A Beginner's Guide to the Steel Construction Manual, 15th ed. Chapter 3 - Tension Members © 2006, 2007, 2008, 2011, 2017 T. Bartlett Quimby |
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Section 3.10 Example Problem 3.1 Last Revised: 04/19/2021 The example problems presented in this section also have a spreadsheet solution. You will need this file to follow along with the presented solutions. You can click on the following link to get the file: Chapter 3: Excel Spreadsheet Solutions Given: A 3/4" x 10" plate that is 5 ft long and has standard holes for 3/4" bolts at each end for attachment to other structural members. Figure 3.10.1.1 shows a face view of the plate. The service level loads that the member will be subject to are 140 kips of dead load and 30 kips of live load.
Wanted: Determine the axial tension capacity of the member. Solution: The problem solution is pursued in the following steps:
Determining the Demand (Required Strength) on the Member A first step is to determine the demand on the member. Pu and Pa represent the design axial tension loads for the LRFD and ASD design philosophies, respectively. These values will be used for all the limit state calculations for this problem. Recalling that there are multiple load combinations listed in ASCE 7 that must be satisfied, you can compute all of them and find out which are the controlling ones. It will soon become obvious that, if the only loads seen by a member are dead and live loads, that LRFD-LC2 and ASD-LC2 will be the controlling load cases. You should take some time to verify this. These two equations may NOT control your design if you have other types of loads on the element being designed. Let us consider LRFD first. The controlling ASCE 7 load case for computing Pu is LRFD-LC2. Pu = 1.2D + 1.6L = 1.2(140 k) + 1.6(30 k) = 216 kips Let's check out ASD. The controlling ASCE 7 load case for computing Pa is ASD-LC2. Pa = D + L = 140 k + 30 k = 170 kips Our member is 5 feet long and the least value of r is computed as:
The correct computation of L/r is (5 ft)(12 in/ft) / (0.217 in). The result is the unitless number 277. If you forgot the unit conversion then you would have gotten 23.1 ft/in. This is not the right answer! The correct expression of the limit state in your calculations is: 277 < 300 ... The limit state is satisfied You will also note that we have written the limit state in an alternate form in the spreadsheet solution. We will be doing this throughout the text as the second form gives the percentage of capacity used. This is useful to know when considering alternate selections for a given member. In the example already stated, the numerical value of the limit state becomes: 0.924 < 1.00 ... the limit state is satisfied This form is useful because you know on observation that you are nearing capacity and will need to make the plate thicker it gets much longer. Another format issue: In your calculations, you should always explicitly state whether or not the limit state is satisfied so that someone reviewing your calculations doesn't have to guess. Our next task is to determine if the member satisfies the limit state of tensile yielding. Consider both LRFD and ASD requirements. Let's tackle the nominal strength, Pn, first. We need to find Fy and Ag. Fy is found to be 36 ksi in Table 2-5 (SCM page 2-50) using the preferred material specification for this bar. In Figure 3.10.1.2 you will notice a section line through the member that is labeled 1-1, tensile yielding failure path. The exact location of this path is not important. It is located anywhere between the end connections. Figure 3.10.1.3 shows a "failed" view with forces and stresses illustrated with arrows and the cross section. This location is where we determine the gross cross sectional area, Ag for the member. Using the requirements of this section and the given plate dimensions, Ag is computed to be 7.50 in2.
Using the values determined for Fy and Ag we can now compute the nominal tensile strength of the member, Pn: Pn = (36 ksi)(7.500 in2) = 270 kips Computing the Limit State: Now that we have Pu, Ps, Pn, ft, and Wt we can compute the limit states.
In this case, there is sufficient tensile yielding capacity to support the anticipated loads for LRFD but not for ASD. Observations:
The next limit state that we will determine is tensile rupture. This occurs at the end connections of the member. For our example problem, we can reduce our computational effort by recognizing that each end has the same rupture capacities since the arrangements of the holes (which reduce the effective section) are mirrored on each end. Consequently, we only need to consider one end. Figure 3.10.1.4 shows the only two valid failure paths for the given end condition. Figure 3.10.1.5 shows the "fractured" connection and cross sectional views. Note that no other failure path is valid since no other conceivable path will see the full force of the member. Another way to look at this is that these are the only paths for which all the bolts will remain with only one of the fractured pieces. As this concept is often difficult to grasp, take some time to work the failure path tutorial if you haven't done so already.
In the general case, it is possible to have different hole patterns on each end, necessitating the computation of the tensile rupture limit state on each end. First let's compute the net area An for each of the two failure paths identified in Figure 3.10.1.4.
The controlling net area is An2 as it has the smaller value. This means that, if tensile rupture were to actually occur, this is the path that it would take. Therefore, for this problem An = 6.19 in2. In this problem we have only one cross sectional element (i.e. one plate element in the cross section) and it is attached to the bolts, so our problem falls under the requirements of case 1 in SCM Table D3.1 leading us to U = 1.0. This means that there is no shear lag for this problem. (Note that example 3.2 illustrates the concept of effective net area better than this problem does.) Consequently, for this problem SCM equation D3-1 becomes: Ae = UAn = (1)(6.19 in2) = 6.19 in2 Fu is found to be 58 ksi in SCM Table 2-5 (SCM page 2-50) using the preferred material specification for this bar. Using the values determined for Fu and Ae we can now compute the nominal tensile strength of the member, Pn: Pn = (58 ksi)(6.19 in2) = 359 kips Computing the Limit State: Now that we have Pu, Ps, Pn, ft, and Wt we can compute the limit states.
In this case, there is sufficient tensile yielding capacity to support the anticipated loads for both LRFD and ASD. Observations:
Block Shear Rupture Limit State: Computing the Nominal Block Shear Rupture Strength, Pn: There are actually three potential block shear failure paths for our problem. Two are shown in Figure 3.10.1.6. The third potential failure path is similar to block shear failure path #2 except that it tears out the "bottom" instead of the "top" of the member. As the areas associated with these two paths are identical their capacities will be identical, so only one of the two paths need be considered.
Block shear rupture on failure path #1: In this case, the failure path runs from the end of the member from center of bolt hole to center of bolt hole around the perimeter of the bolt hole group. There are two shear area and one tension area. It is likely that the tension stress distribution is uniform so Ubs is taken as 1.0. Let's start by computing the quantities needed for equation J4-5:
Using these quantities along with the previously given quantities, equation J4-5 becomes:
This is one of two possible shear rupture values. We now need to look at failure path #2. Block shear rupture on failure path #2: In this case, the failure path runs from the end of the member from center of bolt hole to center of bolt hole and out through one side of the element. There is one shear area and one tension area, causing a stress imbalance on the tension surface. It is likely that the tension stress distribution is not uniform so Ubs is taken as 0.50. Let's compute the quantities needed for equation J4-5:
Using these quantities along with the previously given quantities, equation J4-5 becomes:
Comparing the two failure paths, the second path is the controlling path because it yields the lesser value. Since failure path #2 yields the lower value for Rn, this is the controlling failure path for this limit state. Computing the Limit State: Now that we have Rn, ft, and Wt we can compute the limit states.
In this case, there is sufficient block shear rupture capacity to support the anticipated loads for LRFD but not for ASD. Observations:
Computing the Nominal Bolt Bearing Strength, Rn: In this problem, we will assume that deformation at the bolt holes is not a design consideration. Since standard holes have been specified, this leads us to using Equations J3-6b and J3-6d. The value for Lc is:
This leads to a tear out strength of:
The bearing deformation capacity is:
From this we see that the tear out capacity controls the limit state. The capacity of the member, then, is the capacity/bolt times the number of bolts in the connection (11 bolts in this case). This results in:
Computing the Limit State: Now that we have Rn, ft, and Wt we can compute the limit states.
In this case, there is sufficient bolt bearing capacity to support the anticipated loads for both LRFD and ASD. Indeed, there is great reserved capacity! Summary The following table is a summary of the considered limit states for this problem. Example Problem 3.1 Resulting Limit State Ratios
The ratios in the table represent the applied force divided by the capacity of the member. Values less than or equal to 1.00 indicate that there is sufficient capacity for the indicated limit state. Values over 1.00 indicate that there is not enough capacity to support the predicted loads. For this problem, the member works for the given loads when using LRFD. It does not work for ASD. The critical strength-based limit state (slenderness is not a strength-based limit state) is block shear for both LRFD and ASD. Reviewing the calculations, the capacities associated with the limit state of block shear are:
To express the results in terms of comparable equivalent service loads, we must "unfactor" each result. Let's assume that the applied load, Ps,eq, consists of half dead load and half live load. The controlling ASD load case is ASD-LC2: Pa = D + L = (0.5Ps,eq) + (0.5Ps,eq) = 1.0 Ps,eq < 158 k ASD Ps,eq < 158 k The controlling LRFD load case is LRFD-LC2: Pu = 1.2D + 1.6L = 1.2(.5Ps,eq) + 1.6(.5Ps,eq) = 1.4 Ps,eq < 237 k LRFD Ps,eq < 237 k / 1.4 = 169 k In conclusion, using LRFD in this case will result in allow a greater actual load (169 k vs. 158 k) to be applied to the member than will ASD. You should be able to answer the question: Why is this so? Finally, in the spread sheet solution, there is a comparison of capacity for different percentages of dead load in the total load. This is not a requested part of the problem. It is placed here for you to get a feel for the variable nature of the LRFD factor of safety. Notice that LRFD has a much greater capacity as the load becomes more predictable (i.e. the load becomes predominately dead load). |