PSI - Issue 81

Svyatoslav Gomon et al. / Procedia Structural Integrity 81 (2026) 192–197

195

Fig.6. Structural scheme of truss No. 2 after failure of member 10

Fig.7. Structural scheme of truss No. 2 after failure of member 14

Table 2. Comparison of internal forces in truss No. 2 under element failure Element No.

Design internal forces, kN

Percentage, % Design internal forces after failure of member 16, kN

Percentage, %

Design internal forces after failure of member 15, kN

1 2 5 6 9

-301.6 -301.6 -41.3 301.3 -224.9 -123.4 -344.3 -39.7

-243.6 -243.6 -25.2 341.4 -222.9 241.3 -177.9 -

-19.2 -19.2 -39.0 +13.4

-321.0 -321.0 -3.7 292.5 -211.3 -131.2 325.7

+6.5 +6.5 -91.1 -3.0 -6.1 +11.1

-0.9

10 13 14

-

-29.9

-5.4

+348.6

-

-

As the calculations show, trusses with downward (or diagonal tensile) members, such as the Pratt truss, have certain advantages in the case of accidental failure or local damage to structural elements, which prevents progressive collapse of the entire structural system. The main advantages in this case are: 1. Load redistribution: Truss structures generally possess high structural redundancy - a principle of component reserve. In the event of the failure of a single member, the load can be redistributed to adjacent or parallel members, preventing instantaneous total collapse. 2. Chain mechanism: Following sudden failure of the bottom chord or a diagonal member, a so- called “chain -line mechanism” may be activated, helping to bridge the damaged area and ensuring residual load -bearing capacity of the entire structure. 3. Efficiency of tensile members: In trusses with downward members, vertical elements work in compression, while diagonals work in tension. Steel is more effective in tension, which allows the use of lighter cross-sections for diagonal members in timber trusses, reducing the overall weight of the structure. 4. Spatial effects: In spatial truss designs, accounting for spatial action significantly increases resistance to progressive collapse. For design purposes, the geometric scheme of truss No. 2 is preferable because the change in design forces under member failure is significantly smaller than in truss No. 1. In truss No. 1, the maximum force occurs in member 16, increasing by 756.1% relative to the design value, whereas in truss No. 2, it occurs in member 14, increasing by 348.6%. Therefore, the strength and stability of the elements of truss No. 2 are checked for the maximum forces, and, if necessary, the required cross-sectional dimensions of truss No. 2 members are calculated. After performing the calculations and selecting cross-sections of truss No. 2 members, taking into account the effects of progressive collapse, it was determined that the cross-sections of elements 5 and 6 (bottom chord of the truss) need to be increased by 8.8%, and element 14 (central diagonal) should be increased by 57.14% compared to the original calculations (without considering progressive collapse). It should be noted that the behavior of timber or metal-timber structures under accidental conditions (e.g., fire or explosion) is difficult to predict and depends on many factors, including the specific load configuration and types of joint connections. To ensure reliability and safety, a detailed analysis of potential failure scenarios and, if necessary, reinforcement of critical structural elements is required in each specific case.