A flush collector beam end plate is used to allow for the placement of the floor deck on top of the collector beam.

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1 EARTHQUAKE Eentrially Braed Frames with Removable Links Design Methodology Author: Kevin Cowie a, Alistair Fussell a, Martin Wong a, Charles Clifton b, Dmitry Volynkin b Affiliation: a. Steel Constrution New Zealand In., b. The University of Aukland Date: 16 th January 2013 Ref.: EQK1006 Key Words Eentrially Braed Frame, EBF, Removable Links Introdution Steel eentrially braed frames are expeted to sustain damage during a design level earthquake through repeated inelasti deformation of the ative link. Repair is therefore expeted to be ostly and disruptive, even if the struture has met its goal of providing life safety during an earthquake. The replaeable or removable ative link onept addresses these drawbaks as it allows for quik inspetion and replaement of damaged links following a major earthquake, signifiantly minimising the time to reoupy the building. The development and researh of eentrially braed frames with removable links is overed in Steel Advisor artile EQK1005 (in preparation at the time of writing this artile). This artile presents the design methodology for eentrially braed frames with removable links. It fouses on the design and detailing of the link, and where neessary, modifiations to the design proedure for onventional EBF s. Readers not familiar with the global seismi behaviour and design of eentrially braed frames should onsult HERA report R4-76. Steel Advisor artile EQK1007 provides a worked design example to illustrate the appliation of the proposed eentrially braed frame with removable link design proedure. Anatomy of a Removable Link The onfiguration of an eentrially braed frame with removable links is shown in figure 1. The lear ative length is ideally seleted suh that the EBF behaves in a shear mode, as this makes design of the bolted onnetion more feasible due to the lower moment developed within the ative link. Replaeable links would typially be fabriated from hot rolled universal olumn (UC) or ustom welded I setions. Colletor beams have a depth greater than the removable link setion. The removable link is onneted with an extended end plate bolted onnetion to a flush end plate on the olletor beams. This requires the olletor beam to be a deeper setion than the ative link. Braes may onsist of universal olumns (UC) or SHS setions birdsmouthed over a gusset plate onnetion (figure 1). The ative link end plate onnetion is ommonly onneted to the olletor beam end plate with an eight bolt arrangement using high strength property lass 8.8 bolts. Where the links are long and the apaity derived ative link flange tension fores are large a 16 bolt arrangement may be required. Alternatively, onsideration should be given to using property lass 10.9 bolts. A flush olletor beam end plate is used to allow for the plaement of the floor dek on top of the olletor beam. The gap between the top of the ative link and the underside of the slab should be at least 50mm and suffiient for the following: To allow a timber paker to be plaed between the top of ative link and underside of dek to support the deking during onrete plaement when the deking ribs run parallel to the EBF frame To allow a flat jak to be inserted to push the link out following a severe earthquake if replaement is needed Dislaimer: SCNZ and the author(s) of this doument make no warrantee, guarantee or representation in onnetion with this doument and shall not be held liable or responsible for any loss or damage resulting from the use of this doument Steel Constrution New Zealand In

2 Figure 1: Replaeable Link Configuration, 1a: UC brae, 1b: SHS brae Design Ations on a Removable Link The typial fore distribution in ative links and olletor beams of eentrially braed frames under lateral load is shown in figure 2. The lateral fore is assumed in figure 2 to be applied to both sides of the frame, and thus no axial load is transferred through the ative link. There will be axial load present in the ative link where diaphragm fores annot be transferred to the olletor beams on eah side of the ative link. Where this is the ase then the ative link must be designed for this axial load in aordane with NZS 3404 Clause The ative link beam is hosen suh that the shear apaity is not signifiantly greater than the shear demand. Design ations are then fatored up using the overstrength shear apaity of the ative link. The designer should determine the number of different ative link sizes to be used in aordane with the R4-76 reommendations and then average the design seismi ations aross the levels where the ative links sizes are the same and size the ative link as losely as possible to this average. T C e T C Moment Shear Axial Figure 2: Design Ations in Colletor Beams and Ative Links under Lateral Load Removable link end plate onnetions are subjet to shear, moment and possible axial fores. Where there is no axial fores the link end onnetion is designed for the following apaity design derived ations: 1. Overstrength shear demand V V link oms w where Overstrength fator from NZS3404 oms V Replaeable link nominal shear apaity w Steel Constrution New Zealand In

2. Capaity derived moment demand M V link link e 2 Figure 3: Link End Connetion Fores Geometri onsiderations To minimise endplate apaity derived moment design ations the link length must be kept

3 2. Capaity derived moment demand M V link link e 2 Figure 3: Link End Connetion Fores Geometri onsiderations To minimise endplate apaity derived moment design ations the link length must be kept short. For these short link lengths the allowable interstorey drifts may be limited by the plasti rotation limits. The maximum rotation angle between the olletor beam and ative link is given by the following equation and is illustrated in figure 4. L p p e where p rotation angle of frame inelasti drift interstorey drift for an EBF Figure 4: Plasti Rotation of Link Note that the plasti rotation of the ative link is what is heked against the plasti rotation limits of NZS 3404 Clause To avoid overestimating this, the elasti drift of the EBF must be aurately determined. This an be obtained from equations 3.7 to 3.11 from (Sullivan, 2012). Steel Constrution New Zealand In

4 A seond geometri onsideration is that the link beam depth must be suffiiently smaller than the olletor beam depth to allow an extended end plate on the removable link. As a guide the olletor beam should be 240mm deeper than the ative link. System dutility and member ategories System dutility In keeping with the rapid return to oupany performane riteria for the system, the appropriate strutural dutility displaement dutility fator for EBF s with removable links is =3 (limited dutility). This has been shown from the Christhurh 2010/2011 MCE intensity earthquake series to result in buildings that effetively self entre and with demand on the EBF ative link that is unlikely to require replaement. Note that this will not guarantee no strutural repair is required, however this outome is very likely. Member ategories For onventional eentrially braed frames the ative link and the olletor beam are the same member. The Steel Strutures Standard NZS 3404 permits some inelasti ation in the olletor beam. Beause of this the olletor beam in NZS3404 is lassified as a primary seismi member for material and setion geometry requirements. In the design proedure for the EBF with removable links the olletor beam is designed for the full overstrength design ations i.e. no inelasti ation in the olletor beam. Beause of this the olletor beam an be lassified as a seondary seismi member for material, setion geometry and lateral restraint requirements. Design onsiderations Dutile onnetion design The priniple soure of design guidane for dutile moment endplate plate onnetions is SCNZ report 14.1 Steel Connet (Hyland, Cowie, Clifton, 2008) and HERA Report R4-142 (Clifton, Mago, El Sarraf, 2007). This publiation only onsiders 8 bolt MEP onnetions with property lass 8.8 bolts. The bolted end plate onnetion of the ative link to the olletor beam is designed to ahieve a balaned design joint apaity, with no signifiant dutility demand being fored into any one omponent of the onnetion and with all potential brittle failure modes proteted. The onepts in those douments ould be extended to 8 bolt onnetions with grade 10.9 strutural bolts provided these are appropriately fully tensioned (ontat SCNZ for bolt properties and methods of full tensioning) Connetions between the ative link and the olletor beam must be designed and detailed to resist the ative link overstrength shear apaity and the apaity design derived bending moment (and apaity derived axial fores if present). This is neessary to ensure that dependable shear yielding ours in the ative link before the apaity of the onnetion is exeeded. Welds to end plates Welds between the ative link and the end plate are sized to develop the design tensile apaity of the web and flanges so as to ensure dutile behaviour. Ative link flange and web welds may be either omplete penetration butt welds or symmetrial fillets welds plaed either side of the flanges or web. The ative link web is usually welded to the end plate with balaned double-sided ontinuous fillet welds. The same approah applies for the welds between the olletor beam and the olletor beam end plate. Avoid exessive deformation of the olletor beam end plate It is important to avoid exessive deformation of the olletor beam end plate to failitate ease of removal of the ative link. This is ahieved by ensuring the tension apaity of the olletor beam end plate exeeds that of the ative link end plate. This is ahieved by: 1) Making the olletor beam end plate thiker than the ative link end plate or 2) Adding a baking plate to the seond row of bolts (between the flanges) see HERA Report R4-142 for the inreased design apaity using this baking plate Design ations in bolts The design ations in bolts must make an allowane for the inrease in the bolt tensile loads due to prying ation on the bolts. The design proedure in SCNZ Steel Connet and HERA Report R4-142 inorporates the prying ation when evaluating the various modes of plate behaviour. However, this does not aount for the influene of prying on the atual tension fore in the bolt. For mode 1 behaviour, the bolt tension for this hek an be taken onservatively as the installed bolt tension, as there will be no separation at the bolt line. For mode 2, some separation is possible, however finite element analyses have shown that for fully tensioned bolts this inrease is very minor. So for mode 2 also the installed bolt tension an be used to aount for bolt prying. Steel Constrution New Zealand In

5 The installed bolt tension for a property lass 8.8 bolt is 90% of the design tension apaity so this an be used diretly in the interation equation to aount for bolt prying effets in mode 1 and 2 onnetions. If mode 3 governs for any bolts (whih an apply in 16 bolt onnetions as noted later in this paper) no shear should be assigned to these tension side bolts. The design proedure for moment end plate onnetions typially assume the bolts around the tension flange of the beam take the moment and bolts around the ompression flange take the shear. This is appropriate for moment resisting frames where shear demand is relatively small. However for EBF ative links shear demand is large, relying solely on shear transfer on bolts around the ompression flange may not be suffiient. When this is the ase, onsideration should be given to additional shear transfer via the bolts around the tension flange of the ative link beam. The shear apaity of the bolts is downrated due to the presene of axial load using a square root interation equation, in aordane with NZS 3404 Clause Colletor beam/brae panel zone The panel is subjet to axial load and shear, figure 5. The design ations on the panel are: 1. Fore ouple due to the link moment 2. Axial fore in olletor beam 3. Shear fore from vertial omponent of brae fore 1/3 N C beam N f M C L N C beam 1/3 N C beam V C Link 3 V C Link 3 V C Link 3 V C Link N f 1/3 N C beam (a) (b) () Figure 5: Design ations on olletor beam/brae panel zone, 5a: Link moment, 5b: Colletor beam axial, 5: Shear from brae fore These ations ontribute to a omplex distribution of stresses in the panels zone. A simplified quadrati shearaxial load interation equation is proposed for the olletor beam-brae-ative link panel zone (see Figure 5 for loation of this panel zone). 2 2 * * P NP V VP NP 1.0 To utilise this interation equation simplifying assumptions need to be made about the distribution of load arried by the panel zone web, flanges and horizontal stiffeners. The first simplifying assumption is to onsider the panel zone as an upper and lower stiffened tee setion (figure 6). The tee setion flange and horizontal stiffeners will only be effetive in resisting axial loads. Steel Constrution New Zealand In

Zone 1 Zone 1 Tee Setion Figure 6: Tee setion The priniple of load superposition will be used to determine the axial design ations in eah panel zone tee setion.

6 Zone 1 Zone 1 Tee Setion Figure 6: Tee setion The priniple of load superposition will be used to determine the axial design ations in eah panel zone tee setion. The worst load ondition for a tee setion will our when the axial fore in the olletor beam is additive with the fore ouple generated by the link moment. For example in figure 5 the maximum panel zone ompressive ations will our in the upper tee setion. The seond simplifying assumption is that the olletor beam and brae loads are arried equally by eah flange and web (figure 5b, 5). An example of this panel zone hek methodology is found in Steel Advisor artile EQK This simplified hek is urrently being verified by advaned finite element investigation by the New Zealand Heavy Engineering Researh Assoiation (HERA). The results of the investigation will be reported in HERA Report R Endplate thikness The tension apaity of a moment endplate onnetion is found by onsidering three failure modes (figure 7), the apaity of the onnetion will orrespond to the mode with lowest apaity. These modes are: Mode 1 plate yielding Mode 2 plate yielding and bolt extension Mode 3 bolt failure Figure 7: Three modes of end plate behaviour The endplate is sized to ensure mode 3 behaviour has the highest apaity of the three modes. Design equations for the tensile apaity of eah end plate mode of failure are presented in SCNZ report Steel Connet while those using baking plates are given in HERA Report R To ensure the bolts do not fail in shear, there is an additional requirement to limit the plate thikness. T p <0.9d f Where T p is the plate thikness Steel Constrution New Zealand In

d f is the bolt diameter Design Ations in Colletor Beam For EBFs with removable links, the olletor beam overstrength design ations may not be redued to 80% as per lause 12.11.7.

7 d f is the bolt diameter Design Ations in Colletor Beam For EBFs with removable links, the olletor beam overstrength design ations may not be redued to 80% as per lause in NZS 3404 to avoid inelasti demand in the olletor beam. Detailing Issues Stiffener Arrangements Vertial stiffeners/end plate on the olletor beam must align with brae flanges. Similar horizontal stiffeners on the olletor beam must also align with the ative link flanges. Figure 8 shows a frature in a arpark building in Christhurh where the olletor beam flange fratured in the Christhurh earthquake due to the offset of the vertial stiffener to the brae flange. Figure 8: Frature due to Offset of Vertial Stiffener to Brae Flange Slab support over link The replaeable link setion depth is less than the olletor beam depth. This means that the floor slab is not supported aross the link length, whih ould range from m. Where the floor slab is a omposite metal dek slab, the slab support depends on whether the deking is spanning parallel or perpendiular to the ative link. In all ases during onstrution the deking will be supported off the top of the ative link by a timber paker whih is removed after the onrete has set. When the steel deking is spanning parallel to the ative link, it is effetively supported off the seondary beams on either side of the ative link. When the deking is spanning perpendiular to the ative link (ie as shown in Figure 9), then additional reinforement will be needed to span aross the gap and develop positive moment apaity in the over slab above the gap. This should be designed to arry the onentrated load speified by NZS for the oupany of the floor with this load plaed on the slab over the entreline of the ative link. If the EBF is at the edge of the slab, then there will be a full depth stop-end and the bar an be plaed near the bottom of this stop-end. If the EBF is not near the edge of the building the deking will be spanning over the link as shown in Figure 9 and the reinforement should be plaed diretly onto the top of the deking and span a development length plus the slab depth past the ative link on eah side. Figure 9: Additional Reinforement for Floor Slab aross Link Steel Constrution New Zealand In

8 Column baseplates Columns of seismi loading resisting systems are most vulnerable to plasti hinging at their bases if the base onnetions possess a high degree of fixity and the inter-storey drifts are high. To ensure the low damage performane objetives of the EBF with removable links system are realised, olumn bases should either be pinned or semi-rigid with flexural yielding of the olumn base suppressed by base rotation that ours at a lower moment than the olumn setion moment apaity. Even nominally pinned base details, suh as that shown in figure 10 will provide base fixity that should be modelled. Figure 10: A Typial Nominally Pinned Base Connetion Column base stiffness is best represented used rotational springs. The formulae for rotational springs are as follows (from NZS 3404 Clause (a) and (b) and rearranged): EI ko 1.67 L EI ko 0.1 L (Fixed) (Pin) Semi-rigid base onnetions have been the subjet of researh in North Ameria. There is a urrent researh programme in Taiwan testing pinned and semi rigid base onnetions. New Zealand researh into semi rigid base onnetions is also planned. In pratie, a detail suh as that shown in Figure 9 will provide rotational stiffness loser to that of the fixed base so the rotational stiffness for the fixed base should be used. If onsidering a semi-rigid base whih will allow rotation within the onnetion rather than the olumn itself, provisionally use a stiffness of k o 1.0 EI L Design Aids SCNZ Steel Connet Steel Constrution New Zealand report Steel Connet ontains pre-engineered moment end plate onnetions for a range of setions. These an be used as a starting point in designing the removable link end plate onnetions. The 250UC and 310UC range of standard onnetions use an 8 bolt arrangement (2 bolts in eah row). Steel Connet adapts the design method set out in (SCI P207, 1995). For appliations not overed by these pre-engineered solutions, speifi design an be undertaken using the design methodology in this doument. Final Considerations Not Suitable for Every Appliation The removable link onept presented in this artile is not suitable for every EBF appliation. This will be the ase when the onnetions ations are very high making it diffiult to ahieve pratial onnetion designs. When this ours onsideration should be given to: Use of Property Class 10.9 Bolts The ations on the bolts for the removable link onnetions are large. For some onfigurations it is diffiult to fit the number of bolts required in the onnetion if using property lass 8.8 bolts. If this is the ase property lass 10.9 bolts may be used. Property lass 10.9 bolts are available by indent in New Zealand with 3-5 months lead time. Steel Constrution New Zealand In

9 Or Use of 16 bolt onnetions An example of a 16 bolt MEP onnetion is shown in figure 11. There is limited design guidane for a 16 bolt MEP onnetion. Note that a 16 bolt arrangement requires wide endplates and hene wide olletor beam and ative link flanges. For a short ative link operating in the shear mode, the flanges are not prone to loal bukling hene their slenderness ratio an be taken as that for ategory 3 members if subjet to design axial load aross the ative link and up to the yield limit from NZS 3404 Table 5.2 if the ative link is not subjet to design axial load aross the ative link. Figure 11: 16 Bolt Moment End Plate Connetion Custom Welded Links Using ustom welded links allows the strength of the link be tailored to the demand. This redues the apaity derived ations on the rest of the EBF members. Custom welded links are more expensive than a hot rolled setion of equivalent weight. However the savings in ost of seondary elements may more than offset the inrease in ost. Full strength butt welds are required between the web and flange of welded links. More effiient transportation of frame elements The removable link onept allows the olletor beams and braes to be welded to the EBF olumn. This an be done over 3 4 stories and transported as one element to site. Alternatively the ative link an be bolted into the olletor beams in the fabriation shop and the olletor beam/link/brae subassembly transported to site in the same manner as for a welded ative link/olletor beam/brae system. A variation on this with the SHS brae is that the olletor beam/ative link is shop assembled and brought to site, with the SHS being birdsmouthed, fitted over the gusset plate on site, pinned in plae with a loator bolt as shown in Figure 1 and site welded. That system has been suessfully used on several multi-storey EBF and CBF braed frames in New Zealand (see eg HERA Steel Design and Constrution Bulletin, Issue No 46, pp 16-17). Conlusion A method for designing eentrially braed frames with replaeable links has been presented in this artile. The replaeable link uses moment end plate onnetions to onnet the link member to the olletor beam. Careful detailing of the onnetion is required to ensure good performane. The overall design of EBF members using the replaeable link onept is similar to onventional EBF frames. The EBF with replaeable links has several advantages, the most important being that it allows for quik inspetion and replaement of damaged links following a major earthquake, signifiantly minimising the disruption to the struture and allowing the building to be rapidly returned to servie with any link replaement being undertaken during subsequent sheduled maintenane periods. Steel Constrution New Zealand In

10 Referenes Clifton, G.C., Mago, N. and El Sarraf, R, Eentri Cleats in Compression and Columns in Moment-Resisting Connetions, HERA Report R4-142, HERA, Manukau, 2007 Cowie, K., Fussell, A., Clifton, G. C., Volynkin, D., Eentrially Braed Frames with Removable Links Conept Overview, Steel Advisor EQK1005, Steel Constrution New Zealand, Manukau, intended publiation 2013 Cowie, K., Fussell, A., Eentrially Braed Frames with Removable Links Design Example, Steel Advisor EQK1007, Steel Constrution New Zealand, Manukau, 2013 Hyland C., Cowie K., Clifton C., Strutural Steelwork Connetions Guide: Design Proedures, SCNZ , Steel Constrution New Zealand (In), Manukau City, 2008 Hyland C., Cowie K., Bird G., Strutural Steelwork Connetions Guide: Connetion Tables, SCNZ , Steel Constrution New Zealand (In), Manukau City, 2008 SCI, Joints in Steel Constrution: Moment Connetions, P207, Steel Constrution Institute, Asot, U.K., 1995 SNZ, Steel Strutures Standard (Inorporating Amendments 1 and 2), NZS 3404:1997 Part 1 and 2, Standards New Zealand, Wellington, 2007 Sullivan T.J. Formulation of a Diret Displaement-Based Design Proedure for Steel Eentrially Braed Frame Strutures, Paper no 2121, 15 th World Conferene on Earthquake Engineering, Lisbon, Portugal, Steel Constrution New Zealand In

Suggested Changes to NZS3101:2006 with Amendments 1 and 2

1 Suggested Changes to NZS3101:2006 with Amendments 1 and 2 Rihard Fenwik and Dene Cook Introdution NZS 3101: 2006 Conrete Strutures Standard is a design ode published by Standards New Zealand. Sine the