/ Pinned Connections The following limit states must be checked. Tension on the effective net area T n = 0.75(2 t b eff F u ) where b eff = 2t + 0.63 b (D5-1) Shear on the effective area T n = 0.75(0.6A sf F u ) = 0.75{0.6[2t (a+ d/2)] F u } (D5-2) Bearing on projected area T n = 0.75(1.8 A pb F y ) = 0.75[1.8 (d t ) F y ] (J8-1) where 1.8 A pb F y is based on a deformation limit state under service loads producing stresses of 90% of yield Tension on the gross section T n = 0.9(A g F y ) (D1-1)