Brief data for post-1- tension post

1- A step-by-step introduction to tension members.

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Introduction to Tension members.

Introduction to tension members- tension yielding.

We will begin a new subject, Introduction to Tension Members or Structural Steel Tension Members. The first item covers where to use the tension members.

The second item for Introduction to Tension Members is the AISC chapter that controls tension member design. The third item is the net and gross areas. The net area is the section’s area less the fastener’s area. Later, we shall examine fasteners like bolts and rivets and various considerations or allowances.

For example, imagine one angle with fasteners in one leg; the net area valuation of such an angle will be calculated differently.
There are net and special tables for calculating the final value of the effective net area.
The fourth item covers tensile yielding, tensile rupture, and gauge line information.
The fifth component includes the design parameters for tension members in the LRFD and ASD, φt and Ωt.

The sixth items are the tables for various shapes and the relevant yield and F ultimate stresses for tension members.

Introduction to tension members.

The tension members are used for bridges and roof trusses, towers, and bracings. Any situation, where they are used as tie rods.

The selection of a section to be used as a tension member is one of the simplest problems encountered in design, for instance as compared with columns and beams, so, the design of the tension members is considered to be simple.

As there is no danger of the member buckling, the designer needs to determine only the load to be supported,  like whether the ultimate load is 1.4 D or 1.2 D+ 1.6 L, then choose the biggest values, as for the LRFD design, or D+L as for ASD design.

Where to use tension members?
Where to use tension members?

As an introduction to tension members, the next slide includes the chapter used for the design of tension members, then which section in the chapter for tension members is shown. Chapter D is used. D-2 governs the Tensile strength. AISC-360-16 page 16-1-28.

AISC provision of tension members-chapter D

There are two items, Tensile Yielding and Tensile rupture. For the tensile yielding, the P nominal is estimated as =Fy*Ag, Fy is the yield stress, and Ag is the gross area. For the LRFD, φt =0.90, and for the ASD design Ωt=1.67.

AISC provision of tension members-chapter D2-tensile strength formulas.

Item b) is for tensile rupture at the places of bolts or welds. The nominal load Pn = Fu*Ae, where Ae is the effective area. However, the LRFD parameter is different and is φt =0.75, less than the value given for tensile yielding. and for the ASD design, based on the relation of φ*Ω=1.5, then  Ωt=2.00. Fy is yield strength and Fult is the ultimate strength. D-3 includes the effective net area and is to be estimated from B 4-3, Aeff=Anet*U factor, U, shear lag factor=1.


AISC provision of tension members-chapter D3- effective net area.

In the case of plates, it has various values for other shapes and depends on the way you are using for connecting the fasteners.
For instance, if you have bolts in the upper flange and the web has no fasteners, then the whole section is not fully stressed, since a modification will be used, to be discussed later on.

This is the provision from the AISC code for the net area for tension members and the different terms used.

What is the net area?
What is the net area?

The shear lag factor  U value is obtained from Table D3-1.

Tension yielding.

Tension yield picture Quoted from The external link. A BEGINNER’S GUIDE TO THE STEEL CONSTRUCTION MANUAL, 14TH ED.

As an introduction to tension members, we would want to examine the various modes of failure.

There are many good illustrations, such as a section for the W section; if we deal with tension yielding, there is a failure mode. On the positions distant from the bolts. When tension forces are applied from the ends, the failure will occur at a position other than the bolts’ path.

This failure mechanism focuses on yielding in the gross cross-sectional area.
T=Ag*Fy calculates the tension force at failure. an is the original shape, and b is the shape after the section’s tension yielding has occurred.

What is tension yielding?
What is tension yielding?

Tensile Rupture types.

This is The first failure was a vertical line passing by the line of the bolts in the direction that is perpendicular to the force direction.

Tension rupture  as a vertical line.
Tension rupture as a vertical line.

The second failure is shown here as a zigzag line, from which the net area was estimated.

Tension rupture as a zigzag line.
Tension rupture as a vertical line.

The damage occurs due to the Punching of a hole.

As can be shown punching for bolts, can cause damage around the bolt and create extra diameter that should be taken into consideration while estimating the net area.

Damage due to punching a hole and added diameter.

This is the PDF used for the illustration of both posts 1 and 1A from this link.

The next post contains an Easy introduction to Tension members-part-2.
For a useful external Chapter 3 – Tension Members– Bartlett Quimby

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