QUESTION: (WINTER2014/2015) What is the correct fatigue ‘Detail Category’ for a coped beam detail? S16-14 shows Category E1 whereas W59-13 and S16-09 show Category B
ANSWER: Category E1 should apply to re-entrant corners of copes having a minimum radius of 35 mm and ground smooth as stipulated in S16-14. Category B should not be used unless the design stress range is amplified with an appropriate stress concentration factor, which is a function of the size of re-entrant corner radius and finishing.
Figure: Beam with a Cope Detail
QUESTION: (SUMMER2014) Are bolted moment connections used in a canopy structure required to be slip critical? My question relates to a situation where slip critical connections are not required for deflection control. I have many years of connection design experience but seldom had to provide slip-critical connections for wind-load resisting braced bents or moment frames.
ANSWER: The key question here is whether fatigue is a consideration; will the structure be subjected to repetitive loading and stress reversal? A relatively light canopy type of structure subjected to gusty local wind load may experience stress reversal and a significant number of load cycles to warrant such assessment. The judgement rests with the engineer responsible for the design of the structure. Fatigue design is covered in Clause 26 of S16.
QUESTION: (FALL2010) CSA Standard S6, Canadian Highway Bridge Design Code, requires that cross-frame connection plates be connected to the flanges of bridge girders. The bolted detail, as shown (in Figure 1), appears to be quite popular in rehabilitation work. I heard that this bolted detail qualifies for a “Category B“ fatigue detail, but it is not clear to me how simply bolting the stiffener to the bottom flange makes things better because the web weld is still present.
ANSWER:Where the stiffener also serves as a cross-frame connection plate, both distortion-induced fatigue and load-induced fatigue should be considered. The bolted detail as shown does not alter the stiffener-to-web welded fatigue detail with respect to load-induced fatigue because this welded detail remains “Category C1”. However, connecting the connection plate to the flanges (when done correctly) should improve the distortion-induced fatigue resistance substantially.
In order to avoid welded attachments in the tension flange, many older welded steel bridge girders feature cross-frame connection plates that were either cut short from, or ground to bear on, the tension flange. This outdated practice inadvertently resulted in the web taking out-of-plane stresses due to relative displacements of adjacent girders. These stress ranges, typically unaccounted for in the analyses, have been identified as the common cause of distortion-induced fatigue damage to welded bridge girders. Recent editions of CSA S6 require that cross-frames and diaphragms be connected to each flange for a minimum force of 90 kN.