g 500 Lne Article

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L, N and E Grade 500 Reinforcing SteelL, N and E Grade 500 Reinforcing SteelL, N and E Grade 500 Reinforcing SteelL, N and E Grade 500 Reinforcing Steel

D. BullD. BullD. BullD. Bull 1111, C. Allington, C. Allington, C. Allington, C. Allington 2222

SummarySummarySummarySummary

The advent of the Joint Australian/ New Zealand Standard AS/ NZS 4671:2001, SteelReinforcing Materials has resulted in the introduction of three classes of Grade 500reinforcement into the New Zealand marketplace. A number of issues have beenraised. This paper discusses implications for the classes of Grade 500: L, N and E,with respect to elongation capacities of each, as well as: bond performance, stiffnessof members, flexural overstrength, fatigue resistance and site issues. A number ofrecommendations are made.

1.1.1.1. IntroductionIntroductionIntroductionIntroductionGrade 430 reinforcing steel is being withdrawn from the marketplace in New Zealand by April 2002.

The replacement steel, Grade 500, has a higher strength but lower ductility than the old Grade 430reinforcement. In addition the new joint Australia and New Zealand reinforcing steel standard,AS/NZS 4671:2001 Steel Reinforcing Material [1], has included three different ductility classes ofGrade 500 reinforcement; Class L (low ductility), Class N (normal ductility), and Class E (highductility for Earthquake prone regions). Class L steel includes cold worked wires and welded wiremesh. Class N and Class E steels include hot rolled deformed bars. The mechanical properties ofthe new steels, specified by characteristic values, are summarised in Table1.

Table 1 Characteristic Mechanical Properties of Reinforcing Steels

Property 500L 500N 500E 430

Yield Stress Rek,L 500 500 500 410

Rek,U 750 650 600 520

Ratio Rm/Re 1.03 1.08 1.15 1.15

- - 1.40 1.50

Uniform Elongation Agt (%) 1.5 5.0 10.0 10.0

Rek,L and Rek,U are the lower and upper characteristic yield stress of the material respectively, Rm is theultimate tensile strength, and Agt is the strain in the steel corresponding to the maximum stress in thebar, also defined as the uniform elongation. The change from Grade 430 reinforcement toGrade 500 will have a number of implications in the behaviour of concrete structures, particularly

about ductility of bars and overstrength effects. The uniform elongation, (Agt) of the new Grade500N steel has a minimum requirement that is half that of Grade 500E. In addition, Grades 500L

1 Technical Director, Holmes Consulting Group Ltd

Adjunct Senior Fellow, Department of Civil Engineering, University of Canterbury

2 Design Engineer, Holmes Consulting Group Ltd

Teaching Fellow, Department of Civil Engineering, University of Canterbury


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and 500N do not have an upper limit to the ratio of the ultimate tensile stress to the yield strength(Rm/Re). Both of these issues will affect the available ductility of reinforced concrete members andstructures. This paper investigates the implications of introducing the three classes of Grade 500reinforcement into the New Zealand marketplace.

2.2.2.2. BondBondBondBondIn reinforced concrete members it is essential to have a good mechanical bond between thereinforcement and the concrete to allow stresses to be developed. The two most critical zones forbond in a reinforced concrete building are the lap splice zones between longitudinal reinforcement,and the bond on the longitudinal reinforcing bars passing through beam-column joint regions withyielding elements adjacent to the joint.

The existing design rules in the New Zealand Concrete Structures Standard, NZS3101: 1995 [2] forthe maximum bar sizes passing through the beam-column joint regions adjacent to yielding regionswere derived from research completed on reinforcing steel with different mechanical properties thanthe new grades of 500 MPa steel. Research completed by Lin et al. [3] on eight beam-column

cruciform sections with Grade 500E longitudinal reinforcement in the beam elements found theexisting design rules in NZS3101: 1995 did not conservatively estimate the bond stresses developedon the Grade 500E beam steel. Testing undertaken by Young et al. [4] at the University of Aucklandfound similar results.

Due to the lack of information regarding the bond capacity of Grade 500 reinforcement Pacific SteelLtd have commissioned a series of beam-column joint tests to be undertaken at the University ofAuckland. The results from the testing will be used in the review of the limitations of thelongitudinal bar diameter in beam-column joints for NZS3101: 1995 [2]. Results are expected to beavailable from the testing programme in the later half of 2002.

3.3.3.3. DuctilityDuctilityDuctilityDuctilityThe majority of reinforced concrete structures in New Zealand are designed to be ductile underoverload conditions by forming plastic hinges in the beam elements. Each plastic hinge zone mustbe capable of undergoing large amounts of inelastic deformation with little or no reduction in loadcarrying ability.

The ductility of a concrete member is dependent on the yield curvature, y, which is directly relatedto the yield strain of the longitudinal reinforcement, as shown below:

( )cd

y

y

=

Eqn. 1

Wherey is the yield strain of the reinforcement, d is the depth of the longitudinal reinforcement inthe concrete member and c is the neutral axis depth. When the longitudinal reinforcement is

changed from Grade 430 to Grade 500 the yield curvature, y, increases by the ratio of 500/430

(16%). According the yield displacement,y, of the member will also increase by 16%. The effect ismore pronounced when the reinforcement is varied from Grade 300 to Grade 500 where the yieldcurvature and yield displacement both increase by the ratio of 500/300 or 67%. The increase in yield


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displacement is a function of the increased yield strength of the longitudinal reinforcement andtherefore is independent on the class (L, N or E) of the reinforcement.

The displacement ductility of a concrete member is defined as the ratio ofu/y, whereu is the

ultimate displacement andy is the yield displacement. If the longitudinal reinforcement used in theconcrete member has a sufficiently large value of uniform elongation, Agt, such as Pacific Steel LtdGrade 300, 430 and 500E, then the ultimate displacement of the member may be limited by themaximum allowable inter-storey drift of the structure, in accordance with NZS4203: 1992 [5]. Inthese situations the displacement ductility of a concrete member constructed with Grade 500E

reinforcement will have a displacement ductility,u/y, that is 16% smaller than that of an identicalmember constructed with Grade 430 reinforcement, and 67% smaller than a member constructedwith Grade 300, due to the difference in the yield displacements [6].

Grade 500L and 500N reinforcements have lower allowable uniform elongation, Agt, in the newreinforcing steel standard, AS/NZS 4671: 2001 [1], than Grade 500E reinforcement, as shown in

Table 1. In these situations, assuming the reinforcement has a uniform elongation capacity equal tothe minimum values specified in Table 1, then the ultimate displacement of a beam constructed withGrade 500N steel would be approximately 50% smaller than an identical beam constructed withGrade 500E reinforcement. As a result the displacement ductility of the Grade 500N beam would be

approximately equal to half the ductility of the Grade 500E beam and 43% of a beam constructedwith Grade 430 reinforcement. If the Grade 430 beam was design to achieve a displacement ductilityof 6, the Grade 500E beam would have a ductility of 5.2 and the ductility of the Grade 500N beamwould be 4.2. Various researchers in New Zealand [6, 7, 8] have reported similar findings.

The reduced level of ductility associated with the Grade 500 steel not only affects the main seismicelements in a structure but all elements that are required to undergo inelastic displacements. Thisincludes all members that have redistributed forces, any secondary elements that are tied to theprimary frame, either directly or via a diaphragm, and for slabs or diaphragms that are required to tielateral load resisting elements together. For example, NZS 3101, Cl. 13.4.3.2, requires that adequatereinforcement be provided to tie the columns in to the floor diaphragms, because beam elongation inPlastic Hinge Zones causes large disrupting cracks to form around the columns. Though this

reinforcement is sized, based on fy, it is expected that very large plastic elongation is required of theseties, accommodating cracks in the diaphragm of up to 15 mm or more. The rationale behind thisclause was developed on that fact that reinforcement of high elongation capacity was to be used(Grade 300 E and Grade 430). Similarly, the continuity reinforcement (beam-to-floor) next to PlasticHinge Zones is expected to have to deal with the large cracks that are expected to form because ofbeam elongation.

By using Grade 500L and 500N steel in structures the level of inelastic deformations that can beachieved in these members will be significantly reduced from what is available with either Grade 430or Grade 500E.

In structural elements supporting gravity loads, such as slabs and beams, redistribution of moments(effectively plastic design) is permitted [2]. Studies undertaken by a number of concernedAustralian researchers [9, 10, 11] have shown that the available capacity for moment redistribution issignificantly reduced in a number of situations when Grade 500L and 500N reinforcement is used.

Further, in New Zealand, where continuity reinforcement is provided for deflection control andsome reduction of mid-span moment in precast prestressed concrete floor systems, it is expected thatthis continuity steel needs to yield significantly at the Service Limit State. The plastic elongation ofthese bars is expected on de-propping of long span floors and accommodation of the shrinkage andcreep shortening of the floor systems. These conditions typically result in one or two large cracks in


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the topping concrete at the interface of floor and supporting beam/wall. Therefore, any elongationof the continuity steel is concentrated at one crack. The real in-place compressive strength of theconcrete in the topping and the supporting element can effect the amount of yield penetration eitherside of the dominant crack. This yield penetration could be between 2 and 5 times the diameter ofthe continuity bar. This dominant crack can have widths ranging from hairline up to 3 mm. Thelocal plastic deformation required of the continuity bar could be as large as 4-5% strain. This

demand corresponds with the minimum specified uniform elongation capacity (5% or 20y) requiredof Grade 500 N and is about 3 times larger than the minimum specified uniform elongation capacity

of Grade 500L (1.5% or 6y).

Due to the reduced level of ductility in structure designed with Grade 500L and 500N steel it couldbe recommended that elements which require any form of ductility, either through seismic action,gravity moment redistribution, or imposed displacements be designed with Grade 300E or 500E aslongitudinal reinforcement.

4.4.4.4. StiffnessStiffnessStiffnessStiffnessTo obtain the same flexural capacity a reinforced concrete beam member designed with Grade 500reinforcement would require less area of reinforcing steel than if designed using Grade 430 steel.Intuitively the member with the smaller amount of Grade 500 steel would have a lower stiffness thanthe member with the greater amount of Grade 430 steel. This can be shown using the transformsection method to determine the element stiffness. As a result it is clear that the stiffness of anelement designed with Grade 500 reinforcement will not be the same as the stiffness of a memberwith identical dimensions constructed with Grade 430.

Recent advances in the understanding of member stiffness has determined that the stiffness of aconcrete member is equal to the yield force divided by the yield displacement of the member, asshown below [8, 12];

y

i

iVk

= Eqn. 2

Where ki is the sectional stiffness of the member, Vi is the nominal strength of the element andyisthe yield displacement of the member. The yield displacement of the member is directlyproportional to the yield strain of the longitudinal reinforcement, as described earlier. Therefore iftwo elements with identical dimensions and designed with the same strength but reinforced withGrade 430 and 500E reinforcement respectively they will have different stiffness, with the Grade 430member having a greater stiffness.

When completing a structural analysis of a building it is necessary to estimate the stiffness of themembers to establish the distribution of forces and level of displacements. NZS3101: 1995 [2]provides guidance on how to estimate the stiffness of various structural elements. Theserequirements were derived for members reinforced with Grade 300 and 430 reinforcement. It isunclear whether the same expressions can be used to accurately estimate the stiffness of membersdesigned with Grade 500 reinforcement.


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5.5.5.5. Flexural OverstrengthFlexural OverstrengthFlexural OverstrengthFlexural OverstrengthThe actual material properties of reinforcing steel can differ significantly to the values used bydesigners. Typically, designers use the lower 5th percentile yield strength, fy, when determining thestrength of a concrete member. However in practice the reinforcement will commonly have asignificant higher value of yield stress. In addition the strains that occur in the longitudinalreinforcement in plastic hinge zones will commonly be sufficiently large to result in strain hardeningof the reinforcement. This further increases the strength of the member above that calculated duringthe design process.

When using capacity design procedures it is necessary to be able to predict the maximum likelystrength of a plastic hinge zone so that all other elements in the structure can be designed to remainelastic [14]. In order to accurately determine the maximum likely overstrength capacity of a plastichinge zone it is necessary to statistically determine the properties of the reinforcing steel used in thedesign. In 1986 Adriono and Park [15] completed a detailed statistical analysis of a sample of 5 yearsof production of the reinforcing steels available in New Zealand. The study determined statisticaldistributions of the major material parameters of the reinforcement that influence the behaviour ofconcrete members, including the yield stress and ultimate tensile strength. The results from the studywere used to derive a flexural overstrength factor for use in seismic design, taking into account the

likely variations in the steel properties.

The new steel reinforcing material standard, AS/ NZS 4671: 1992 [1] does not place an upper limiton the ratio of the ultimate tensile stress, Rm, to the yield stress, Re for Grade 500N and 500Lreinforcement. If the reinforcement has a large, uncontrolled Rm/ Re ratio then there is no certaintyon how much strain-hardening and flexural overstrength is generated. Hence the surety associatedwith capacity design very difficult to achieve. Consequently, the overstrength factor of 1.25,derived by Adriono and Park for Grade 430 reinforcement, may not be applicable for the Grade 500steels. Park postulated that the overstrength factor might be higher than for Grade 430reinforcement, citing the previous overstrength factor of 1.4 used for Grade 380 reinforcement [7].

This could have a large implication on the design actions that are required for all elastic members in aductile structure to ensure they remain elastic under overload conditions.

Further concerns are, with respect to a too low Rm/Re ratio, that the region of yielding in a bar doesnotpropagate along the length of the bar, but rather concentrates in one location. In a reinforcedconcrete member, this results in a small number of large cracks developing. This effect is morepronounced in slab elements where one large crack will often occur at the interface with the beamelements. The formation of one large crack will result in large strains being induced in thereinforcement at the location of the crack. Grade 500N and Grade 500L reinforcement have lowuniform elongation capacities making them less capable of resisting large levels of imposed strainbefore fracture. The Rm/Re ratio of Grade 500 E of 1.15 is considered the minimum value forestablishing the formation of a Plastic Hinge Zone.

After the Northridge earthquake (1994, Los Angeles), reinforcement in topping slabs which was colddrawn wire with similar mechanical properties as L grade, was found to have fractured [13]. This was

prevalent at the interface between slabs and supporting structure. The cold drawn wire was acting asdiaphragm reinforcement well as continuity reinforcement (not common practice in New Zealand).Particularly in the situation of starter bars/ continuity reinforcement, it is recommended that Grade500E be used with stricter controls on the ratio of Rm/Re to ensure yield penetration and a largeruniform strain capacity to resist the imposed displacements without fracturing.

Pacific Steel have been completing a comprehensive testing programme on the Grade 500Ereinforcement to develop a database of material properties that can be used to complete a similarstatistical analysis as undertaken by Adriono and Park. The results from the database will be used to


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determine a new overstrength factor for the Grade 500E steel. A conclusion is expected from thestudy by the second quarter of 2002. The overstrength factor will only be applicable for Grade 500Esteel.

6.6.6.6. FatigueFatigueFatigueFatigueThe fatigue strength of reinforced concrete is of interest for members that are subjected to repeatedcyclic loading, such as heating and cooling cycles, repetitive wind loading and traffic loads. Thefatigue strength of a reinforced concrete member is highly dependent on the fatigue strength of thereinforcement.

Helgason et al. [16] conducted a series of experimental test to determine the fatigue resistance of highstrength hot rolled reinforcing steel. From the tests, Helgason concluded that the fatigue strength ofthe reinforcing steel with a yield strength of 520 MPa was greater than an identical bar with a yieldstrength of 450 MPa. MacGregor et al. [17] found that the fatigue resistance of a sample of highyield strength American steels (fy greater than 600 MPa) was similar to reinforcing steel with yieldstrength of 300 MPa. All of the reinforcement tested by MacGregor was hot rolled deformed

reinforcing bars. Similar results were found by Zhang and Able from tests completed on highstrength inline quenched TEMPCORE reinforcement [18].

All of the steels tested by Helgason et al., Zhang and Able, and MacGregor et al. had relatively largevalues of uniform elongation. Very little research has been completed on the fatigue resistance ofhigh strength reinforcing steel with relatively low uniform elongation capacity, such as Grade 500Land 500N. However it was observed after the Northridge earthquake that a large number parkingstructures with concrete roofs had fractured all of the reinforcement tying the roof to the wallelements. In a number of buildings it was determined that the reinforcement had fractured beforethe earthquake. The probable cause of the fracture of the reinforcement was fatigue due to theheating and cooling cycles felt by the roof. The steel in these structures was cold drawn wire withsimilar properties to the new class L reinforcement in AS/NZS 4671: 2001 [13].

Due to the lack of information regarding the fatigue resistance of high strength reinforcement withlow uniform elongation capacity it could be recommended that any element subjected to cyclic loads,such a thermal movements or traffic loads, should be constructed with either Grade 300E or 500Ereinforcement.

7.7.7.7. Site Issues / ClassificationSite Issues / ClassificationSite Issues / ClassificationSite Issues / ClassificationIt was shown earlier that Grade 500L and 500N reinforcement should not be used for concretemembers that are required to undergo plastic deformations, either from imposed lateral loads,moment re-distribution, imposed displacements or cyclic loads. However a number of contractors

are now enquiring about using Grade 500N reinforcement for non-ductile members. This raisesthe issue of steel identification on a construction site and the consequences of misidentification.

Previously the only grades of reinforcement on a construction site would have been Grade 300 and430. If a Grade 300 reinforcing bar was misidentified and placed into a yielding element designed forGrade 430 reinforcement, then the member would have a lower strength than calculated. This wouldresult in the member yielding at a lower level of load and therefore requiring a higher level ofductility. However both the Grade 300 and the Grade 430 reinforcement have large uniformelongation capacities thereby ensuring that the yielding element could withstand the increase ductility


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demand without fracture of the reinforcement. The ultimate consequence of a Grade 300reinforcing bar being placed in a member designed for Grade 430 reinforcement would thereforelead to a more flexible and ductile element but would not result in the premature failure of theelement.

With the introduction of three classes of Grade 500 steel onto the market place there is a possibilitythat both Grade 500N and 500E will be used on the same construction site. If a Grade 500N bar

was misplaced into a primary yielding element instead of Grade 500E then the strength of theelement would not be affected but the displacement ductility of the member would be reduced, asdiscussed above. The reduced level of ductility could result in the member failing prematurelythereby placing additional loads on the surrounding elements. The overall consequence ofmisidentifying a Grade 500N and 500E reinforcing bar would be significantly more serious than withGrade 300 and 430.

8.8.8.8. ConclusionsConclusionsConclusionsConclusions1. It is unclear whether the existing bond requirements of NZS3101: 1995 are directly applicable

for use with Grade 500 reinforcement.

2. Due to the reduced level of ductility in structure designed with Grade 500L and 500N steel, itcould be recommended that elements that require any form of ductility, either through seismicaction, moment redistribution, or imposed displacements be designed with Grade 300E or 500Eas longitudinal reinforcement.

3. The stiffness of concrete elements designed with Grade 500 reinforcement are different to thosedesigned with Grade 430. This needs to be addressed by NZS3101: 1995 to ensure the realisticestimates of member stiffness are obtained when using the guidelines.

4. For certain regions in slabs, plastic elongation capacity for reinforcement is desirable. It isrecommended that in these situations, reinforcement should be constructed from Class Ereinforcement with the strict controls on the ratio of Rm/Re to ensure that there is reasonableyield penetration and a larger uniform strain capacity to resist the imposed displacements withoutfracturing.

5. More research needs to be completed to determine if the current overstrength factor of 1.25 isapplicable to Grade 500 E reinforcement.

ReferencesReferencesReferencesReferences

[1] AS/NZS 4671: 2001, Steel Reinforcing Materials, Standards Association of New Zealand,

2001[2] NZS3101: 1995, Concrete Structures Standard, Standards Association of New Zealand,

1995

[3] Lin. C. M, Restrepo. J. I, Park. R, Seismic Behaviour and Design of Reinforced ConcreteInterior Beam-Column Joints, Department of Civil Engineering, University of CanterburyResearch Report 2000-1, University Press, 2000


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[4] Young. K, L, Anchorage Plates and Mechanical Couplers in Seismic Resistant ConcreteFrames reinforced with threaded bar, Department of Civil and Resource Engineering,University of Auckland, Master of Engineering Thesis, October 1995.

[[3] Sozen. M. A, Moehle. J. P, Development and Lap-Splice Lengths for DeformedReinforcing Bars in Concrete, Report, The S &M Partnership, Urbana, 1990

[5] NZS4203: 1992, General Structural Design and Design Loadings for Buildings, Standards

Association of New Zealand, 1992

[6] Paulay. T, A Note on New Yield Stress, Journal of Structural Engineering Society of NewZealand, Volume 13, No. 2, September 2000

[7] Park. R, Use of the New Grade 500E Reinforcing Steel, Journal of Structural EngineeringSociety of New Zealand, Volume 14, No. 1, April 2001.

[8] Paulay. T, What Stiffness?, Journal of Structural Engineering Society of New Zealand,Volume 12, No. 2, September 1999

[9] Gilbert. R. I, The Impact of 500 MPa Reinforcement on the Ductility of ConcreteStructures - Revision of AS3600, Proceedings, Concrete Institute of Australia (2001)Conference, 2001.

[10] Patrick. M, Warner. R.F, Utilisation of Ductile of 500 MPa Steel Reinforcing in Reinforced-Concrete Structures Designed to AS3600: 2001, Proceedings, Concrete Institute ofAustralia (2001) Conference, 2001.

[11] Gravina. R. J, Warner. R. F, Moment Re-Distribution in Indeterminate RC Beams andSlabs Constructed with 500 MPa Grade, Class L and Class N Reinforcing Steels,Proceedings, Concrete Institute of Australia (2001) Conference, 2001.

[12] Fenwick. R, Davidson. B, Hunt. R, Bull. D, Stiffness of Structural Walls for SeismicDesign, Unpublished Paper, 2001

[13] Wood, S.L., Stanton, J.F., Hawkins, N.M., New Seismic Design Provisions for theDiaphragms in Precast Concrete Parking Structures , PCI Journal, Jan.-Feb. 2000.

[14] Park. R, Paulay. T, Reinforced Concrete Structures, Wiley Interscience Publications, JohnWiley and Son, 1975

[15] Adriono. T, Park. R, Seismic Design Considerations of Properties of New ZealandManufactured Steel Reinforcing Bars, Department of Civil Engineering, University ofCanterbury Research Report 86-9, University Press, 1986

[16] Helgason. T, Hanson. J, Somes. N, Corely. G, Hogstad. E, Fatigue Strength of High-YieldReinforcing Bars, NCHRP, Research Results Digest 73, June 1975

[17] MacGregor. J, Jhamb.I, Nuttall. N, Fatigue Strength of Hot Rolled Deformed ReinforcingBars, Title NO. 68-18, ACI Journal, March, 1971, pp 169-179.

[18] Zhang. H, Abel. A, Fatigue Properties of Reinforcing Steel Produced by TEMPCOREProcess, Journal of Materials in Civil Engineering, Vol. 11, No. 2, pp 158-165.

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