Use of intermediate RCmoment frames in moderateseismic zones

Alpa Sheth

In theaftermathof theBhuj earthquakefor 2001, the scopeofIS 13920 was extended to include all buildings in zone III,whereas, prior to that, it was applicable for high and veryhigh seismic hazard areas (zones IV and V) and for buildingsgreater than five storeys in moderate seismic hazard area(zone III). The IS 13920:1993 lays down the same conditions

for seismic detailing of frame members and shear wallsregardless of the level of seismic hazard (moderate to very

high). This needs to be rationalised. It may be possible tosimplify some of the ductile detailing in zone III andcompensate for the reduced ductility and toughness by suitablyincreasing the seismic force levels. This is already addressedin the ACI 318 and IBC codes. The paper makes a casefor asimplified methodology of detailing for ordinary buildings

in zones with moderate seismic hazard which will greatlyease the application of earthquake engineering for buildingsin zone III. As per IS 1893 : 2002, of the 79 important townsin seismic zones III to V in India, 49 towns are in zone III. Some

of the ductility provisions of the IS 13920 are cumbersomeand difficult to execute and as a result there is a resistance toits application. The author argues that the simplification ofthe ductile detailing in zone III would greatly encourage itswidespread implementation.

The IS 13920 : 19931covers the requirements for design anddetailing of monolithic special reinforced concrete (RC)moment resisting frames (SMRF) so as to give them adequatetoughness and ductility to resist severe earthquake shakingwithout collapse and moderate shaking with some non-structural damage and some non-structural damage.

Prior to 1993, the IS 4326 : 19762 (Codeof practiceforearthquakeresistantdesignand construction of buildings) laiddown requirements for ductile detailing in seismic areas of

November 2003 * The Indian Concrete Journal

moderate to high hazard. The IS 13920 : 1993 differs from the. IS 4326: 1976 in the following.

(i) Material specifications have been introduced for thelateral force-resisting elements

(ii) Geometric constraints have been introduced for thelateral force-resisting elements as also detailing andlocation of splices, provisions of minimum andmaximum reinforcement

(iii) Requirements for the transverse reinforcement inbeams and columns and their detailing have beenexpanded

(iv) Provisions for reinforced concrete shear walls areincluded

The IS 13920 : 1993 specifies the same level of ductility forall buildings regardless of the level of seismic hazard thebuilding is subjected to and makes no distinction betweenzones of moderate, high and very high seismic hazard in theenergy dissipation requirements. Thus, seismic zones III toV require the same ductile detailing.

Need for varying toughness requirementsin moderate versus high seismic zonesThe level of ductile detailing in concrete structures directlyaffects its energy dissipation capacity (toughness). It is fairlyapparent that structures in higher seismic zones or thoseassigned to higher seismic performance or design categoriesshould possess a higher degree of toughness. The degree ofenergy dissipation, and therefore the detailing, shouldlogically increase fqr structures progressing from ordinarythrough intermediate to special categories, based on theseismic zone they are located in as well as the level of seismicperformance that is required of them. .

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The Bhuj earthquake of 2001 demonstrated that ordinaryRC buildings (with well defined frames) in seismic zone IIIperformed badly when they were marginally designed andpoorly detailed even for gravity loads or had a soft storey,stiffness discontinuities, torsion, plan or vertical or massirregularities? Most buildings in zone III performedreasonably well as long as they did not possess any majorirregularities, even though many of them were notspecifically detailed as per with the ductility provisions of IS13920. One of the reasons for this behaviour was the

contribution of the infill panel walls to the stiffness andstrength of the building. The intensity of shaking in zone IIItowns and cities was also much lower.

Based on lessons learnt from past earthquakes, forordinary buildings (with importance factor = 1), a case maybe made for a varying level of required energy dissipationcapacity (toughness) the structure is called upon todemonstrate, depending on the seismic zone it is located inand which is assumed while computing the design seismicloads. By reducing the response reduction factor and thusdesigning for a higher seismic force, the detailingrequirements may be relaxed in zone III.

Thus, a separate type of frame, say the intermediate momentresistingjrame (IMRF) whose performance would be betweenthat of the ductile moment resisting frame (detailed as per IS13920: 1993) and the ordinary moment resisting frame couldbe considered for application in moderate seismic zone. Theseismic detailing requirements of this type of frame wouldbe less stringent than that required as per IS 13920 : 1993 forspecial RC moment resisting frames. To compensate for thereduction in the toughness due to a relaxation of the ductilitycriteria, the response reduction factor R may be less than thevalue of 5 for (ductile) special RC moment resisting framebut may be more than 3.0 which is the value of R for ordinaryRC moment resisting frame. The factor could be consideredto be between 3.5 to 4. Such a relaxation in energy dissipationrequirement is only for the ordinary, regular buildings inzone III which do not fall in the category of irregularbuildings.

ACI 318-023 (Building Code Requirements for StructuralConcrete) also has a similar provision and allows for the useof "Intermediate moment frames" in regions of moderateseismic hazard. For such intermediate frames, the detailingrequirements are greatly relaxed in ACI 318. Therequirement of toughness is reduced by reducing the responsemodification coefficient R from 8 in special moment resistingconcrete frames to 5 in intermediate concrete moment frames

in the International Building Code (!BC 2003)4,thus increasingthe seismic load by about 1.6 times. The International BuildingCode (2003), like the ACI 318, allows the use of intermediatemoment frames for buildings in seismic design category C.

The suggestions for revised requirements of ductiledetailing for zone III made herein are based on those of ACI318-02 for intermediate frames.

Requirements of intermediate momentresisting frameAs distinct from the requirements of special RC momentresisting frames spelt out in IS 13920, the intermediate

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(reinforced concrete) moment resisting frames (IMRF) willperform at a lower level of toughness with an appropriatecompensating modification in the response reduction factor.

The suggested relaxations in the various ductilityrequirements of IS 13920 : 1993 for intermediate frames arediscussed herein.

Flexural membersLongitudinal reinforcement

It is necessary to provide flexural members with a thresholdlevel of toughness. To achieve this, the IS 13920 requires thatminimum positive strength at a joint face should be equal toat least half the negative strength at the joint face. It furtherrequires that minimum steel at any face should not be lessthan 25 percent of the maximum negative steel at any jointface.

This ctiteria may be relaxed for intermediate frames suchthat the positive moment strength at the face of the joint isnot less than one third the negative moment strengthprovided at that face of the joint and the negative or thepositive moment strength at any section along the length ofthe member is not less than one fifth the maximum moment

strength provided at the face of either joint.

Transverse reinforcement

IS 13920 requires the web reinforcement in flexural membersto have a 135 degree hook and an additional 10 diameterlength of the hoop reinforcement beyond the hook. The 135degree hook is rather difficult to execute and in the case of a200 rom wide flexural members with 50 rom clear cover, the10 diameter additional length of the hoop bar beyond thehook practically touches the vertical leg of the hoop on theother side (for an 8 rom or higher diameter bar), Fig l(a).This causes various construction difficulties such as difficultyin the placement of these hoops, in the placing of concreteand in the vibration of the concrete. This requirement shouldbe eliminated for intermediate moment resisting frames(IMRFs) in view of the reduced requirement of toughnessand a 90 degree hook with a 8d extension instead may beallowed, Fig l(b).

Design shear strength

It is necessary in all seismic zones to avoid a shear failure inframe members. Hence, the shear capacity of any framemember should be larger than the sum of actual shear on themember due to vertical loads and that associated with the

development of the full moment capacity of the member.

Presently, in IS 13920, the shear capacity required inflexural members is given as:

Vu,a = VaD+L- 1.4(

M ~Urn + M ~Urn

)LAB

v: D+L

(

MAS +MBh

)u,b = Vb + 1.4 u,Urn U,Urn

LAB

The Indian Concrete Journal * November 2003

r 200 -I r 200 -I

0'"....

"0<X)

0'"....

50 50 50

(a) (b)

Fig 1 Transverse reinforcement in flexural members(a) 1350 hook is difficult to execute (b) A 900 hook with aad extension would be better

V V+L

(

MAh MBS

)Uft = Va + 1.4 u,Hm+ u,Hm

LAB

V - V V+L

(

MAh. +MBs

)u,b - b - 1.4 u,lnn u,Hm

LAB

where,

Vu,a,Vu,b = the design shear force,

Va, Vb = the shear due to vertical loads,

M/s, MuBs = the sagging and hogging moment capacitiesof the beam,

M/h, MuBh = the sagging and hogging moment capacitiesof the beam,

LAB = the clear span of the beam.

The factor of 1.4 for the moment capacity of the flexuralmember is meant to take care of the design steel stress of0.87Jyand the fact that in reality steel can take upto 1.25Jydueto strain hardening, Thus, moment capacity of the memberis taken as 1.25/0.87=1.44 say 1.4 times the calculated capacity,

For IMRF, the required shear capacity of flexural membersmay be reduced to following:

V - V V+L

(

MAs. +MBh

)u,a - a - u.hm u,Hm

LAB

Vub = VV+L

(

MAs. +MBh

), b + u,hm u,Hm

. LAB

November 2003 * The Indian Concrete Journal

V - V+L

(

MAh +MBs

)u,a - Va + u,lnn u,Hm

LAB

V - V+L

(

MAh M BS

)u,b - Vb - u,Hm+ u,Hm

LAB

Note that the 1.4 factor for the moment capacity of thebeams as given in IS 13920 is proposed to be eliminated inIMRF. The rationale behind this proposed change is that thestructure is not likely to be called upon to demonstrate ashigh a level of toughness as in high and very high seismiczones

Spacing of transverse reinforcement

The maximum spacing of the hoop reinforcement as specifiedin IS 13920 : 1993, may be retained for IMRFs. That is,maximum hoop spacing over a length of 2d at either end of abeam shall not exceed the smallest of:

(i) d / 4

(ii) Eight times the diameter of the smallest longitudinalbar enclosed

(iii) 100 mm.

Stirrups shall be placed at not more than d/2 throughoutthe length of the member

Columns

Longitudinal reinforcement lap splicing

IS 13920 requires that the vertical column bars should bespliced only in the central half of the member length andproportioned as a tension splice. Further, not more than 50percent of the bars may be spliced at a section. The latterrequirement in conjunction with the former is extremelydifficult to follow especially for short floor heights where thecolumn bars are perforce required to be of at least two andsometimes even three floor heights to ensure compliance ofthe above requirements. Maintaining the plumbness of suchlong bars then becomes a challenge.

The requirement of not more than 50 percent of the barsto be spliced at a section also does not take into account theactual construction systems and practice. In recent times withincreased mechanisation at sites, column cages are oftenfabricated at the ground level and placed at the final locationby means of cranes or other devices. In such a case, the lapsof all vertical bars are at one location. The requirement alsodoes not account for future provision of a floor where all thebars may be lapped at one location.

While ACI 318 requires that the laps of column bars inhigh and very high seismic zones should be located withinthe central half of the member length, it does not have arequirement that maximum of 50 percent of bars may belapped at one location.

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A more practical approach would be to eliminate therequirement of maximum 50 percent of bars to be lapped atone location in IMRF and require a higher, punitive lap length,say 1.3 times that required presently, when more than 50

.percent of the bars are to be lap spliced at one location. Thesame may in fact be extended for the SMRFs also.

Transverse reinforcement

IS 13920has rather stringent requirements for the requiredarea of confining steel. As per IS 13920: 1993,

for circular hoops,

{

(

A -10

)A = 0.09 S D li..- g .sh k

J. Ay k

while for rectangular hoops steel,

ASh = 0.18 ShDk fck _(

Ag -1.0

)fy Ak

where,

ASh = area of the bar cross section

Dk = diameter of core measured to the outside ofthe hoop

S = spacing of hoops

Ag = area of gross section of column

Ak = area of concrete core

fck = characteristic compressive strength ofconcrete cube

fy = yield stress of steel.

The formula yields unrealistically high diameter of hoopreinforcement steel for small columns. As per the aboveformula, the diameter for hoop reinforcement of confiningsteel works out to 16 mm for a column of 200 mm x 200 mm.

Similarly, for very large columns (for instance, in case ofbridge piers), the equation gives very low transverse steel.This needs to be addressed in the code appropriately.

The above clauses were formulated with the intent that

the column would have the same axial strength after spalling'of the cover concrete as before spalling.

In zones with moderate seismic hazard, such a situation isnot expected or envisaged for ordinary buildings and theabove requirements for area requirement of hoop steelshould be eliminated as long as the detailing requirementsmentioned below are adhered to, in order to ensure that theconcrete is properly confined

Spacing of transverse reinforcement

To ensure the minimum level of toughness and ensure thatconcrete is well-confined, the spacing criteria for hoop steelin IS 13920 may be maintained, that is, at both ends of the(column) member hoops shall be provided at spacing, So,over a length, 10, measured from the joint face.

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Spacing, So, shall not exceed the smallest of (i) and (ii)

(i) one fourth of the minimum member dimension ofthe frame member

(ii) 100 mm

Length, 10, shall not be less than the largest of (iii),(iv) and (v)

(iii) One-sixth of the clear span of the member

(iv) Larger lateral dimension of the member

(v) 450 mm.

Elsewhere along the column length the spacing of thehoops shall not exceed one half of the minimum memberdimension of the frame member.

Shear capacity of transverse reinforcement

As with flexural members, it is very important to preventshear failure in columns. Hence the shear capacity of thecolumn should exceed that shear associated with the

development of the moment capacity of the column at bothends.

IS 13920 requires this to be as follows:

(

MbL bR

)Vu = 1.4 uJim+ Mu.lim

hSI

For IMRF, the above equation may be revised to :

-

(M~Llim + MbR.

)Vu - . u.lim

h"

Vu = the shear force to be resisted

M}L, MubR = the top and bottom moment capacities ofthe column and

hSI = the height of the column.

Note that the 1.4 factor for design shear force for columnsas per IS 13920 is missing. The rationale for this change is thesame as for beams mentioned earlier

Structural wallsProperly designed and detailed RC shear walls haveconsistently performed well in moderate seismic zones inthe past earthquakes and these should be encouraged. It issuggested that in IMRF, the ductility requirements of Section9 of IS 13920 for boundary elements and coupling beamsmay be eliminated.

Flat slabsFlat slabs have Qecome extremely popular structural systemsfor office buildings in the past few years. While flat slabstructures have not demonstrated very good behaviour in

The Indian Concrete Journal * November 2003

---

Table 1: Recommendations for new proposed IMRF

Clause ofIS 13920

Present requirement for SMRF in IS 13920 Suggested recommendation for new proposed IMRFPertaining to

6.2.3 Flexural members-Longitudinal reinforcement

Minimum positive strength at a joint face should beequal to at least half the negative strength at the joint face

6.2.4 Flexural members -Longitudinal reinforcement

6.3.1 135° hook + 10d extension of hoop bar

Minimum steel at any face should not be less than25 percent of the maximum negative steel at any joint face

Web reinforcement -

6.3.3 Web reinforcementShear capacity

=shear required for design gravity loads + sheardeveloped by 1.4 times sum of moment capacities ofboth ends of member

7.2.1 Columns lap splices Maximum 50 percent of bars to be lapped at one location

7.3.4 Transverse reinforcementShear capacity in columns

V,= 1.4(M~im +M~im )h"

7.4.7and7.4.8

Area of special confiningsteel in columns

for circular hoops,

f., (Ag-1.0 )A'h=0.09SD,-'-- -fy A,

While for rectangular hoops steel,

f., (Ag-1.0 )A,h=0.18ShD,-'-- -fy A,

9.4-9.5 Shear walls Boundary elements, coupling beams

??? Beamless slabs (flat slabs) Detailing clauses for flat slabs (for example, provisions ofintegrity steel) required

past earthquakes and are not very desirable in high and veryhigh seismic zones, they may be used with caution inmoderate seismic zones. However there are no requirementsfor the placement, location and detailing of reinforcement inflat slabs in IS 13920. This needs to be rectified immediatelyand some minimum requirement of provision of integritysteel needs to be included on priority basis for seismic zonem.

ConclusionThere appears to be a need to differentiate between theductility criteria in zones of moderate seismic hazard fromthat in zones of high and very high seismic hazard as providedfor in ACI 318-02. In zones II and III, buildings may bedesigned with less stringent ductility detailing but with anincrease in the design seismic force.

Thus for zone III, there should be a choice of using either

(i) SMRF as per the existing IS 13920with correspondingresponse reduction factor R value as 5.0 as spelt outin IS 1893 : 2003s

or

(ii) IMRF zone III with a corresponding reduced R say 4

Similarly, in zone II, an engineer may detail as per IS 456only (OMRF with R value of 3.0, as IMRF (with R value of4.0), or as SMRF (with R value of 5.0t

November 2003 * The Indian Concrete Journal

I

Minimum positive strength at a joint face should be equal toat least one-third the negative strength at the joint face

Minimum steel at any face should not be less than 20 percent ofthe maximum negative steel at any joint face

90° hook + 4d extension of hoop bar

=shear required for design gravity loads + shear developed by sumof moment capacities of both ends of member

This requirement to be relaxed

v, =(M~1im +M:~Iim )h,t

This requirement to be eliminated

This requirement to be eliminated

Table 1 summarises the suggested changes required in IS13920 for IMRF for use in zone III or lower.

References

1. _Indian standard codeof practicefor ductile detailing of reinforced concrete

structures subjected to seismic forces, IS 13920 : 1993, Bureau of IndianStandards, New Delhi.

2. _Indian standard code of practice for earthquake resistant design and

construction ofbuildings, IS 4326: 1993,Bureau of Indian Standards, New Delhi.

3. _Building coderequirementsfor structural concrete,AC1318-02, AmericanConcrete Institute, USA.

4. _International building code,IBC2oo3, International Code Council, USA.

5. _Indian standard criteria for earthquake resistant design of structures,Part 1 General provisions and buildings (fifth revision), IS 1893 (Part 1) :2002, Bureau of Indian Standards, New DeIhi.

6. _Indian standard code of practicefor plain and reinforced concrete (fourthrevision), IS 456: 2000, Bureau of Indian Standards, New Delhi.

7. JAIN,S K, LETIlS,W R, MURTY,C V R, and BARDET,J.P. (editors), 2001 Bhuj, India

Earthquake Reconnaissance Report, Supplement A to Volume 18, Earthquake

Spectra, Earthquake Engineering Research Institute, USA, July 2002.

Ms Alpa Sheth obtained her Master's fromUniversity of California, Berkeley. She is currentlyPartner, Vakil Mehta Sheth Consulting Engineers,Mumbai.

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