A Beginner's Guide to the Steel Construction Manual, 13^th ed. (old)

Chapter 3 - Tension Members

© 2006, 2007, 2008 T. Bartlett Quimby

Section 3.10

Example Problem 3.3

Last Revised: 06/16/2011

For this problem, we want to determine the tensile capacity of the WT sections in the connection shown in Figure 3.10.2.1 for example problem 3.2. We want to be able to compare the results from both LRFD and ASD, so the capacity will need to be in comparable equivalent service load terms for both methods.

Given:  The tension splice connection shown in Figure 3.10.2.1 connects two WT sections with two splice plates attached to the web.  Assume that the system supports a tension load that is 40% dead load and 60% live load.

Wanted:  The capacity of the splice plates in terms of equivalent service load, P_s,equiv, using both LRFD and ASD.

Solution:  The chapter spreadsheet has a solution to the problem.  You will need to review that solution as this discussion progresses.

In this case, each of the WTs carries the full tension force and they are mirrored images of one another.  Consequently we only need to find the capacity of one to find their limit on the connection.  The provided spreadsheet solution gives all the relevant computations.  Table 3.10.3.1 summarizes the results.

Observations:

• WTs are W shapes that have been cut longitudinally down the middle of the web, so when looking for material properties in SCM Table 2-3 you need to look under the W shape column.
• The section properties for the WT are obtained from SCM pg 1-54.  Since the bolts are only on the web, the only section properties that are needed are A_g and t_w.  
• Since we don't know how long the members are, we used the slenderness limit state to compute a maximum length for the members.
• Figures 3.10.3.1 and 3.10.3.2 show the only valid unique failure paths for tensile rupture and block shear, respectively.  You should be able to explain why these are the only valid failure paths.  Review the failure path tutorial if you have difficulties.
• When computing A_e, U is no longer 1.0 since the flange is not transferring force directly to connectors.  In this case, you have two options for determining U.  Our situation falls under either case 2 or case 7.  We chose to use case 7 to simplify the problem.  If you use case 2 you will need to take the time to compute the centroidal distance x.  We can avoid this since we have at least four bolts in a line in the direction of force.
• The capacity of the WTs is significantly less than the capacity of the plates that connect them.  From a designer's stand point, you would consider downsizing the splice plates if the WTs have sufficient capacity for the application.
• Note the controlling limit state is block shear and it is significantly less than the other limit states.  Some consideration should be made to increasing the spacing the bolts (i.e. increasing the tension and shear areas) in order to use more of the capacity of the members.

Table 3.10.3.1
Example Problem 3.3 Results

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