Page 29

NSCJuly2016digi

NSC 29 July/Aug 16 the whole base in a pocket, or welding a shear nib on the underside (to be located in a pocket in the foundation). 5. Nominally pinned connections invalidate the original assumption of full fixity to the column. In this situation, the designer had assumed an effective length of 0.7L for the column, yet the permitted connections are nominally pinned, with only shear loads provided. The scenario seems unlikely – the choice of 0.7L must have been based on full fixity at both ends – both ends held in position and restrained in direction according to Table 22 of BS 5950. But nominally pinned connections do not provide full restraint in direction, so a longer effective length would be the correct choice. In the scenario described, it seems the original designer has made an error in choosing the effective length. Practice probably varies amongst designers, but an effective length equal to the system length or an effective length factor of 0.85 are common choices when nominally pinned connections are anticipated. 6. High shear and bending. The last situation presented in Verulam was a member with high shear – sufficiently high to reduce the moment capacity. In the (hopefully hypothetical) scenario, the necessary strengthening was considered to be part of the connection design. Clearly, the connection plays no part in the combination of member design forces and the responsibility for selecting a member with sufficient strength lies squarely with the structure designer. A relatively common (real) situation is when a floor plan is prepared, possibly indicating certain shear loads for major beams, but also with a general note stating that if no force is given, the connection must be designed for a certain minimum shear. This note can easily become too general, with the connections for small beams supposed to be designed for a shear force that exceeds the resistance of the beam itself. In general, the critical check for a beam is likely to be the bending resistance or deflection, with the shear force no more than about 60% of the beam’s shear resistance. High shears at the end of a beam are generally only produced if there is a concentrated load near the end of the beam. You can lead a horse to water … The proverb continues … but you can’t make them drink. There are very many resources available covering the sorts of topics raised in Verulam, if only designers knew of them and read them. A good place to start is the Steel Industry Guidance Notes (SIGNS), which cover a wide variety of topics. Searching for “SIGNS” on Steelbiz will produce a complete list, which could form the background to a succinct library of “good practice” guidance. You can also go to www.newsteelconstruction.com and search the Advisory Desk articles. Technical Figure 3: Typical base detail 1 Volume 94, Issue 4. The Institution of Structural Engineers, April 2016 2 Joints in steel construction: Simple Connections, SCI and BCSA, 2009 3 Joints in steel construction: Simple joints to Eurocode 3, SCI and BCSA, 2014 4 SN48 Design of welded joints using structural hollow sections. Available on Steelbiz 5 SN51 Design responsibility – simple connections. Available on Steelbiz


NSCJuly2016digi
To see the actual publication please follow the link above