COMPARATIVE DESIGN OF CERTAIN STEEL CONNECTIONS ACCORDING TO USA AND EUROPEAN PROVISIONS

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1 COMPARATIVE DESIGN OF CERTAIN STEEL CONNECTIONS ACCORDING TO USA AND EUROPEAN PROVISIONS Anna Marinopoulou Phd. Structural Engineer - CCS S.A., Greece annamar@ccs.gr Euripidis Kardaras Structural Engineer, Greece e-akarda@otenet.gr Ioannis Palamas Phd. Structural Engineer - CCS S.A., Greece jpalamas@ccs.gr ABSTRACT The main subject of this research is the comparative esentation of different actices used in America and Europe for the design of commonly used connections, and their complete design in accordance with the ovisions of the respective codes. The work focuses on the comparative esentation of the European and U.S. design codes on structural steel connections and specifically on the beam to column moment connections. Beam to column moment connections of multi story buildings are designed and checked in accordance with European and U.S. standards. Included is the design in accordance with EC3 and AISC, where either the connection configuration is identical in both cases (EC3 and AISC) or it is different actices applied in U.S. and Europe. In each case the corresponding failure ratios are calculated and conclusions concerning the most economical design are deducted. 1. INTRODUCTION Beam-to-column moment connections of multi-storey steel building frames are examined. Hot-rolled sections of the European and the American industry are implemented in the examples esented herein. Specifically, a joint of IPE 450 beam section to HE 500A column section, IPE 600 beam section to HE 500A column section and W27x94 beam section to W14x311 column section are considered. For gravity and wind loads, the joints are designed with the internal forces that have been computed from the analysis. On the other hand, for the seismic applications according to the European and the American standards the joints are designed with the beam over-strength, as it is explained in detail further down. 2. CONNECTION OVER-STRENGTH DEMANDS ACCORDING TO EAK 2000 (Greek seismic code) AND AISC341 The exterior joint of a frame of a multi-storey building shown in Fig. 1, is examined. The design shear force for the connection is:

2 V = V + V f 0 m The design bending moment of the connection is: M = M + V L f q L V 0 = 2 f V p 2M m = yb xb L M = c f Z (1) (2) q q Lp L Lp L Fig. 1 Frame of a multi-storey building (part elevation) Where q: the uniformly distributed permanent and live loads compatible with seismic design, L : the distance between the plastic hinges of the beam, L : the distance from the point where the plastic hinge occurs to the face of the column, c: over-strength factor, specified in the codes for connection of dissipative zones (beams) to the rest of the frame (columns). These non-dissipative connections should have sufficient over-strength to allow for the development of cyclic yielding in the dissipative parts. In this capacity design the possibility that the actual yield stress of steel is higher than the nominal value is taken into account by this material overstrength factor. f yb : the nominal yield stress of the steel material of the beam and Ζ xb : the plastic modulus of beam section. According to EAK 2000 [2]: c =1.2 irrespective of steel grade (i.e. the same for all grades of steel). According to AISC 341 [4, 5, 7]: c = 1.10 x R y where 1.10: factor which takes into account mainly the strain hardening of the material and R y : the ratio of the expected yield stress to the specified minimum yield stress. This ratio is taken 1.10 for steel grade with yield stress f y = 50 ksi = 345 MPa and 1.50 for steel grade with f y = 36 ksi = 248 MPa. Thus c = 1.21 for steel grade with f y = 50 ksi (corresponding to the steel grade S355) and c = 1.65 for steel grade with f y = 36 ksi (apoximately corresponding to the steel grade S235). In the calculations of the following paragraphs, where certain numerical examples are examined (concerning seismic design), in case in which hot-rolled American sections are used, the corresponding American over-strength factors, described in AISC 341, are used, while in cases where hot-rolled European sections are used, then the corresponding factor 1.20, described in the Greek seismic code, is utilised. According to the European ovisions, the design forces of the joint (shear force and bending moment) are significantly less for beams of steel grade S235 than p

3 those for beams of steel grade S355. For this reason, it is often quite wise to use beams from S235 steel grade (whereas the column section could be of S355 grade) in order to decrease the design forces of the joint. On the contrary, according to AISC 341, the difference in the design forces of a joint with beams of steel grade with f y = 36 ksi and f y = 50 ksi is insignificant. Thus, usually, the same steel grade for beams and columns is specified. This is derived quite easily by comparing the oduct 1.10 x R y x f y for both steel grades (for yield stress f y = 36 ksi it is 1.65 x 36 = 59.4 ksi, while for f y = 50 ksi it will be 1.21 x 50 = 60.5 ksi 59.4 ksi). 3. CONNECTION DESIGN ACCORDING TO ΕC3 / EAK 2000 AND AISC 341 SEISMIC PROVISIONS/ AISC Steel Construction Manual 3.1 Beam ΙΡΕ 600 to column ΗΕ 500Α for seismic design q=28.6 kn/m q=28.6 kn/m HE 500A IPE600 Lp L Lp HE 700A 7200 Fig. 2 Frame of a multi-storey building The exterior joint of Fig. 2 is investigated. For this beam ΙΡΕ 600 / S235 to column ΗΕ 500Α / S355 connection, the permanent and live loads compatible with seismic loading are taken kn/m. American connection (according to AISC): The distance between the beam plastic hinges is: L = = 5.90 m. The design shear force is: Vf = V0 + Vm = = kn The design bending moment of the connection is: Mf = M + Vf Lp = knm European connection (according to EN): The distance between the beam plastic hinges is now: L = = 4.95 m different from the one calculated for the American connection, because of the larger length of stiffening. The design shear force according to EAK 2000 is: Vf = V0 + Vm = kn and thus the hogging design bending moment is:

4 Mf = M + Vf Lp = knm For the positive design bending moment, we have: Vf = Vm V0 = 1 71 = 330 kn The positive design bending moment will be: M f = M + Vf Lp = knm American configuration (Fig. 3(α)) [3, 4, 5, 6]: End plate: 35 x 2 x 970 (mm) Material: S235 (f y = 235 MPa and f u = 360 Mpa) Bolts: 2 x 8, 1 A490N d bolt = 1 in. (corresponding european bolts Μ24 Grade 10.9). Beam web welds: double-sided fillet 8 mm. Flange welds: full penetration groove weld. Height of triangular stiffener: 185 mm. Length of triangular stiffener: 320 mm Thickness of triangular stiffener: 15 mm x120x x444x A490N 970x2x35 320x185x IPE M27, x30 t=12 IPE x120x x2x30 (a) USA (b) Europe Fig. 3 Beam ΙΡΕ 600 to column ΗΕ 500Α connection European configuration (Fig. 3(b)) [1]: End plate: 30 x 2 x 1550 (mm) Material: S235 (f y = 235 MPa and f u = 360 Mpa) Bolts: 2 x 11, Μ Web welds: double-sided fillet 8 mm. Flange welds: double-sided fillet 12 mm. Height and length of stiffening: 800 mm Thickness of stiffening flange: 30 mm Table. 1: Failure ratios Check AISC EN (hogging moment) EN (positive moment) Bending resistance

5 3.2 Beam ΙΡΕ 600 to column ΗΕ 500Α for gravity and wind loading Design shear force (factored force): V = 210 kn Design bending moment (factored force): M = 296 knm American configuration (Fig. 4(α)) [3, 4, 6]: End plate: 20 x 2 x 760 (mm) Material: S235 (f y = 235 MPa and f u = 360 Mpa) Bolts: 2 x 5, ¾ A325N d bolt = 3/4 in. (corresponding European bolts Μ20 Grade 8.8) Web welds: double-sided fillet 6 mm. Flange welds: double-sided fillet 10 mm /4 A325N M20, 8.8 IPE IPE x2x20 760x2x20 (α) USA (b) Europe Fig. 4 Beam ΙΡΕ 600 to column ΗΕ 500Α connection European configuration (Fig. 4(b)) [1]: End plate: 20 x 2 x 760 (mm) MAterial: S235 (f y = 235 MPa and f u = 360 Mpa) Bolts: 2 x 5, Μ Web welds: double-sided fillet 6 mm. Flange welds: double-sided fillet 10 mm. Table. 2: Failure ratios Check AISC EN Bending resistance Beam ΙΡΕ 450 to column ΗΕ 500Α for seismic design The exterior joint of Fig. 5 is investigated. For this beam ΙΡΕ 450 / S235 to column ΗΕ 500Α / S355, the permanent and live loads for seismic loading are equal to kn/m. American connection (according to AISC): Because there is no stiffening at the joint, it is assumed that the plastic hinges occur at a distance equal to 20 cm from the column flange ( t p + db 3 = / m ). The distance between the beam plastic hinges is: L = = 6.20 m. The design shear force is:

6 Vf = V0 + Vm = = 243 kn The design bending moment is: M = M + V L = = 529 knm f f p q=28.6 kn/m q=28.6 kn/m HE 500A IPE450 Lp L Lp HE 700A 7200 Fig. 5 Frame of a multi-storey building European connection (according to EN): The plastic hinges occur at the end of stiffening: L = = 5.60 m. The design shear force is: Vf = V0 + Vm = = 251 kn The hogging design bending moment is: Mf = M + Vf Lp = = 605 knm. For the positive design bending moment, we have: Vf = Vm V0 = = 91 kn and Mf = M + Vf Lp = = 525 knm. American configuration (Fig. 6(α)) [3, 4, 5, 6]: End plate: x 230 x 670 (mm), material: S235 (f y = 235 MPa and f u = 360 Mpa). Bolts: 2 x 4, 1 & 1/8 A490N d bolt = 1 & 1/8 in. (corresponding European bolts: between Μ27 and Μ30 Grade 10.9). Beam web welds: double-sided fillet 6 mm. Flange welds: full penetration groove weld. European configuration (Fig. 6(b)) [1]: End plate: x 230 x 970 (mm), material: S235 (f y = 235 MPa and f u = 360 Mpa). Bolts: 2 x 7, Μ Web welds: double-sided fillet 6 mm. Flange welds: double-sided fillet 10 mm. Height and length of stiffening: 450 mm. Thickness of stiffening flange: 25 mm Table 3: Failure ratios Check AISC EN (hogging moment) EN (positive moment) Bending resistance

7 & 1/8 A490N IPE M27, x120x IPE x25 t= x444x10 670x230x 444x120x25 970x230x (α) USA (b) Europe Fig. 6 Beam ΙΡΕ 450 to column ΗΕ 500Α connection 3.4 Beam ΙΡΕ 450 to column ΗΕ 500Α for gravity and wind loading Application for gravity and wind loading Design shear force: V = 194 kn Design bending moment: M = 252 knm American configuration (Fig. 7(α)) [3, 5, 6]: End plate: 25 x 210 x 620 (mm), material: S235 (f y = 235 MPa and f u = 360 Mpa). Bolts: 2 x 5, ¾ A490N d bolt = ¾ in. (corresponding European bolts Μ20 Grade 10.9). Beam web welds: double-sided fillet 6 mm. Flange welds: double-sided fillet 10 mm. European configuration (Fig. 7(b)) [1]: End plate: 20 x 210 x 620 (mm), material: S235 (f y = 235 MPa and f u = 360 Mpa). Bolts: 2 x 5, Μ Web welds: double-sided fillet 6 mm. Flange welds: double-sided fillet 10 mm. Table 4: Failure ratios Check AISC EN Bending resistance M20, /4 A490N IPE450 IPE x210x x210x20 (α) USA (b) Europe Fig. 7 Beam ΙΡΕ 450 to column ΗΕ 500Α connection

8 3.5 Beam W27x94 to column W14x311 for seismic design W14x311 q=18.6 kn/m W14x311 q=18.6 kn/m W27x94 Lp L Lp 85 Fig. 8 Frame of a multi-storey building The exterior joint of a frame of a multi-storey building of Fig. 8 is investigated. The beam span is 28 ft, i.e. apoximately 8.54 m. The column section is W14x311 and the beam section is W27x94, steel grade Gr50 with yield stress f y = 50 ksi (345 MPa) and tensile stress f u = 65 ksi (450 MPa). The permanent and live loads compatible with seismic loading are considered to be 18.6 kn/m. American connection 1 (extended end plate according to AISC): The distance between the plastic hinges of the beam is: L = = 7.30 m. The design shear force is: Vf = V0 + Vm = = 589 kn The design bending moment is: M f = M + Vf L p = = 2138 knm. American connection 2 (flange plated moment connection according to AISC) [7]: The outer joint of the evious example is considered again. According to AISC, a flange plated moment connection is utilized. This type of connection is widely used in USA. The bending moment is undertaken by the flange plates and it is transferred through them to the column. The shear force is undertaken by the web shear plate (shear tab). After formation of plastic hinges, we consider yielding of the beam. For this reason, thick flange plates are used in order to resist and transfer the design bending moment. Bolts are allowed to slip which contributes to absorption of a significant amount of seismic energy. The plastic hinge is assumed to occur at the point of the last bolt row of the flange plates. The most distant bolt row is at a distance of 720 mm from the column flange. Therefore, the distance between the plastic hinges of the beam is equal to L = = 6.66 m. The design shear force is: Vf = V0 + Vm = = 633 kn The design bending moment is: M = M + V L = knm. f f p =

9 European connection (according to EN): The plastic hinges occur at the end of the stiffening. The distance between the plastic hinges of the beam is L = = 6.42 m. The design shear force is: Vf = V0 + Vm = = 652 kn The hogging bending design moment is: M = M + V L = knm. f f p = In order to calculate the positive bending design moment, we have: = V V 533 kn Vf m 0 = and M f = M + Vf L p = = 2350 knm W14x311 W14x x350x x160x x213x15 W27x A490N 1 & 1/4 A490N 1 & 1/4 A490N 600x120x x160x W27x94 780x350x 1135x320x x290x 1220x320x10 (a) USA 1 (b) USA 2 W14x311 W27x M30, 10.9 t=15 254x30 320x160x30 16x290x (c) Europe Fig. 9 Beam W27x94 to column W14x311connection American configuration 1 (Fig. 9(α)) [3, 4, 5, 6]:

10 End plate: x 290 x 1110 (mm), material: Grade 50 (f y = 345 MPa και f u = 450 Mpa). Bolts: 2 x 8, 1 ¼ A490N d bolt = 1 ¼ in. (corresponding European bolts: between Μ30 and Μ33 Grade 10.9). Web welds: double-sided fillet 8 mm. Flange welds: full penetration groove weld. Height of stiffening: 213 mm. Length of stiffening: 370 mm. Thickness of stiffening: 15 mm. American configuration 2 (Fig. 9(bβ)) [3, 4, 5, 6, 8]: Shear plate (shear tab): 10 x 120 x 600 (mm), material: Grade 50. Bolts: 6, 1 A490N d bolt = 1 in. (corresponding European bolts Μ24 Grade 10.9). Shear tab-to-column flange welds: double-sided fillet 6 mm. Flange plates: x 350 x 780 (mm), material: Grade 50. Bolts: 2 x 7, 1 & ¼ A490N d bolt = 1 & ¼ in. (corresponding European: between Μ30 and Μ ). Flange plates-to-column flange welds: full penetration groove weld. European configuration (Fig. 9(c)) [1]: End plate: x 290 x 16 (mm), material: Grade 50. Bolts: 2 x 11, Μ Web welds: double-sided fillet 8 mm. Flange welds: double-sided fillet 12 mm. Height and length of stiffening: 800 mm. Thickness of stiffening flange: 30 mm. Table 5: Failure ratios Check AISC (1) AISC (2) EN (hogging moment) EN (positive moment) Bending resistance Beam W27x94 to column W14x311 for gravity and wind loading Design shear force: V = 580 kn Design bending moment: M = 0 knm American configuration (Fig. 10(α)) [3, 6, 7]: Shear plate (shear tab): 10 x 120 x 600 (mm), material: Grade 50 (f y = 345 MPa and f u = 450 Mpa). Bolts: 6, 1 A490N d bolt = 1 in. (corresponding European bolts Μ24 Grade 10.9). Shear tab-to-column flange welds: double-sided fillet 6 mm. Flange plates: 20 x 300 x 550 (mm), material: Grade 50. Bolts: 2 x 6, 1 A490N d bolt = 1 in. (corresponding european Μ24, 10.9). Flange plates-to-column flange welds: full penetration groove weld. European configuration (Fig. 10(b)) [1]: End plate: 30 x 270 x 810 (mm), material: S355 (f y = 355 MPa and f u = 510 Mpa). Bolts: 2 x 7, Μ Web welds: double-sided fillet 8 mm. Flange welds: double-sided fillet 12 mm. Table 6: Failure ratios Check AISC EN Bending resistance

11 W14x311 W14x x300x x120x A490N 1 A490N 144 M24, W27x W27x94 550x300x20 810x270x30 20 (α) USA (b) Europe Fig. 10 Beam W27x94 to column W14x311 connection 4. CONCLUSIONS The esent investigation concludes that for the joints with end plate moment connection for gravity and wind loading, the design of the connection according to AISC and EN is quite similar. On the other hand, for end plate moment connections under seismic loading, the design was quite different in order to reflect the different actices followed in Europe and America. According to the American actice, the end plate connection (with or without stiffening) is symmetrical top and bottom, while according to the European actice the connection is not symmetrical and a triangular stiffening is added only at the bottom flange. This inevitably leads to a larger height of stiffening. Consequently, the length of the stiffening is also large, which means greater design bending moment for the connection (due to the additional moment due to shear force), and hence more bolts in general. Thus, the European connection, as used in actice, is found to be % more expensive (in material) compared to the American corresponding connection. It is, besides, worthy to say that the American configuration esents less architectural oblems due to the smaller stiffening at bottom flange (the stiffening at top flange is almost always covered by the concrete slab). Apart from the connection with end plate, an alternative connection configuration according to the American actice is esented. It is a connection where bolted flange plates are used. This type of connection is very popular in USA, much more than the one with end plate. This type of connection is designed for gravity and wind loading and for seismic loading also. It is oved to be more expensive in material compared to the corresponding American beam-tocolumn end plate connection, but it is more convenient for the erection. This paper esents a comparison between the different actices followed in USA and Europe in the design of beam-to-column moment connections of multistorey buildings, through specific numerical examples. Of course, it is not complete and it is considered to be just an initial investigation and assessment. More examples with the same types of connections as well as different actices also could further be examined. Moreover, it would be useful to investigate the individual checks that each standard code demands, in order that conclusions about the conservatism of the corresponding codes in the design of moment connections in

12 USA and Europe could be drawn. Finally, this paper could also be extended further in the investigation of different types of connections other than moment connections. REFERENCES [1] Eurocode 3: Design of steel structures Part 1-8: Design of joints. [2] EAK 2000, Greek seismic design code [3] Steel Construction Manual, American Institute of Steel Construction, 13th Edition. [4] AISC Seismic Provisions for Structural Steel Buildings. [5] AISC Steel Design Guide 4. Extended End Plate Moment Connections. 2 nd edition. [6] AISC Steel Design Guide 13. Stiffening of W-flange columns at Moment Connections: Wind and Seismic Applications. [7] Akbar R. Tamboli. Handbook of Structural Steel Connection Design and Details, 2nd edition. McGraw Hill, [8] Robert Englekirk. Steel Structures, Controlling Behavior through Design, John Wiley and Sons, [9] Instant 2010 Struconnect, Computer Control Systems, S.A.