Computational Technology Resources - CCP

Keywords: structural engineering, semi-rigid behaviour, steel structures, cyclic behaviour, tension joints, pin joints.

Summary

Pin connections are commonly used in temporary structures. In this paper, a typical joint detail commonly used by PERI - Pericofragens, Lda - Cofragens e Andaimes in scaffolding structures is analysed.

By allowing free rotation about the axis of the pin and no lateral confinement, pin joints present distinctive behavioural aspects when compared to ordinary single bearing bolt connections loaded in shear. Additionally, because of the temporary application of these pin joints, the issue of reuse becomes crucial. Although PERI has used this type of joints in many applications, and despite the existence of specific code provisions for pin joints in Eurocode 3, the actual safety level and partial safety factors to adopt in temporary structures and under repeated utilisation is repeatedly the object of discussion between Owner and Contractor. It is noted that not unrelated to this controversy is certainly the frequent occurrence of accidents with temporary structures, usually the object of far less scrutiny and structural checking and certainly more prone to partial or total collapse because of their usually statically determinate nature. Additionnally, the ongoing conversion of the ENV version of Eurocode 3 to EN status has introduced changes in the specific code clauses for pin connected tension joints.

The analysed prototypes consist of the connection of circular hollow section tubes 120x120x5 to 15 mm thick double Gusset plates by superimposing 10 mm thick double plates using circular pins. All the models and coupons were fabricated by PERI. The two models tested were similar, composed with the same materials (steel grade S275 for the plates and CK45 in pins). Prototype 1 has a 36 mm diameter pin (d₀ = 38 mm), while prototype 2 has a 20 mm diameter pin (d₀ = 22 mm). The geometry of the first model was established to represent a joint used in a real structure; in the second model a reduced diameter pin was used to force the failure mode to be the bending in the pin.

For each grade, the materials properties of the steel plates and pins were determined from standard uniaxial tensile tests. The two prototypes were instrumented with electrical strain gauges (linear and rosettes) and displacement transducers (LVDTs). The tensile forces were applied through 20 mm thick plates. For pin connections, with the geometry of the plates in accordance with the dimensional requirements given in Table 3.9 of Part 1.8 of Eurocode 3 (prEN 1993-1-1 : 2003) or in Table 6.5.6 of of Eurocode 3 (prEN 1993-1-1 : 1992), the code provisions for the design of plates and pins, already described in Table 1, consist of (i) shear resistance of the pin ( $F_{v.Rd}$ ), (ii) bearing resistance of the plates and pin at serviceability ( $F_{b.Rd.ser}$ ) and ultimate limit ( $F_{b.Rd}$ ) states, (iii) bending resistance of the pin at serviceability ( $F_{(MRd.ser)}$ ) and ultimate limit ( $F_{(MRd)}$ ) states and, (iv) combined shear and bending resistance of the pin ( $F_{(MRd, Fv.Rd)}$ ).

Based on the actual mechanical properties and geometrical dimensions, the the resistance of the various failure modes was evaluated according to the two design code, with and without partial safety coefficients, and are listed in Table 67.1.

Table 67.1: Resistance of the various connection components

Prototype	Code version		$F_{v.Rd}$ (kN)	$F_{b.Rd}$ (kN)	$F_{(MRd)}$ (kN)	$F_{(MRd, Fv.Rd)}$ (kN)
	EC3:1992	$\gamma$ = 1.0	808.1	347.9	213.8	206.2
P1		$\gamma$ code	646.5	278.3	170.6	165.0
( $\phi36$ mm)	EC3:2003	$\gamma$ = 1.0	808.1	347.9	399.9	358.4
		$\gamma$ code	646.5	278.3	399.9	340.8
	EC3:1992	$\gamma$ = 1.0	286.8	237.4	51.4	50.6
P2		$\gamma$ code	229.4	189.9	41.2	40.5
( $\phi20$ mm)	EC3:2003	$\gamma$ = 1.0	286.8	237.4	96.4	91.4
		$\gamma$ code	229.4	189.9	96.4	88.9

Failure of prototype 1 presented high ductility and corresponded to combined bearing of the 10 mm plate and yielding of the pin in bending. Prototype 2 exhibited significant ductility and corresponded to failure in combined bending and shear of the pin, bending being the dominant effect. Comparing the two code implementations proposed in EC3, it is noted that the current version allows, at ULS, a bending moment resistance of the pin 1.5x its elastic moment, while the ENV version limited this value to 0.8x the same elastic moment.

The experimental tests carried out have shown that whenever bending of the pin was a limiting factor, the actual resistance was always in excess of the (unfactored) design value. This observation supports the safety of the proposed changes in the EN version of EC3. However, it is worth mentioning that in both tests the pins fully yielded in bending, a result that was possible because of the considerable steel ductility. Should the pins be made of high strength steel with reduced ductility, the results and conclusions might be different.

Finally, for both prototypes (but especially P2), the outer plates were bent out-of- plane together with bending of the pin (an interactive failure mode). This leads to the observation, to be further researched, that if the plates were transversely connected such that parallelism was imposed (a similar effect to bolted joints) the global behaviour could be improved.

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