309 Application

What follows is a step-by-step design procedure for simple tubular trusses, applying the charts presented in the foregoing section:

Step 1. Lay out the truss and calculate member forces using statically determinate pin-end assumptions. Flexibility of the connections results in secondary bending moments being lower than those given by typical rigid-joint computer frame analyses. Step 2. Select members to carry these axial loads, using the appropriate governing design code, for example, AISC. While doing this, consider the architecture of the connections along the following guidelines:

A. Keep compact members, with especially low D/T for the main member (chord).

B. Keep branch/main thickness ratio (tau) less than unity, preferably about 0.5.

C. Select branch members to aim for large beta (branch/main diameter ratio), subject to avoidance of large eccentricity moments.

D. In K connections, use a minimum gap of 2 in. between the braces for welding access. For small tubes, this may be reduced to 20% of the branch member diameter. Connection eccentricities up to 25% of the chord diameter maybe used to accomplish this. Reconsider truss layout if this gets awkward.

Step 3. Calculate and distribute eccentricity moments and moments due to loads applied in-between panel points. These are not secondary moments and must be provided for. They may be allocated entirely to the chord, for connection eccentricities less than 25% of the chord diameter, but should be distributed to both chord and braces for larger eccentricities, portal frames, or Vierendeel-type trusses. Recheck members for these moments and resize as necessary.

Step 4. For each branch member, calculate Ay, utilization against member-end yield at the joint. For ASD:

or where fa is the nominal axial stress and fb is the bending in the branch. Where used, the 1 increase is applicable to the denominator.

Step 5. Also calculate chord utilization, using the formula in Figure 30.10 with chord nominal stresses and specified minimum yield strength. Use the appropriate chart in the figure to determine the derating factor Qf. At heavily sheared gap K connections and at eccentric bearing shoes, it may (rarely) also be necessary to check beam shear in the main member and its interaction with other chord stresses, for example, using AISC criteria. For circular sections, the effective area for beam shear is half the gross area.

Step 6. For each end of each branch member, calculate the joint efficiency, Ej, using Equation 30.4 and the appropriate charts for punching shear efficiency, Ev. Joint efficiencies less than 0.5 are sometimes considered poor practice, rendering the structure vulnerable to incidental loads that the members could resist but not the weaker joints.

Step 7. For axial loading alone, or bending alone, the connection is satisfactory if member-end utilization is less than joint efficiency, that is, Ay/Ej < 1.0. For the general case, with

FIGURE 30.10 Derating factor Qq for (a) axial loads in branch, (b) in-plane bending, and (c) out-of-plane bending.

combinations of axial load and bending, the connection must satisfy the following interaction formula:

Step 8. To redesign an unsatisfactory connection, go back to Step 2 and do one of the following:

A. Increase the chord thickness.

B. Increase the branch diameter.

C. Both of the above.

Consider overlapped connections (AWS Section 2.24.1.6) or stiffened connections only as a last resort. Overlapped connections increase the complexity of fabrication but can result in substantial reductions in the required chord wall thickness. Step 9. When the designer thinks he is done, he should talk to potential fabricators and erectors. Their feedback could be valuable for avoiding unnecessary difficult and expensive construction headaches. Also, make sure they are familiar with and prepared to follow AWS Code requirements for special welder qualifications and that they are capable of coping the brace ends with sufficient accuracy to apply AWS prequalified procedures. Considerable savings can be realized by specifying partial joint penetration welds for tubular T, Y, and K connections with no root access, where these are appropriate to service requirements. Fabrication and inspection practices for welded tubular connections have been addressed by Post [11].

A more satisfactory version of the ''Truss Note'' mentioned earlier might read as follows:''The designer has sized the main member (chord) at truss connections to meet the provisions of AWS D1.1-2002 Section 2.24. All welded truss connections (intersecting pipe-to-pipe) with either member less than 0.375 in. thick shall be provided with fillet welds meeting the requirements of AWS D1.1-2002 Figure 3.2. All welded truss connections with both members thicker than 0.375 in. thick shall be provided with partial joint penetration welds meeting the requirements of AWS D1.1-2002 Figure 3.5. Weld strength calculations are generally not required for these prequalified joints, but (if provided) shall be in accordance with AWS D1.1-2002 Sections 2.23 and 2.24.1.3. All bevels and their fit-up shall be inspected before welding commences. Completed welds shall be inspected to verify the full leg lengths as tabulated in AWS. Magnetic particle inspection may be applied at the toes of completed welds, where specified elsewhere in contract documents. Ultrasonic testing is not applicable to welds that are designated to be less than full penetration or to tube thickness less than 0.50 in.''

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