60 CODE PROVISIONS FOR CONFINING STEEL IN POTENTIAL PLASTIC HINGE REGIONS OF COLUMNS IN SEISMIC DESIGN

A summary is given of the provisions for both circular and rectangular confining steel in potential plastic hinge zones of columns, as specified in the seismic codes of various countries. In particular, a comparison is made between New Zealand and overseas recommendations. The background to the confining steel provisions recommended in the draft SANZ Concrete Design Code is outlined.


INTRODUCTION
Energy dissipation provided by the development of ductile plastic hinges in columns is essential to the satisfactory response under seismic loading of many structures.
In particular, a large proportion of modern bridge structures constructed in zones of high seismic activity are supported by piers consisting of one or more columns.Inelastic response of these bridge structures under seismic attack will invariably involve plastic hinging of the columns, unless mechanical energy dissipators are incorporated in the design.
Although current seismic design philosophy for multistorey reinforced concrete framed buildings, as specified by the SANZ Loadings Code , is directed towards ensuring the formation of plastic hinges in the beams rather than in the columns, it is necessary to develop plastic hinges at the base of the columns to obtain a full plastic mechanism.
Also, for one or two storey frames, and for the top storey of multistorey frames, plastic hinging in columns leading to column sidesway mechanisms,is permitted by the SANZ Loadings Code { } .
It is now widely accepted that adequate ductility of column plastic hinges can only be obtained if sufficient transverse confining reinforcement is provided to confine the concrete core of the column, to prevent lateral buckling of the longitudinal flexural reinforcement, and to provide adequate shear reinforcement.
During the ^) San Fernando earthquake of February 9, 1971 failure of the columns of several bridges and buildings could be directly attributed to inadequate confinement of the plastic hinge regions.
Nevertheless, the amount and distribution of confining reinforcement necessary to ensure adequate ductility without significant strength degradation is still a matter of some controversy.Very significant differences exist between the provisions for confining steel specified in the seismic codes of various countries.rectangular confining steel in commonly used seismic codes and makes a comparison between New Zealand and overseas recommendations .

THE CODES CONSIDERED
The overseas codes for reinforced concrete design best known to New Zealanders are probably the ACI Building Code( 3 > , the SEAOC Code) , and the tentative provisions for buildings of the ATC ^ .
In addition there is available an ACI Committee 343 report on the analysis and design of bridge structures ^ , and the California Department of Transportation has design rules ^ .An indication of current Japanese practice can be obtained from publications in English, for example ( 8 ' 9 ).
In New Zealand, the Ministry of Works and Development have developed provisions for confinement of bridge piers (10,11).also, the Standards Association of New Zealand has prepared a draft Concrete Design Code { ', which has been available for comment during 1978 and is now being redrafted into its final form in the light of comments received.The summary given in the following sections includes the provisions for confining steel of both circular and rectangular shape.This summary will not include references to design for shear reinforcement, it being understood that the transverse steel placed for confinement will also act as shear reinforcement, and that there may be cases where further transverse steel is necessary to provide adequate shear strength.
Generally the quantity of circular confining steel is expressed in terms of p g , which is the volumetric ratio of spiral or circular hoop reinforcement confining the concrete core.
A ird 4A _ sp s _ sp This paper summarises the present recommendations made for both circular and * Department of Civil Engineering, University of Canterbury.
For confining steel of rectangular shape, with or without supplementary cross ties, the quantity of transverse steel is expressed by A g ^ which is the total area of hoop bars and supplementary cross ties in the direction under consideration within longitudinal spacing s b .Sometimes ( 10 ) p is used to define rhe total volumetric ratio of rectangular hoops plus supplementary cross ties in the section confining the concrete core.AC I 318-77 Confining steel consisting of spiral or hoop reinforcement is required over the end regions of columns adjacent to moment resisting connections over a length from the face of the connection equal to the greater of the overall thickness h (h being the larger sectional dimension for rectangular columns or the diameter for circular columns), one-sixth of the clear height of the column, or 457 mm (18 in).
whichever is greater.
In both cases, the spiral bar diameter should not be less than 9.5 mm (-f in) , and the clear spacing between spirals should not exceed 76 mm C3 in) nor be less than 25 mm (1 in).
For rectangular hoop reinforcement, with or without supplementary cross ties, if P^ < <J>0.4PB , the transverse steel should be designed as for beams but the hoop bar diameter should not be less than 9.5 mm (--in) and the spacing should not exceed d/2.
If P^ > <j>0.4P b , and a single rectangular hoop is used, the area of one leg of the hoop bar in the direction considered within spacing s^ should be at least equal to where p is as calculated by the greater of Eqs. S 2 and 3 with A taken as the area of concrete core measured to the outside of the peripheral hoop.
The value of s, used should not exceed 102 mm (4 in).Supplementary cross ties of the same bar diameter as the hoop may be used to reduce the unsupported length 1, .
The hoop and cross tie bar diameter should not be less than 9.5 mm in) for longitudinal bars 31.8 mm Q~ in) diameter or smaller, or 12.7 mm Cy in) for larger longitudinal bars or bundled bars.
For spiral reinforcement, p is taken as the greater of Eqs. 2 or 3 regardless of the axial load level and detailing is as in ACI 318-77.
For rectangular hoop reinforcement, the total area of hoop bar and supplementary cross ties (if any) in the direction under consideration within spacing s^ should not be less than A , = 0.12s.h"f Vf u sh h c yh (6) whichever is greater.
The value of s^ used should not exceed 102 mm (4 in), and supplementary cross ties or legs of overlapping hoops should not be spaced at more than 356 mm (14 in) between centres transverselv.

ATC (1978)
( The SEAOC provisions appear to have been followed for fully ductile frames.ACI COMMITTEE 343 (6)   For spiral steel Eq. 2 is specified up the whole height of the column without any limit on axial load level.
There is no mention of Eq. 3.
Spacing and spiral bar size requirements are as in ACI 318-77.
For rectangular hoop steel the transverse bar diameter is specified as in ACI 318-77.
THe quantity of transverse steel is not specified, but it is stated that the tie spacing should not exceed the lease dimension of the member or 3 05 mm (12 in), except that when bars larger than 31.8mm (IT in) diameter are bundled the tie spacing should be reduced to one-half of that value.

CALTRANS • (7)
It is understood v ' ' that the current practice of the California Department of Transportation involves for circular columns the use of 12.7 mm (~ in) diameter spirals at 8 9 mm (3^-in) centres from steel with a value for f , of at least 414 MPa (60,000psi).^ These provisions apply regardless of column diameter or axial load level.

JAPANESE PRACTICE
No information was available to the authors regarding current Japanese practice , for bridges.
However, available literature indicates confining ratios are substantially less than ACI 318-77 requirements, and spiral contents for bridges are often as low as p = 0.001.
AIJ requirements for builSingsspecify a minimum rectangular hoop diameter of 9 mm (0.35 in) with spacing not to exceed the lesser of 150 mm (5.9 in), one-half of the smaller column dimension, or 7.5 longitudinal bar diameters.

N.Z. MINISTRY OF WORKS AND DEVELOPMENT, CIVIL DIVISIONiro,iiJ
Until recently the design of confining reinforcement for bridge columns was governed by the MWD provisions , 11) .This involves providxng sufficient confining steel to ensure that the available structure displacement ductility factor \i is at least 6, based on the following steps: 1.
The ratio of the structure displacement ductility factor, u, to curvature ductility factor, <f> /<J> , within the plastic hinge region i¥ calculated.
The ratio depends on geometric considerations, and the plastic hinge length which is taken to be that given by Baker's formula^-*).

2.
Calculate the yield curvature tj) , and hence the required ultimate curvature y u 3.
Calculate the "ultimate" compression strain e cu corresponding to <|> , based on a conservative idealisation for the stress-strain curve of confined concrete.

4.
Calculate the required volumetric ratio of confining reinforcement.For spiral or circular hoop steel p is obtained using s = 0.0033 { 0.29 + 150p + (0.7 -lOp )-} s s c (7) For rectangular hoops and supplementary cross ties p g is obtained using Eqs. 7 and 8 are based on the .work of Baker and Amarakoneand Chan^1 4 ), but were made significantly less conservative than the results of that work in the light of the findings of more recent tests (see the December 1977 amendment ^10 ^) .
It is understood that the MWD will be adopting the requirements of the new SANZ Concrete Design Code (at present in draft form (12)) when that SANZ code is available in its final form.
DRAFT SANZ CONCRETE DESIGN CODE (11)   The confinement provisions of the draft SANZ Concrete Design Code, DZ3101, are based on the ACI/SEAOC requirements modified to take account of the effect of axial load level.

(a) First Draft
In the first draft of DZ3101, issued for comment in 1978, the potential plastic hinge regions were specified as in ACI 318-77.
The volumetric ratio for spiral or circular hoop reinforcement in potential plastic hinge regions, when P < 0,7f^A^ was required to be not less tnan eg' = 0.45 fa (10 where P was not to be taken as less than 0.If 1 A .The diameter of spiral or eg circular hoop bar was to be at least 10mm CO.3 9 in), and the maximum centre to centre spacing of spirals or circular hoops was not to exceed the smaller of one*-fifth of the member diameter, 125 mm (4.9.in)., or six times the diameter of the longitudinal bars.
The total area of rectangular hoop reinforcement, including supplementary cross ties if any, in the direction under consideration in potential plastic hinge regions, when P Q <0.6f^A^, was required to be not less than less than The diameter of hoop or tie where P was not to be taken as O.lfiA ?c g bar was to be at least 10 mm (0.39 in), and the maximum centre to centre spacing of hoop sets was not to exceed the smaller of one-fifth, of the smaller member section dimension, 150 mm (.5.9 in),, or six times the diameter of the longitudinal bars.The yield force of the hoop bar or supplementary cross tie was to be at least one-sixteenth of the yield force of the longitudinal bar or bars it was to restrain.
Other rules were also given to ensure adequate lateral support of the longitudinal bars.
It is to be noted that for low P^f^A^ ratios, Eqs, 2 to 12 result in lower confining steel contents than the SEAOC recommendations, but that higher confining steel contents than the SEAOC recommendations are.required at high axial load levels.Eqs, 9 to 12 were the result^1 5 ^ of an assessment of moment-curvature analyses of typical column sections (16,17,18,19), which were based on idealised stress-strain curves for concrete confined by either circular or rectangular shaped confining stee.
These idealised stress-strain curves were based on a limited number of tests on small specimens but the curves used were generally considered to be conservative.
The complete stress-strain curve was used in the analyses, That is, the full extent of the "falling branch" of the stress-strain curve after the maximum stress was reached was utilised and hence no arbitrary value for the"ultimate" concrete compressive strain was assumed.
Instead, the available curvature ductility factor of the section was judged by assessing the curvature after maximum moment was reached when the section was still carrying a reasonable proportion of the maximum moment.This assessment indicated that the level of confinement provided by Eqs. 9 to 12 should be sufficient to ensure curvature ductility factors well in excess of 5 without the moment capacity decreasing to less than 80% of the moment capacity at an extreme fibre concrete compressive strain of 0.003.

(b) Revised Draft
More recently, experimental results have been obtained( 20 r^l).from tests conducted on a range of near full size reinforced concrete columns using the 10 MN capacity DARTEC servo-hydraulically controlled universal testing machine installed at the University of Canterbury in 1978.
The column sections were either 550 mm (21.7 in) square or 600 mm (23.6 in) octagonal, with a total height of 3.3 m (.10.8 ft).
A range of axial load levels between 0.21f'A and 0.7Of'A was applied eg eg and cyclic flexure imposed to simulate earthquake type loading.
The confining steel was designed according to the first draft of the SANZ Concrete Design Code '^2) or in one case to the MWD provisions^1 0 ).The square columns had transverse steel consisting of overlapping rectangular or octagonal hoops; the octagonal columns had transverse steel consisting of circular spirals.
The loading arrangement and the sections of two of the column units tested are shown in Fig. 1.Load-displacement hysteresis loops for these two units tested in the programme, which had been designed according to the draft SANZ code, are shown in Figs. 2 and 3.
In these figures the theoretical ultimate load*decreases with increasing deflection as a result of the P-A effect.
It will be seen that stable hysteresis loops were obtained at displacement ductility factors of 6 for the square column and 8 for the octagonal column.
It is of interest that maximum measured core concrete compression strains of 0.026 and 0.080 were obtained from the square and octagonal units tested, respectively, compared with ultimate compressive strains of about 0.01 predicted by the MWD provisionst 10 ).
The columns tested could have reached much higher maximum displacement ductility factors than those applied, since at the end of the tests the measured horizontal load-displacement curves were still rising.That is, the maximum load carrying capacity of the columns was not fully reached and the strength was not degrading with further increase in deflection, and this in spite of the fact that strain gauge measurements indicated that yielding of the confining steel had occurred at moderate displacement ductility factors.
Thus, the columns tested performed well on the whole; v At ^Calculated using ACI column design charts using actual material strengths and with a strength reduction factor <f> = 1.low axial load levels the concrete was confined satisfactorily, although there was some tendency for spirals to straighten between longitudinal bars.
At high axial load levels the columns demonstrated very good behavious, and confirmed the required increase in confining steel with axial load level recommended by the draft SANZ Concrete Design Code.
As a result of these test results, and a general reassessment of confining requirements, the confining requirements have been modified as follows in the revised draft of Ref.12.
The potential plastic hinge region for P < 60.3f 1 A is now recommended as e Y eg not less than the longer column section dimension in the case of a rectangular section or the diameter in the case of a circular section, or where the moment exceeds 0,8 of the maximum moment at that •end of the member.
When P e > <J)0.3f^A the potential plastic hinge region g is increased to 1.5 times the tests(20,21) that at high axial load levels the plastic hinge region tends to spread along the column, because the flexural strength at the critical section is enhanced by the larger confining steel content.Thus flexural failure could occur in the less heavily confined adjacent regions of column unless the heavy confining steel was spread over a longer length of column.
In potential plastic hinge regions, when spirals or circular hoops are used and either P @ < cj>0.7f^Ag or P g < cj)0.7PQ , the volumetric ratio should not be less than rA" P, P 3 =0 -45 te-1 fe C0 -5 + 1 -2 5 ^ = 0.12.yh CO.5 + 1.25 _ ' A c g (12) <|>f'A Y c g In potential plastic hinge regions when rectangular hoops, with or without supplementary cross ties, are used and either P < <J>0.7f'A or P <<|>0.7P, e r eg e o .the total area of transverse steel within f 1 A sh = °-12s h h " f yh (14) (.0.5 + 1.25 .£ T A ) (15) c g When the load P has been obtained using a capacity design procedure, the value of the strength reduction factor cj> in all the above equations can be taken as unity.

For both circular and rectangular 550mm Cover to hoops -40mm Cover to hoops 40mm
Pe/fcAg = 0.260 p e /f c A g s shaped transverse steel the centre to centre spacing in potential plastic hinge zones should not exceed the smaller of one-fifth of the least lateral dimension of the cross section, six longitudinal bar diameters, or 200 mm (7.9 in).

°-237 (a) LOADING ARRANGEMENT (b) SECTION OF SQUARE COLUMN (c)SECTION OF OCTAGONAL COLUMN UNIT 1 IN PLASTIC HINGE REGION UNIT 1 IN PLASTIC HINGE REG/ON
For rectangular hoops the centre to centre spacing between hoop legs or supplementary cross ties across the section, and between longitudinal reinforcing bars, should not exceed the larger of one-third of the section dimension or 200 mm (7.9 in).
Also for rectangular hoops, each longitudinal bar or bundle of bars should be laterally supported by a corner of a hoop having an included angle of not more than 135° or by a supplementary cross tie, except that longitudinal bars are exempted from this requirement if either (1) they lie between two bars supported by the same hoop where the distance between the laterally supported bars does not exceed 200 mm (7.9 in) between centres, or (2) they lie within the concrete core of the section centred more than 200 mm (7.9 in) from the inner face of the hoop.
In additon, for rectangular hoops the yield force in the hoop bar or cross tie should be at least one-sixteenth of the yield force of the longitudinal bar or bars it is to laterally restrain, including the contribution by any bar or bars exempted above from the direct support.
Eqs. 12 to 15 provide a little extra confining steel, particular at low axial load levels, mainly to compensate for the possibility of the actual value of f 1 significantly exceeding the specifieS value for f 1 (as will generally be the case) and a?so to better control yielding of the transverse steel.
It is expected that confining steel provided in accordance with Eqs. 12 to 15 will be adequate to provide for a curvature ductility factor of 4> u /<j> of at least 20, which should allow topical bridge piers or building columns in one storey to reach a displacement ductility factor of at least 8.
The revised draft of DZ3101 also allows the amount of confining steel in potential plastic hinge regions to be reduced to one-half of that required by Eqs. 12 to 15 (but without any relaxation of the other provisions) where a design procedure is used to provide the column with sufficient flexural strength to ensure a high degree of protection against plastic hinging.This reduction in confining steel content is not permitted at the bases of columns or piers, or in any storey of a building frame or any bridge pier portal where a column sidesway mechanism will occur with plastic hinges forming in the columns during a severe earthquake.

COMPARISON OF CODE REQUIREMENTS FOR TYPICAL COLUMNS
The difference between the confinement ratios required by the above codes are illustrated in Figure 4 for a typical 1.5 m (59 in) diameter circular column confined by a spiral such as would be used in a bridge pier, and in Figure 5 for a typical 700 mm (27.6 in) square column confined by an arrangement of square and octagonal hoops such as would be used in a building.

(a)
Comparison of spiral steel requirements Figure 4 Figure 4 illustrates that the Japanese and CALTRANS quantities of spiral steel are very low, particularly at high axial load levels, and that ACI Committee 343 also recommends only moderate amounts of spiral steel.
The step change in the ACI 318 requirements between Eqs. 2 and 3 has been taken to occur at P = O.lf^A in Figure 4, which is a reasonable approximation for the specified level of 0.44>P,.
The SEAOC and ATC recommendations do not permit a reduction in the spiral steel content at low axial load levels.
The SANZ (revised draft) quantity shows a linear increase in spiral content with axial load from 50% of the SEAOC amount at zero axial load to 1.38 times the SEAOC amount at P = 0.7f'A .
It is to be noted that the e quantiSy^of spiral steel required by the SEAOC code for the column shown in Figure 4 could be met using a spiral or circular hoops from 2 0 mm (0.79 in) diameter bar at 74 mm (2,9 in) centre to centre spacing.The longitudinal steel used in this example consisted of twenty one 40 mm (1.57in) diameter bars.
It is obvious that large diameter columns require very large spiral steel bars at close centres for confinement.The linear variation of this quantity of spiral steel with axial load level, as recommended by the draft SANZ code, is more logical than the step function adopted by the ACI 318 approach.Figure 4 shwos that the MWD approach requires substantially more steel than the other approaches at high axial load levels when the displacement ductility demand is high.
For bridge piers the displacement ductility demand for the overall pier structure is normally taken to be p = 6 by the MWD.
According to their recommendations, a column with rigid base and monolithic pier/superstructure construction would be designed for y .
=6.However, if the foundation is ^ier flexible, or additional flexibility results from bearings between the pier and superstructure, y ^er is required to be greater than 6. p  This is because the additional flexibility results in an increased yield displacement, while the inelastic displacement will be solely provided by flexural yielding of the columns, resulting in an increased ductility demend on the columns for a given overall structure ductility demand.
It is evident that at high axial load levels the MWD requirements can result in extremely high quantities of spiral steel, particularly if the foundations and/or bearings are reasonably flexible.
The case of y .=8 illustrated in Figure 4 is for a pier ^oundation with only very small foundation and/or bearing flexibility.
For y .=4, which is the usual requirement for building columns, the MWD approach does not result in high spiral contents.For low axial load levels the MWD approach results in uncomfortably small quantities of confining steel, due to the high emphasis of the d/c ratio on E in Eq, 7.
For example, Eq. 7 indicates t£at e = 0.010 will be available if c/d < 0.26 even if p =0. Clearly Eq. 7 is overly optimistic for low c/d ratios.Note however that the requirements of transverse steel for shear force would mean that the quantity of spiral reinforcement actually present in the column at low axial load levels would exceed that shown in Figure 4.

CM Comparison of Rectangular Hoop Steel
Requirements' ' CF±"gureH5 i. ' Figure 5 illustrates that the Japanese requirements apparently result in very low quantities of hoop steel.
The step change in the ACI 318 requirements has again been taken to occur at = O.lf'A in Figure 5, which is a reasonable approximation for 0«4$P^S The SEAOC and ATC recommendations do not permit a reduction in the hoop content at low axial load levels.
The SANZ (revised draft) quantity shows a linear increase in hoop content with axial load from 50% of the SEAOC moment at zero axial load to 1.38 times the SEAOC amount at P g = 0.7fA , Using the arrangement of one square hoop c ^ plus one octagonal hoop per set, shown in the column section in Figure 5, the total effective area of hoop bars per set, A^, is taken as twice the area of the square hoop leg plus 1.414 (i.e.twice 1//2) times the area of the octagonal hoop leg.Hence if both hoops are of the same size bar with leg area A h , the value of A , would be 3.414A sb .
The SEAOC code requirements for the column shown in Figure 5 could be met using 16 mm (0.63 in) diameter square and octagonal hoop bar with the hoop sets placed at 88 mm C3.46 in) centres.
The longitudinal steel used in this example consisted of twelve 32 mm (.1.26 in) diameter bars.
Again, as illustrated in Figure 5, the MWD approach requires substantially more hoop steel than does the other approaches at high axial load levels.At low axial load levels only very small quantities of confining steel are again required because of the high emphasis of the d/c ratio on e in Eq. 8.
For example, Eq. 8 indicates tha£ e = 0.010 will be available if c/d < 0.19 CU even if p g = 0. However shear would goven the transverse steel quantity in this case.
Again there are large differences between the hoop requirements for various y .demands in Figure 5.

CONCLUSIONS
The various overseas and New Zealand code recommendations for transverse confining steel in potential plastic hinge regions of columns and piers in seismic design show vast differences in the required quantities of transverse steel and it is evident that this is still a matter of some controversy.
Recent tests on near full scale reinforced concrete columns containing spiral steel or rectangular hoop steel, under simulated seismic loading, at the University of Canterbury, have shown that the quantities of confining steel recommended in the draft SANZ Concrete Design Code, with slight modifications mainly to take into account the effect of the possible increase of actual concrete strength over the specified f and to avoid the spread of plastic hinging into less heavily confined regions, will result in available displacement ductility factors for columns of at least 8.
The provisions for confining steel which have been used by the Ministry of Works and Development for ductile bridge piers appear to be very conservative when axial load levels are high and are in need of revision to avoid the use of excessive quantities of confining steel.This observation is made not only from comparison with the quantity of confining steel required by the draft SANZ Concrete Design Code but also in the light of the results of the column tests recently conducted at the University of Canterbury.At low axial load levels the MWD confinement provisions may be unconservative but the requirements of transverse steel for shear will govern in that case and result in a greater quantity of transverse steel being placed.

Loose cover concrete removed after testing completed
Fig.
OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING VOL 13.NO 1 MARCH 1980

Fig. 1 LoadingFig. 2
Fig. 1 Loading Arrangement and Sections of Two Test Columns i

Fig. 5
Fig. 5 Comparison of Code Hoop Steel Requirements for a Square Column