34233 Hollow Structural Sections

There have been numerous publications regarding the fatigue strength of rectangular and circular hollow structural sections (HSS), as summarized in Van Wingerde et al. [43]. The majority of recent work has taken place in Europe under the advisement of the European Coal and Steel Community (ECCS), the British Department of Energy (DEn), now called the Health and Safety Executive, and the Comite

International pour le Developement et l'Etude de la Construction Tubulaire (CIDECT) [44]. The research has resulted in design recommendations that have been adopted by the International Institute of Welding (IIW Document XIII-1804-99 and XV-1035-99) [45] and CIDECT Design Guide #8 [46] and have been proposed for inclusion into AWS D1.1 [18] and EC3 [20]. The CIDECT fatigue design guidelines include both the nominal-stress approach and the hot-spot stress approach. AWS D1.1 [18] also has nominal-stress and hot-spot stress S-N curves for HSS, although these are not considered as accurate as the CIDECT guidelines. Unlike AWS, the CIDECT guidelines distinguish between circular hollow sections (CHS) and rectangular hollow sections (RHS).

Many sign, signal, and light supports are fabricated from HSS. AASHTO has published a new specification for these types of structures [47] that includes fatigue design using the nominal-stress approach and classification of the common details with respect to the AWS S-N curves, based on research in NCHRP Report 412 [14]. In addition, this specification has fatigue design loads for natural wind gusts, galloping, and vortex shedding. Many aspects of this specification could also be applied to wind turbines, communication towers, and other flexible structures exposed to wind.

The hot-spot stress approach is similar to the nominal-stress approach and is also based on full-scale tests, except that the S-N curves in this approach are based on the hot-spot stress range [48]. The hotspot stress is the stress normal to the weld axis, originally defined at some small distance (about 6 mm) from the weld toe. The hot-spot stress can be measured, calculated using finite-element analysis, or determined from parametric formulas derived from the finite-element analyses and measurements.

A distinction is made between the geometric stress concentration, that is, that associated with the arrangement of the members, and the local stress concentration, that is, that associated with the weld toe. In the hot-spot approach, the effect of the geometric stress concentration, expressed as the SCF, is taken out of the fatigue resistance and instead is included in the analysis. This has the advantage of collapsing all the S-N curves for different categories into a single baseline S-N curve, but it has the disadvantage of increasing the complexity of the analysis. A simple structural analysis is usually performed to obtain the axial and bending forces entering a joint, from which nominal stresses are calculated. The hot-spot stress at a weld is determined by multiplying the appropriate nominal stress by the SCF. The test data and the baseline S-N curve still include the effect of the local stress concentration, which is impossible to calculate accurately and therefore must still be treated empirically.

A simple approach that works well is to define the hot-spot stress as the stress measured with a 3-mm strain gage placed as close as practically possible to the weld toe, that is, centered about 6 mm from the weld toe [49]. This is essentially the definition originally used by AWS [48]. The baseline S-N curve used with the hot-spot stress defined this way is essentially the same as the nominal-stress S-N curve (Category C) for a transverse butt or fillet weld in a nominal membrane stress field, that is, a stress field without any geometric stress concentration [49]. This makes sense since the stress at the weld toe of this detail would include the local stress concentration but would not be affected by geometric stress concentration. In other words, the SCF is equal to 1.0, and the hot-spot stress is equal to the nominal stress in this detail.

The CIDECT research updated parametric equations used for SCF calculations and unified the definition of hot-spot stress. The definition of hot-spot stress is more complex than the simple AWS definition and involves extrapolating the stress from multiple strain gage measurements or analysis points [46]. However, the CIDECT design guidelines contain parametric equations that can be used to calculate SCF based on nondimensional parameters (brace to chord diameter ratio, brace diameter to thickness ratio, brace to chord thickness ratio, and chord length to diameter ratio) for different types of loading. A minimum SCF of 2.0 is recommended.

Obviously, the SCF and the baseline curve are dependent on the definition of the hot-spot stress [50]. The CIDECT baseline hot-spot S-N curve is the T' curve from BS 7608 (equivalent to the Eurocode class 114 curve) for use with 16-mm-thick sections (see Figure 34.5). This is about 25% higher than the baseline S-N curve used with the AWS-defined hot-spot stress (6 mm from the weld toe). In hollow structural joints, there is a pronounced thickness effect, with thinner sections having higher fatigue lives. CIDECT included the thickness effect in the definition of the hot-spot S-N curves, as shown in Figure 34.5.

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