Truss Member

The Trusses mode described above provides the most common but not complete set of truss structures. To analyze members of an arbitrary truss, the application contains a Truss Member mode which enables to analyze the load-bearing capacity or to find the section for a truss member (it is assumed that the static problem has been solved previously and the forces in the member are known). The following checks are performed in this mode:

Check	SNiP II-23-81*	SNiP RK5.04-23-2002	SP 53-102-2004	SP 16.13330	DBN B.2.6-163:2010	DBN B.2.6-198:2014	ShNK 2.03.05-13
Strength	Sec. 5.1	Sec. 5.1	Sec. 8.1.1	Sec. 7.1.1	Sec. 1.4.1.1	Sec. 8.1.1	Sec. 7.1
Stability in the truss plane	Sec. 5.3	Sec. 5.3	Sec. 8.1.3	Sec. 7.1.3	Sec. 1.4.1.3	Sec. 8.1.3	Sec. 7.3
Stability in the truss plane (post-buckling behavior)	Sec. 5.3, 7.20*	Sec. 5.3, 7.30	Sec. 8.1.3, 8.3.5	Sec. 7.1.3, 7.3.6	Sec. 1.4.1.3, 1.4.3.5	Sec. 8.1.3, 8.3.5	Sec. 7.3, 9.20
Stability out of the truss plane	Sec. 5.3	Sec. 5.3	Sec. 8.1.3	Sec. 7.1.3	Sec. 1.4.1.3	Sec. 8.1.3	Sec. 7.3
Stability out of the truss plane (post-buckling behavior)	Sec. 5.3, 7.20*	Sec. 5.3, 7.30	Sec. 8.1.3, 8.3.5	Sec. 7.1.3, 7.3.6	Sec. 1.4.1.3, 1.4.3.5	Sec. 8.1.3, 8.3.5	Sec. 7.3, 9.20
Web slenderness based on local stability constraint	Sec. 7.1, 7.2, 7.14, Table 27	Sec. 7.1, 7.2, 7.23	Sec. 8.3.1, 8.3.2, Table 8, Sec. 8.3.10	Sec. 7.3.1, 7.3.2, Table 9, Sec. 7.3.11	Sec. 1.4.3.1, 1.4.3.2, Table 1.4.3	Sec. 8.3.1, 8.3.2, Table 8.3	Sec. 9.1, 9.2, 9.3, Sec. 9.15
Flange overhang (flange plate) slenderness based on local stability constraint	Sec. 7.22, 7.23, Table 29, Sec. 7.26, 7.27*	Sec. 7.32, 7.33, 7.37	Sec. 8.3.7, Table 9, Sec. 8.3.10	Sec. 7.3.8, Table 10, Sec. 7.3.11	Sec. 1.4.3.7, Table 1.4.4	Sec. 8.3.7, Table 8.4	Sec. 9.22, 9.23, 9.27
Pipe radius to thickness ratio based on local stability constraint	Sec. 8.6	Sec. 8.6	Sec. 12.2.2	Sec. 11.2.2	Sec. 1.10.2.2	Sec. 14.2.2	Sec. 10.6
Local stability of the pipe wall based on closed circular cylindric shell calculation	Sec. 8.5-8.13	Sec. 8.5-8.13	Sec. 12.2.1-12.2.8	Sec. 11.2.1-11.2.9	Sec. 1.10.2.1-1.10.2.9	Sec. 14.2.1-14.2.9	Sec. 10.5-10.13
Stability of the curved member
Slenderness	Sec. 6.1-6.4,6.16	Sec. 6.1-6.4, 6.15	Sec. 11.1.1-11.1.4, 11.4.1	Sec. 10.1.1-10.1.4, 10.4.1	Sec. 1.9.1.1-1.9.1.4, 1.9.4.1	Sec. 13.1.1-13.1.4, 13.4.1	Sec. 8.1-8.4, 8.18

Limitations

The following values of the service factor γ_c are used for finding and checking the truss members:

0,95 for chords, support diagonals, lattice members in tension, lattice members in compression with cross-shaped sections;
0,8 for lattice members in compression of tee section, the slenderness being greater than 60.

Truss members are calculated for longitudinal forces by default. A special checkbox enables to take into account the bending moments and shear forces as well. In this case the calculation is similar to that performed in the Resistance of Sections mode, but the values of effective length and limit slenderness are selected automatically in accordance with the recommendations of the codes for truss members.

Since the standard requirement to the limitation of the slenderness of a tension member is related to the limitation of its self-weight sag, the slenderness check of tension members is performed only in the vertical plane (for example, according to Note 1 to Table 33 of SP 16.13330).

The truss plane is assumed to be the vertical one in the Truss Member mode. Slenderness of the tension truss members out of the truss plane is not checked. Thus, the out-of-plane bracing of the bottom (tension) chord does not affect the result of its slenderness check.

If the user has to check the slenderness of the tension members out of the truss plane (for horizontal and inclined trusses), the Resistance of Sections mode can be used. When the calculation of a tension member is performed in the Resistance of Sections mode, both slenderness values are checked, because the very concept of a vertical plane is absent in this mode.

The Section tab is used to assign a cross-section to a truss member. The sections are made from double equal or unequal angles arranged as a tee (the latter come in two variations) or a cross (of equal angles), or from round and rectangular pipes. The sections are selected from a database of rolled profiles and a gap between the angles is entered. The type and the length of the member are also specified. When a chord member is considered, you need to specify the panel length and the length between out-of-plane restraints. The following member types can be analyzed: a chord member, a lattice member, a support diagonal, a support vertical.

A separate tab is used to specify forces (longitudinal forces) in a member under various load cases. It is possible to indicate the presence or absence of the dynamic loads or crane loads on the truss. If all loads are static, the slenderness check of tension members is performed only in the vertical plane.

The Web instability is forbidden checkbox is used to perform the check of the section taking into account its post-buckling behavior (after the local buckling of the web). The checked checkbox enables to reject the post-buckling behavior of the section if the check indicates local buckling of the web.

If a truss member has additional bracing or weakening, you can use the Effective length factors differ from those recommended by SNiP checkbox to enter nonstandard effective length factors in the truss plane and out of the truss plane.

The mode enables to perform the check analysis taking the corrosion into account. Moreover, you can specify the values of initial imperfection for members made of double angles, and if equal angles are used, you can also analyze local defects such as notches and bents. To do it, check the Allow for corrosion and initial imperfections checkbox in the Section tab. This will add one more tab to the dialog, Defects, where you have to specify the data on the thickness of the corrosion layer, the value of the initial imperfection both in the truss plane and out of the truss plane, the data on the local notches or bents.

Using the button $image\calc.png$ , you can invoke the calculation of the corrosion layer thickness.

It should be noted that the standard checks of the load-bearing capacity of bar elements of steel structures depend on the internal forces and geometric properties with respect to the principal axes of inertia of the section. Thus, the stability checks of the truss members under central compression are performed in the Truss Member mode depending on the geometric properties of the section (in particular, depending on the moments of inertia and the radii of inertia) with respect to the principal axes of inertia.

One of the principal axes of inertia of most types of cross-sections implemented in the Truss Member mode lies in the plane of the truss, and the other one lies in the plane perpendicular to the truss plane. Thus, the buckling of a truss member that has lost its stability under central compression can occur either in the truss plane or out of the truss plane. Therefore, the stability checks of the truss member under central compression are related to the truss plane, i.e. the Stability in the truss plane and Stability out of the truss plane factors are calculated. The only exception is a compound section made of two equal angles arranged as a cross with its principal axes of inertia lying neither in the truss plane, nor in a plane perpendicular to the truss plane. The buckling of a truss member made of two equal angles arranged as a cross at the loss of its stability is not related to the truss plane. Nevertheless, in order to unify the names of the factors, the Stability in the truss plane and Stability out of the truss plane factors are used for this type of section as well.

The behavior analysis of the damaged structure will be performed according to the recommendations of SNiP 2.08.01-85 and the Guide to design of reinforcing for steel structures (to SNiP II-23-81*). The analysis takes into account the possibility for the bar with an initial imperfection to experience spatial buckling, therefore the set of factors generated in the result of the analysis may not include the results of in-plane or out-of-plane stability checks, or the results of both of these checks.

Clicking the Calculate button will display the value of K_max and indicate the type of check (strength, stability, slenderness) in which this maximum took place. You can also browse all the other utilization factors of restrictions by clicking the Factors button.

The search is performed according to the same rules as in the Trusses mode.