Bahen/Tanenbaum Professor of Civil Engineering, University of Toronto, Ontario, Canada
Tubular steel sections are commonly used as cantilevered structural members to support electricity distribution lines, cellular phone equipment, road and highway signs, lighting and luminaires, traffic signals, advertising boards, and in a host of similar monopole applications (Figure 1). The structural design of such poles is normally based on consideration of the strength, serviceability and fatigue limit states under all applicable load combinations. For the strength limit state, the pole needs to be designed for axial load and bending, including P-Δ effects, with moment amplification due to second-order effects. For strength-critical designs, square/rectangular HSS, round HSS, tapered round and polygonal (multisided) tubes have all been used. Fatigue considerations arise with welded attachments to poles, such as base plates. AISC 360 Appendix 3 on Fatigue (AISC 2016) does not cover welded details such as hollow sections-to-base plates, and AWS D1.1 Clause 9 on Tubular Structures (AWS 2015) covers the fatigue of round-to-round welded T-, Y- and K-connections based on offshore practice for tubulars. Guidance for the design of monopoles, however, is given by AASHTO (2015).
Figure 1: Applications of HSS as poles: (a) round section used for a high-level electrical power line tower;
(b) square section used to support low-level plaza lighting.
Detailed design provisions are provided for steel structural supports for highway signs, luminaires and traffic signals (AASHTO 2015), but for hollow sections, these only cover round and multisided shapes. Clause 5.6.2 specifies that multisided tubular sections shall have a minimum number of sides, n, where
Dis the outside distance from flat side to flat side of a multisided tube (in.) and nhas a minimum value of 8. The Commentary states that square or rectangular sections are susceptible to early fatigue cracking, leading to poor fatigue performance, hence these sections should not be used for highway sign, signal and high-level luminaire support structures (Dexter and Ricker 2002). Increasing the number of sides and/or increasing the internal bend radius improves the fatigue performance of multisided sections (Roy et al. 2011), and since pole members are frequently galvanized, larger corner radii also reduce the possibility of liquid metal embrittlement during hot-dip galvanizing. A minimum bend radius of five times the tube wall thickness mitigates the possibility of such embrittlement according to AASHTO (2015). Due to the prohibition of square and rectangular HSS for highway signs and associated structures, such shapes are predominantly used for low-level poles in urban situations (e.g., Figure 1[b]).
Fatigue Design of Welded Base Plate Connections
High-cycle fatigue design of base plate connections is normally performed using the nominal stress method in conjunction with a classification for the base plate welded joint detail. Accurate load spectra for defining fatigue loads are hard to prescribe with environmental loadings, so faced with this dilemma, engineers typically resort to an “infinite fatigue life” design. This involves keeping the maximum elastic normal stress range below a constant amplitude fatigue threshold, FTH, which corresponds to the horizontal line(s) in Figure 2. HSS fillet-welded to a base plate, with the weld transverse to the direction of cyclic stress, are relegated to the lowest fatigue category, E/ in Figure 2, for which FTH has a very low value of 2.6 ksi (18 MPa). This also corresponds to the lowest fatigue threshold stress in Tables 18.104.22.168-1 and C22.214.171.124-1 of AASHTO (2015). The infinite life fatigue design approach should ensure that a structure performs satisfactorily for its design life to an acceptable level of reliability without significant fatigue damage and propagation of cracks (AASHTO 2015). The fillet-welded HSS-to-base plate fatigue detail is specifically included in the Canadian steel design code (CSA 2019), and experimental research on such connections (Mashiri et al. 2002) has confirmed the linearly decreasing low-cycle portion of the fatigue design S–N “curve.” (A design line with a slope of m = -3, passing through FSR= 6.3 ksi [43 MPa] at 2 million cycles, was obtain from experiments.) The fatigue threshold value of 2.6 ksi (18 MPa) could not be confirmed because of the very long testing time required and is instead inferred from experience.