In this article we will discuss about the types and design of tension members used in steel structures.
Types of Tension Members:
1. Minor Types of Tension Members:
A. Bars and Rods:
These are commonly attached to other members by bolting or welding. These are slender and are meant to resist tension only.
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These minor tension members are of the following types:
(i) Eye Bars:
These members are used where flexible end connections are desired. They are used as members of pin connected truss bridges. Eye bars are made by first upsetting each end of a bar of rectangular section to a nearly round shape and then boring holes of the desired sizes on the enlarged ends.
A pin is passed through the eye or the hole in the bar and also through corresponding holes in the other members meeting at the joint. The pin provides the means of transmission of load from the eye bar to the other members at the joint. Eye bars are also used at the end of wire rope hangers.
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(ii) Loop Bars:
Loop bars are made by bending each end of a square or round section back upon the bar itself and then welding it to form a loop. Stress transmission is similar to the eye bar.
(iii) Threaded Bars:
Threaded bars consist of round bars whose ends are threaded. Nuts are attached on the threaded ends after the bar has been placed in its proper position. The ends of the rod are first, upset and then threaded so that the sectional area at the root of the thread is not less than the sectional area of the bar.
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After upsetting usually the sectional area at the ends will be about 20 per cent greater than the sectional area of the bar. If a non-upset threaded bar is to be selected, the designer must select a bar in which the diameter at the root of the threads will be at least 1.5 mm greater than the normally required diameter.
Threaded rods have many applications in structural work.
Some of the common applications are the following:
(a) Purlins of trusses spaced at large distance are laterally supported by threaded rods in order to minimize the bending moment about their minor axis. Successive pairs of purlins are connected by threaded bars.
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(b) Threaded rods are used as hangers to support walkway slabs and balconies.
(c) Tie rods can be used to connect the lower ends of arches to minimize or prevent horizontal thrusts.
(d) Tie rods can be used as diagonal bracings.
(iv) Welded Bars:
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These are flat bars. These are flat bars carrying light tensile loads. These are welded to other members at their ends.
Structural Tubings are nowadays used in modern constructions. These are usually referred to as hollow structural sections. In situations where a smooth closed section is desired and when resistance to torsion is required, structural tubing has its good application. Further structural tubing will be an economical choice for compression members subjected to light to moderate loads. Square and rectangular tubing and seamless round tubing are the usual types in use.
Welded Connection between Tubes:
Let a secondary tension member consisting of a tube of external diameter d be welded to a main tension member consisting of a tube of external diameter D (Fig. 6.7).
If s = size of the weld, it can be determined for the condition
Design strength of the length of wed = Factored tension in the secondary tube.
2. Major Types of Tension Members:
Structural sections generally used for tension members are the following:
(i) Open sections like angles, channels tees.
(ii) Closed sections like circular, square and rectangular hollow sections.
(iii) Built-up sections like double angles, double channels. Built sections like four angle members are also used as tension members.
Sings, angle tension members are commonly used in roof trusses carrying light loads. They are also used as bracings for members of composite section. A single member transfers its load eccentrically to the gusset plate and is subjected to a bending moment. This factor should also be taken into account in the design.
Double angle tension members are often used, connected on either side of a gusset plate, at each end. If provided in this manner eccentric load transfer to the gusset plate is avoided and the member is practically free from bending stresses. These are most commonly used in roof trusses and foot bridge trusses.
Double channel tension members may also be used in a manner similar to double angle members. In view of considerably greater depth of web available two or even three rows of rivets can be provided. These members therefore require less length of gusset plate.
Besides the above, four angle members with or without a plate may be used as tension members in heavily loaded bridge trusses.
Design of Tension Members:
The design of a tension member is based on several factors like the factored tensile force, the type of member (Plates, rods, structural sections like single or double angles, single on double channels etc.).
The following procedure may be adopted:
(i) The gross-sectional area required from consideration of yielding of the gross-section is determined from the equation-
(T – Factored tension, ƒy = yield stress. γmo = Partial safety factor for failure by yielding = 1.1)
(ii) A suitable section is provided having at least the gross area.
(iii) The net area An of the section is determined making deductions for bolt holes,
(iv) The design rupture strength of the net section is determined from the equation-
α may be assumed as 0.7 or 0.8, ƒu = ultimate stress, γml = Partial safety factor against failure against rupture = 1.25. Tdn from block shear consideration may also be determined. For the design to be safe Tdn should be greater than the factored tension T. If found necessary the section can be suitably increased and the design strength is recalculated.
Effective Net Area:
The strength of a tension member is liable to be affected due to its connection at its ends. There are various factors affecting the strength of the net section of the member.
The tension capacity of the member is determined corresponding to the net effective area Anc given by-
Anc = K1 K2 K3 K4 . An
where, An = net area of the section determined by making deduction for bolt holes
K1 = Ductility factor
K2 = Hole forming factor
K3 = Geometry factor
and K4 = Shear leg factor
As per studies made by, Gaylord and Kulak, the ductility factor K1 and the geometric factor K3 are found to be practically unity. The hole forming factor K2 depends on the manner the bolt hole is made. For drilled holes K2 = 1 and for punched holes K2 = 0.85.
For all practical purposes we may take Anc = K4 An
As per studies made by Munse and Chesson the net section tension capacity of an angle connected by a leg to the gusset plate is given by-
where, x̅ = Distance of centroidal axis from the face of the gusset plate,
and l = length of the connection.
The IS code adopts the factor α in place of the coefficient K4. Accordingly Anc = α An
α = 0.6 when the number of bolts ≤ 2
α = 0.7 when the number of bolts = 3
α = 0.8 when the number of bolts ≥ 4
For welded connection, we may take α = 0.8
To accommodate the necessary number of bolts (or to accommodate a certain length of weld) for the connection of a member to a gusset plate a certain length of the member is utilized. When the load in the member is large many bolts may be required (or a large length of weld may be required) and the length of the member utilized for the connection itself may be large and may involve a considerable length of gusset plate. By using lug angles the length of the connection can be decreased.
Fig. 6.55 shows a lug angle used for the connection of an angle member to a gusset plate. Similarly for the connection of a channel member two lug angles may be used as shown in Fig. 6.56.
In the case of angle members, the lug angles and their connections to the gusset or other supporting member shall be capable of developing a strength not less than 20 per cent in excess of force in the outstanding leg of the angle and the attachment of the lug angle to the angle member shall be capable of developing 40 per cent in excess of that force.
Lug angles connecting a channel shaped member shall as far as possible be placed symmetrically with respect to the section of the member. The lug angles and their connection to the gusset or other supporting member shall be capable of developing a strength of not less than 10 per cent in excess of the force not accounted for by the direct connection of the member and the attachment of the lug angles to the member shall be capable of developing 20 per cent in excess of that force.
In no case shall less than two bolts be used for attaching the lug angle to the gusset or other supporting member.