Suggested Changes to NZS3101:2006 with Amendments 1 and 2

Transcription

1 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 publiation of the seond amendment to NZS 3101: 2006 a number of questions about speifi lauses in the standard have been sent to Standards New Zealand and members of the Standards ommittee. The authors have sighted these questions and believe that a number of hanges should be onsidered to orret errors, simplify interpretation of lauses and prevent unintended onsequenes or lashes with other lauses. The suggested hanges, whih are detailed below, are made by the authors and they have not been onsidered by Standards New Zealand or the NZS3101 Standards Committee. The authors hope that in time the suggested hanges will be put forward by SESOC, as a NZS3101 ommittee nominating organisation, and subsequently onsidered for the next amendment to the Standard. We hope this paper will be of assistane to designers using the Strutural Conrete Standard in the interim. Readers are referred to the dislaimer on page 2 of this journal for the onditions of use. In addition to questions raised by pratiing strutural engineers the Department of Building and Housing (DBH) has imposed several modifiations and limitations on the Standard in order for it to be used under the New Zealand Building Code in their B1 Struture Compliane Doument, B1/VM1, effetive from September These hanges are noted and in some ases the rational for these hanges is questioned by the authors. The suggested hanges are grouped aording to the main setions in the Standard. s and omments on the suggested hanges are in italis. Setion 1 Clause 1.5 Add to the end of definition of Pier the sentene, For design purposes piers shall be onsidered as olumns. A number of other hanges are suggested to the setion on olumns to make the provisions more appropriate for the design of bridge piers and olumns where the ratio of length to width of a setion is greater than 2. Setion 2 Clause (b) (ii) This sub lause relates to the alulation of rotation in unidiretional plasti hinges. Replae the equations in (b) (ii) by; for for u This hange redues the disrepany in the alulation of the plasti hinge rotations for unidiretional plasti hinges alulated using NZS and NZS3101: The equations

2 2 are based on a limited number of analyses, whih are reported in referene 2.20 in the ommentary to NZS 3101: Setion 3 Clause 3.10 and C3.10 Questions have arisen about the ommentary to It is aepted that as the number of yles the onrete is subjeted to below zero temperature inreases so should the strength of the onrete. However, the ommentary to 3.10 provides a table whih details the number of frost yles expeted in various loations. What is not explained is the relationship between these yles and the number of yles the onrete experienes. The thermal mass of the ground and the onrete itself should have some impat on reduing the number of yles. The question has arisen as many driveways onstruted of 20MPa onrete are performing quite adequately in areas prone to many frost yles. The issue needs further debate amongst the durability group. Setion 5 Clause This lause gives the minimum speified design onrete strength, f, as 25MPa. This is in onflit with setion 3, whih deals with durability, where the permitted range of design onrete strengths is given as 20 to 100MPa. The tentative suggestion is that the minimum design onrete strength speified in be redued to 20MPa. Setion 6 Clause ; Change title to Setion properties for seismi analyses This is an editorial hange. Clause : ULS defletions to allow for post-elasti effets Change the text to: Assessment of strutural defletions and inter-storey drifts for the ultimate limit state involving seismi ations shall make due allowane for the elasti and inelasti deformation as speified in NZS Setion 7, or other appropriate referened loading standard, and for raking in onrete setions and the reinforement grade that is used as speified in NZS3101: 2006, lauses 6.8 and 6.9. This is mainly an editorial hange but with the addition of the referene to NZS as the soure riteria defining the inelasti deformation. C6.9.1: Linear elasti analysis Immediately below the title insert- C6.9.1 and C6.9.2: Setion properties and deformation of strutures Replae the 4 th paragraph by; As an alternative to the values in Table C6.6 for T and L beams the effetive seond moment of area of the beam may be taken as the value alulated from Equation 6-2, assuming the flexural tension reinforement in the potential plasti hinges at eah end of the beam just reahes its design yield stress and that the beam is supporting the gravity load assoiated with the seismi load ases. Where there is a major differene in the effetive seond moments of area for zones subjeted to negative and positive flexure the effetive stiffness properties of a

3 3 beam may be alulated using first priniples from setion properties alulated at regular intervals along the member. The effetive setion properties for individual setions may be alulated from Equation 6-2 with the 2 terms (M r /M a ) 3 being replaed by (M r /M a ) 4, see referene 6.16 in the ommentary. This hange brings the assessment of setion properties into line with the values for non seismi ases as desribed in Clause C6.9.1 on page C6-19 Replae equation C6-11(b) by- Vy L Ashear (Eq. C6-11(b)) G Where G is the shear modulus (0.4E ). y This hange orrets an error in Equation C6-11(b). Setion 7 Clause 7.1 In the definition of t, replae 0.75 A o /p o by 0.75A o /p In the definition of t o replae 0.75 A o /p by 0.75 A o /p o This hange orrets errors in the definition of the above two variables. Clause C Add two new paragraphs after the first three paragraphs in this ommentary lause. Where torsional moments are indued in members due to twist assoiated with ompatibility requirements the torsional moments may be negleted provided the requirements of are satisfied. This step may be made as the torsional stiffness of a member dereases to a small proportion of its initial stiffness when torsional raks form, whih allows torsional moments to be redistributed. A method of assessing the torsional stiffness for both equilibrium and ompatibility indued torsion after torsional raks have formed is given in C Clause : Minimum reinforement for ompatibility torsion Replae Equation 7-10 and the portion of the lause above this equation by; Where required by (b), losed stirrup and longitudinal reinforement meeting the requirements of shall be provided for a minimum torsional moment, T n, equal to or greater than the smaller of: (a) T n T, where T is alulated negleting redution in stiffness due to torsional raking, or (b) Tn 0.10 Ao t N f A g f (Eq. 7-10) Where N is taken as positive for axial ompression. This hange redues the torsional moment required by Equation 7-10 in (b). Clause C7.6.2 Replae the seond paragraph in this lause by-

4 4 The minimum stirrup and longitudinal reinforement required to satisfy Equation 7-10 orresponds to a torsional moment of approximately 22% of the average torsional raking moment. This reinforement ensures that the member has adequate dutility to enable redistribution of the torsional ations, whih ours due to the loss of torsional stiffness assoiated with torsional raking, but does not signifiantly derease the flexural or shear strengths of the member. With the suggested hanges to the minimum amount of torsional reinforement for ompatibility indued torsion is signifiantly redued from that previously required in lause This hange brings the requirements of the lause into line with ACI-318: The previous edition of NZS3101 (1995) stated that the torsional reinforement should be designed to sustain the torsional raking moment, though the equation for alulating this moment was only given in the ommentary. Clause Design moment for torsion Replae existing lause by; Where the torsional ation, T, exeeds the limit given by , losed stirrup and longitudinal reinforement shall be designed to meet the requirements of for a nominal torsional moment, T n, whih is equal to or greater than the larger of: (a) Tn 0.10 Ao t N f A g f or, (b) T n T, where T is required for equilibrium and N is taken as positive for axial ompression. C and C : Torsional design moment and torsional reinforement Replae the first paragraph by- Where a torsional moment required for equilibrium exeeds the limit given in , the required nominal torsional strength is taken as the larger of T or the torsional moment orresponding to the value given by the equation in (a). The equation given in (a) orresponds approximately a quarter of the average torsional raking moment. In pratie there is a wide variation in the torsional shear stress that initiates diagonal raking. The reinforement requirements orresponding to equations in (a) and (b) ensure that the member has adequately dutility to prevent a brittle failure in the event of an over-load or in a situation where torsional raking ours at a low torsional shear stress. Setion 9 Clause ; Minimum area of shear reinforement Replae () by; () In insitu slabs with a depth equal to or less than 400mm, and in omposite floor slabs ontaining preast prestressed units where the maximum lear spaing between the webs is equal to or less than 750mm and the overall depth is equal to or less than 400mm; The hange () should remove possible onfusion in the interpretation of this sub-lause. The DBH requires a hange to the sub-lause () and this is addressed together with other DBH requirements later in this paper. Setion 10 Clause 10.2: Sope To the end of the paragraph add-

5 5 The requirements for piers shall be the same as those for olumns. The referenes to piers in , , and are redundant and ould be deleted. Clause Replae the line below (Eq. 10-1) by; () The slenderness ratio, whih is given by k L u, is equal to or less than 100; r Where L u is defined in , k is defined in and r is defined in The slenderness limit, whih was in earlier versions of the standard, had been omitted in the urrent version of the Standard. Clause Replae the existing (b) by; (b) Within the dutile detailing length for dutile plasti regions the nominal shear stress resistane of onrete, v, is given by- N o v 3vb (Eq ) A g f This hange orrets an error in the existing (b) in this lause. Clause Add a new sub lause after sub lause (a) and re-label the urrent sub lause (b) to (). The new sub lause is- N (b) For retangular olumns where h/b >2.0 and o and the seismi fores A f indue bending moments about the strong axis, the reinforement normal to the longer side, h, shall be equal to or greater than the appropriate value given below. (i) In the end regions of the olumn, whih are outside the mid region defined in Ash , the minimum area of a single tie, A t1, is given by At 1 st ; h (ii) In the mid region defined in the minimum area of a tie may be redued to Ash At st ; h (iii) In all ases the total area of transverse reinforement shall equal or exeed the area speified in ; Where A sh is given by Equation 10-40, and s t is the spaing of the ties normal to the longer side of the olumn. This is a suggested addition to the lause, whih brings the onfinement requirements of onrete into line with the existing relaxation allowed for the spaing of longitudinal bars in olumns with high length to width ratios, see lause C to C Add to the end of the existing ommentary- The sub lause () in is intended to over the ase whih frequently arises in bridge piers. In Setion 10 bridge piers are treated as olumns. In these strutural elements g

6 6 the predominant seismi fores often at in the plane of the pier induing high ompression stresses in the end zones of the pier setion. Consequently in the dutile detailing length these zones need to be onfined to ensure that dutile behaviour an be sustained. However, the mid regions of the pier are only required to sustain relatively low ompression stresses and onsequently the level of onfinement to the onrete in this area may be redued. It should be noted that all longitudinal bars in the pier should be restrained against bukling as required in sub lause (). In addition, the minimum requirements of apply over the whole setion and the full length of the olumn. Setion 11 General omment on hapter 11 This hapter overs detailing requirements for walls, whih range from slender tilt up walls to major strutural walls in multi-storey buildings. There has been limited researh on the stability of tilt up walls and on the shear performane of major strutural walls. Consequently this hapter was diffiult to write and not all the situations that may arise in pratie have been adequately overed by the Standard. Clause Move the label for Equation to the RHS of the page; k ft Ln 65 t This is an editorial hange. (Eq ) Clause Replae title and lause by Shear strength provided by onrete and maximum shear strength (a) Shear strength provided by onrete In wall subjeted to an axial load,. N, the onrete shear strength provided by onrete, V, in the end region defined by shall not exeed N V 0.27 f 0.0 A 4 v (Eq ) Ag Where λ = 0.25 for dutile plasti regions λ = 0.5 for limited dutile plasti regions N the design ation axial fore is taken as negative for tension. (b) Maximum nominal shear strength The nominal shear strength, V n, shall be equal to or smaller than- ow Vn 0.15 f Av vmax Av (Eq ) Where v max is given in α = 3.0 for limited dutile plasti regions α = 6.0 for dutile plasti regions as defined in Table 2.4. Linear interpretation for α may be used between the values given above when the alulated urvature dutility lies between the limits provided in Table 2.4 for limited dutile and dutile plasti regions. The suggested hanges to the text are editorial in nature and they should improve the larity of this lause.

7 7 Setion 12 Clause Two lines below Equation 12-8, replae 1.0 k 0. 5 by 0.5 k This orrets an error in the Standard. Setion 17 ds ds Clause 17.1 Notation In the definition of 2, replae by 1. Replae the existing definition of 1 by- 1 the distane from the entre of an anhor to the edge of the onrete in the diretion in whih the load is applied, mm Clause , page 17-8 The urrent wording for the 1 is a opy of ACI-318: However ACI has amended the definition so the intention is to adopt the latest definition from ACI-318. In the last sentene of the definition of 1 delete shall be limited to h/1.5 and replae with shall not exeed the greater of 2 /1.5 in either diretion, h/1.5, and one third the maximum spaing between anhors within the group. Clause , page 18-6 Replae the first two lines of text by- Where hollow-ore flooring is adjaent to a beam, a wall or other strutural element, whih may deform in a diretion normal to the plane of the floor, either; As written in the Standard a flexible linking slab is required where a hollow-ore unit is adjaent to a beam whih is parallel to the span of the hollow-ore units. When a plasti hinge forms in suh a beam it deflets in a vertial diretion relative to the floor. This differential movement has been observed to ause extensive raking in the webs of adjaent hollow-ore units separating the top and bottom flanges of the hollow-ore units and ontributing to the premature ollapse of a floor. However, other strutural elements suh as walls or beams in eentrially braed frames may also generate differential displaement relative to the plane of the floor. The suggested modifiation generalises the requirement for linking slabs to be used in all situations where vertial displaement an our between a strutural element and an adjaent hollow-ore unit. Clause C on page C18-9 In (a) replae or 1.0 where lower dutility reinforement is used by or 1.25 where lower dutility reinforement is used The smallest value of strutural dutility fator in NZS3101 is APPENDICES Appendix A Clause A5.2: Effetive ompressive strength of onrete strut Replae α 1 is given by () by α 1 is given by () Appendix D Clause D3 2 3: Dynami magnifiation and modifiation fators In part () replae the seond paragraph by-

8 8 In the top storey the minimum value of shall be equal to or greater than 1.2. This value had been omitted in error from the Standard. Restraint of topping onrete above preast units Questions have been asked about the need to restraint of topping onrete above preast units where this topping onrete is subjeted to a shear stress of more than 0.3 f due the transfer of lateral fores to the lateral fore resisting elements (walls or frames). NZS310:1995 lause required onrete topping to be restrained by a nominal amount of ties when the shear stress sustained by the topping onrete exeeded the limit of 0.3 f. In the urrent edition this requirement has been removed exept for the ase for dutile diaphragms, whih are required to dissipate energy by inelasti deformation, see Clause For these unusual elements, nominal ties are required regardless of shear stress that the dutile diaphragm is required to sustain. This requirement will seldom be ritial as it is only in exeptional situations that a dutile diaphragm is used. Where a dutile diaphragm is used nominal ties are required as yli inelasti deformation in the reinfored onrete topping an lead to extensive delamination between the topping and preast floor units. The general requirement for tie reinforement between topping and preast units when the shear stress exeeded the limit of 0.3 f was not maintained in NZS 3101: 2006 for the following reasons; Any tie between preast and topping onrete is unlikely to be fully effetive due to the diffiulty of anhoring the ties in the thin layer of topping onrete; Design for a diaphragm is in general based on a strut and tie analysis, whih makes it diffiult to assess the equivalent shear stress for a diaphragm, or of a region in a diaphragm. No theoretial or experiment evidene was found to justify the requirement for these ties. Amendments by DBH for use with B1/VM1 (refer to Clause 4.8 When AS/NZ1170 was ited by the DBH, some modifiations were made to the load ases required to be onsidered during and after fire. These neessitate some small modifiation to lause 4.8 of NZS3101. The amendment to B1 of the ompliane douments inluded the itation of NZS3101 and inluded a small modifiation to lause 4.8 of NZS3101: It is our intention to inlude these modifiations in the amendment of NZS3101. Clause : Minimum area of shear reinforement In many situations for beams and slabs nominal shear reinforement is required if the design shear fore exeeds 50 perent of the design shear strength provided by the onrete V 0.5V. This lause details the situations where this requirement need not be applied. The DBH amendment hanges part () of this lause, whih relates to the ase of a floor slab that may be onstruted either, entirely with insitu onrete or, from preast pretensioned units, suh as suh as ribs, double tees and hollow-ore units, joined together with insitu onrete that generally takes the form of a reinfored onrete topping. The urrent sub-lause () indiates that nominal shear reinforement is not required where the total slab thikness is equal to or less than 400mm. This limit applies to both the insitu floor

9 9 and the omposite preast floor provided in the latter ase the web spaing does not exeed 750mm. For floors whih satisfy this ondition when the shear fore, V, is less than V, nominal shear reinforement is not required. The DBH modifiation hanges this to; () in slabs, inluding floor slabs ontaining pretensioned units, where the maximum lear spaing between webs is equal to or less than 750mm; and the depth of the preast unit is equal to or less than 300mm. Comment The DBH modifiation has 2 signifiant effets, whih are noted below. 1. The previous requirement to have nominal shear reinforement in one way insitu slabs for beam type shear where the depth is greater than 400mm and the design shear fore, V, lies between 0.5V and V has been removed. This hange is very signifiant as some deep slabs that would have required shear reinforement aording to the existing lause will no longer require suh reinforement. This redues the fator of safety of these strutural elements. The provision in NZS3101: 2006 was added due to analytial and experimental work arried out by Collins and others, whih showed that deep slabs ould have signifiantly lower shear strengths than had previously been assumed, see referene [1] and other referenes in the ommentary to NZS3101: With omposite slabs there is also a signifiant hange in that the depth of insitu onrete added above the preast units ould be greatly inreased without invoking the V 0.5V rule, whih requires nominal shear reinforement to be used. A onsequene of this is that greater depths of insitu onrete an now be used. For the ase where shear is ritial near a support in negative moment zones this is a rational hange, whih will safely redue the level of onservatism inherent in the existing provisions in the Standard for the ase where the insitu onrete depth is greater than 100mm. This redution in onservatism with the DBH hange arises as nominal shear reinforement is no longer required if 0.5V V V. However, where shear is ritial in positive moment zones and the flexural shear ondition is ritial the inrease in depth redues the shear stress that an be sustained at shear failure. In this situation the shear stress that results in failure dereases with an inrease in the total depth (preast and insitu onrete topping). Hene exluding the overall depth from the 0.5V rule redues the fator of safety where the overall depth of the hollow-ore unit and insitu onrete topping exeeds 400mm and high onentrated loads at in the mid-span positive moment region of the floor. This situation may only our in unusual situations. The shear stress that an be transferred aross a rak redues as the rak width inreases. With inreasing slab depths the spaing of primary raks inreases and this results in wider raks in the mid depth region of the flexural tension zone and a redution in shear transfer by aggregate interlok ation in these regions. This results in a redution in the shear stress level that an be sustained prior to diagonal raking (shear failure) as the depth inreases in insitu onrete slabs, see referene [1]. Hene with the DBH modifiation the fator of safety is redued. The flexural shear strength of prestessed onrete beams and slabs depends to an appreiable extent on shear transfer aross raks by aggregate interlok ation. Close to a support negative moments an at and flexural raks an extend through the insitu onrete into the top of the preast units and into their webs. However these web raks are loated lose to the neutral axis, and onsequently the rak widths are small and high shear stresses an be

10 10 sustained aross this region of these raks. This enables the omposite floor to sustain a relative high shear stress in the zone where the web width is a minimum. In the region where the raks are wide the width of onrete available to sustain the shear flow is large and onsequently the shear stress is low. Hene the shear strength is high ompared with that of typial a retangular or Tee beams from whih the shear design equations were developed. The situation is different for flexural raking in positive moment zones. In this ase the flexural raks develop from the bottom fibre and extend into the web. Consequently, the ritial raks in the flexural tension zone are in a region of high tensile strain, whih results in wide raks and a redution in the shear stress that an be sustained by aggregate interlok ation. For the positive moment region the greater the onstrution depth (preast plus insitu onrete) the lower the shear stress that an be sustained by the onrete in the ritial region of the web. In NZS3101: 2006, the limit of 400mm was used to over both ases. This was a onservative assumption for negative moment shear strength near a support but a reasonable limit for shear strength provided by onrete in the mid region positive moment zone. The DBH hange redues the onservatism for floors with thik insitu onrete toppings in the negative moment zones, whih is not an issue, but does not aount for the antiipated redution in flexural shear strength in positive moment zones where a thik onrete topping is used. Hene the DBH hange ould be un-onservative in some unusual situations for omposite floor slabs. Clause : Floor or roof members supported by bearing on a seating The DBH require an amendment to this lause, whih is to add to the end of lause (e) to following sentene. The details given by C19.6.7(e) may be applied to hollow-ore units where the depth of the preast unit is equal to or less than 300mm. Comment There is no lause or ommentary lause (e). However, there is a ommentary lause C18.6.7(e), whih is probably what was intended by the DBH. The intent of this modifiation is to prevent the detail desribed in this sub lause from being used where the depth of the hollow-ore units exeeds 300mm. No rational explanation was given for this modifiation. The lause C18.6.7(e) detailed a method of support for hollow-ore units. It was tested in a large sale floor test using 300mm deep hollow-ore units [2]. Experimental results showed that this detail was superior to all other support details that were examined, whih is not surprising as this an also be shown using standard strutural theory. The reommended detail, whih was illustrated in Figure C18.4 is also illustrated below in Figure 1. Two ells broken out and filled with insitu onrete and one 16mm plain round bar in eah ell Figure 1: Reommended support detail for hollow-ore floor units Conept behind reommended detail Two ells at the end of eah unit are broken out and filled with insitu onrete. Eah of these ells is reinfored with a single a 16mm plain round bar, whih is plaed in the bottom of the

11 11 ell. The addition of the insitu onrete in the ells inreases the flexural raking strength of the hollow-ore unit and hene it inreases the likelihood that when relative movement ours between the unit and supporting beam, due to relative rotation or elongation, a rak will form at the bak fae of the hollow-ore unit rather than in the unit lose to the fae of the supporting ledge. The 16mm bar in effet laps the pretension strands, whih gives the support zone to the unit some positive moment flexural strength. Without this reinforement the eentri gravity load from the unit indues torsion in the support beam, whih an be a problem as the development of plasti hinges in a beam destroys torsional resistane. The positive moment flexural strength provided by the 16mm bars allows redistribution to our with the torsional moment in the support beam reduing and the reation from the hollowore unit effetively moving to the entre-line of the beam setion. The 16mm bar needs to be plaed at the bottom of the ell. In this position it laps the pretension strands and it is in the optimum position for inreasing the positive moment flexural strength. Plaing this reinforement higher up in the ell has the adverse effets of; Inreasing the negative moment that an be transferred to the hollow-ore floor, whih ould require additional reinforement to be plaed in the insitu onrete in the span of the floor to resist the indued negative moments; Inreasing the length of the negative moment zone that an result in a redution of the shear strength of the hollow-ore floor. Plain round 16mm bars are used to allow yielding to extend along the bar in the event that elongation of beams parallel to the span of the hollow-ore units opens up wide raks between the floor and supporting beams. In addition in the unlikely event that the hollowore unit is pulled off the supporting ledge the 16mm bars an prevent omplete ollapse by developing a kink in the bars, whih enables eah unit to safely resist a gravity load of 120kN without ollapsing. Sale effets our in reinfored onrete members. These arise: When in modelling a member the ourser aggregate sizes are redued. This hange an lead to the ratio of tensile strength to ompressive strength inreasing, whih in turn results in the model sustaining a higher shear stress at failure than the full sized member; The flexural tensile stress, whih results in the formation of a flexural rak, dereases with an inrease the depth of the member due to non-linear behaviour of onrete in tension [3]. Consequently, where a strength depends on the flexural tensile strength of the onrete inreasing the depth of a member redues the stress whih results in failure; Inreasing the depth of a member inreases the spaing of primary flexural raks and this inreases the rak width, whih redues shear transfer by aggregate interlok ation. As a result the shear stress sustained at diagonal raking in members without shear reinforement redues with inreasing depth of the member [1]. None of the above effets influene the flexural strength of reinfored onrete and onsequently there appears to be no rational explanation of why the reommended support detail in C18.6.7(e) should be limited to hollow-ore units with a depth equal to or less than 300mm. This modifiation by the DBH is likely to redue the robustness of hollow-ore floors built with hollow-ore units with a depth greater than 300mm. Referenes 1 Collins, M P and Kuhmas, D., How safe are our large lightly reinfored onrete beams, slabs and footings?, ACI Strutural journal, Vol. 96, No. 4, July-Aug. 1999, pp

12 12 2 MaPherson, C J., Seismi analysis of preast onrete hollow-ore floor super-subassembly, ME Thesis, Civil Engineering, University of Canterbury, CEB-FIP, Comite Euro-Internationale du Beton and Federation Internationale de la Preonstrainte, CEB-FIP Model Code, 1990, Thomas Telford, London, 1993 Aknowledgements The authors aknowledge with thanks the ontributions made by Stuart Ng from Standards New Zealand, Ashley Smith and Don Kirkaldie from the NZS3101 ommittee and the other strutural engineers who raised questions on the validity or interpretation of lauses in the Standard. UNREFERREED