The Influence of Transfer Plates on the Lateral Behaviour ...

ENGINEER - Vol. XXXX, No. 04, pp. 22-30, 2007

© The Institution of Engineers, Sri Lanka

The Influence of Transfer Plates on the LateralBehaviour of Apartment Buildings

S. S. Balasuriya, K. M. K. Bandara, S. D. Ekanayake, M. T. R. Jayasinghe

Abstract: The skylines in the major cities of Sri Lanka are fast changing with many apartment buildingsbeing constructed. One of the primary requirements is the provision of car parking for which the lowerfloors are generally used. The car parks will have a certain grid requirement for vertical load carryingmembers primarily arising due to guidelines specified in building regulations. Often, this gridarrangement is not suitable for apartments which generally consist of concrete walls that can becomepart of the partition wall system of the apartments. Therefore, it is usual to have transfer girders or atransfer plate to allow acceptable load paths for the vertical loads. When transfer plates are used in tallapartment buildings, the wind induced acceleration could be an issue that needs careful consideration.In this paper, it is shown that the transfer plates can be used as outriggers of the building is plannedcarefully. Such outriggers have a tendency to reduce the wind induced acceleration to reasonably lowlevels and hence become a useful feature despite the extra cost involved

Keywords: Transfer floor, Thick plates

1. Introduction

When considering the present situation of thecountry, the land becomes a scare resource. Soevery bit of land is precious and it is used forsome important purpose. Thus, high riseapartment buildings are in demand. The currenttrend is to use the whole land for the structureand provide several parking floors within thebuilding rather than providing parking outside.The column grid of an apartment floor willgenerally not match that of a parking floor,which gives rise to the issue of connecting thetwo sets of columns together.

There are several ways of connecting the twofloors. Either girders or plates can be used as thetransfer system. These transfer floors aredifficult to design and cost a vast sum of money.Use of transfer girders is the moststraightforward solution for transfer floors, butthere may be many construction difficulties.Thus, this research mainly focuses on the use ofa thick plate for transfer which would be easierto construct.

One particular feature of apartment buildingswhen compared to commercial buildings is thatthey have a lesser number of lifts due to lowerlevels of service generally provided than in anoffice building (Smith, 1991). Therefore when astructural system of 3D reinforced concreteframes with shear walls is used as often done inSri Lanka, lateral stability may not be enough by

just providing shear walls around lifts and stairwells specially with respect to controlling thewind induced acceleration. Often, other meanssuch as outriggers may be needed to provideadditional lateral stability in tall buildings of 30-40 storey range. However, the presence of atransfer plate may be effectively used forcontrolling the lateral behaviour to a certainextent. This is investigated in detail in thisresearch.

2. Objective

The objective of this research is to determine theeffectiveness of a thick plate for transfer and itsinfluence on the lateral behaviour of thestructure. Determination of a proper way tomodel a thick transfer plate in finite element willalso be investigated.

3. Methodology

First, a typical apartment layout and a parkinglayout were selected that satisfied the buildingand parking regulations (Building Regulations1999). Particular attention was paid to select thetwo plans in such a way that their column gridsclosely resembled those in practical use at

Eng. (Prof.) M.T.R. Jayasinghe ,B.Sc. Eng (Moratuu'a), Ph.D.(Cambridge), C.Eng., MIE(SL, )Professor, Department of CivilEngineering, University ofMoratuiua.

S.S. Balasuriya, K.M.K. Bandara, S.D. Ekanayake; Final yearstudents, Department of Civil Engineering, University ofMoratuiva.

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.

present. The effect of other changes to theparking floor layout will be studied later.

The analysis was done based on a finite elementmodel in SAP2000. Changes were made to thebasic model to study the effects of differentmeshing techniques. Thus, two models werecreated with Model A having a coarse mesh inthe transfer plate with Model B having a finemesh. A third model, C was created with thesame building height but with apartment levelsfrom ground floor to roof only (without the needfor a transfer plate since no car parking wasprovided). Comparison with this model willyield the effectiveness of the thick plate as anoutrigger. Lateral stability was measured interms of fundamental period of vibration, windinduced acceleration and deflection for windloads. The wind load analysis was carried outbased on loads evaluated from AS1170.2-1989 asit enables calculation of dynamic windresponses including acceleration.

4. An overview about apartmentbuildings

4.1 General layout

Most modern high-rise structures that are usedas apartment buildings in Sri Lanka follow aregular pattern. Many such buildings consist ofconcrete walls which are part of the partitionwalls. Due to the advancement of architecturalfeatures in modern day buildings, moststructural elements are provided in amanner such that they will not obstruct thespaces within the building. As a result, a regularcolumn grid is not present in the residentialareas of the apartment building as such a

column grid often violates the architecturalfeatures.

Some layouts of typical apartment buildings areshown in Figure 1. They reflect the factsmentioned above and highlight the point thatusing concrete walls with short beams willpreserve the architectural qualities.

4.3 Car parking regulations

It is necessary to allow space for vehicle turningcircles, parking stalls, ramps, driveways, etc,when arranging a parking floor. This must bedone satisfying the parking regulations in SriLanka. As a result, the vertical load carryingmembers should have a certain gridrequirement. Columns are the most suitablestructural elements to transfer vertical loads andthe dimensions of a suitable column grid arelargely dependent on the parking regulations.

This section mainly focus on the relevantregulations for standard vehicle type asindicated in Table 1 (Car Equivalent, also to beused for two and three wheelers), since multi-storey car parks in apartment buildings mainlycontain of those vehicle types. Otherrequirements are given in Table 2 and 3.

Table 1 - Minimum plan dimensions of parkingstalls (Building Regulations, 1999)

Vehicle Type

Standard(Car Equivalent, alsoto be used for two andthree wheelers)

Stall width(m)

2.4

Stall length(m)

4.8

Figure 1: General layouts of typical apartments (Mills, 1985)

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Table 2 - Minimum width of aisles(Building Regulations, 1999)

Parking Angle(Degree's)

00° Parallel30° Angle45° Angle60° Angle90° Angle

One-waytraffic

one sidedbays (m)

3.63.64.24.86.0

One waytraffic twosided bays

(m)

3'.64.24.84.86.0

Two-waytraffic

(m)

6.06.06.26.46.0

Table 3 - Minimum inner and outer turning radius(Building Regulations 1999)

Inner turning radius (m)Outer turning radius (m)

Passenger Car

7.34.7

Some typical car parking arrangements areshown in Figure 2. According to thosearrangements, it is evident that spacing ofcolumns fall within a regular pattern mainly dueto the clearance required for drive ways andparking bays. In addition, there are ramparrangements needed for access to upper floors.The possible arrangements are shown in Figure 2.

Multi-storey structurewith full ramps

Half-story ramo car-park(D'Humy system)

Full ramps, no loss ofspace Gradient < 6%

Spiral ramp car-park

Figure!: General layouts of parking arrangements(Neufert, 1980)

4.2 The need for different grids

To cater to the different needs of the apartmentfloors as well as to the parking floors, it isnecessary to have different grids in the samestructure. According to the building regulationsand traffic regulations, there are some specificspace allocations for those floors. In order tokeep those, different grid systems are ut,ed inthe same structure.

4.3 Solution with transfer members

For a typical apartment building shown below,it is possible to have either a transfer plate ortransfer girders as the solution.

Figure 3: General architectural drawing forapartment floors

LTU

Figure 4: Column layout for apartment floors

UNLuJ

Figure 5: General architectural drawing forparkingfloors

4.3.1 Solution with transfer plates

A transfer plate is similar to a flat slab, of highthickness, often in the range of 1.0m. Withsuitable reinforcement for shear and flexure,such a thick plate can withstand the large point

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fJ , ^ ^ ^ ^_ ^ ^

[f -$i i

m m

-^ . ^ ^ ^ - ^ - - -^iI I ! I i '

Figure 6: Column layout for parking floors

loads from the columns above. This may be thebetter solution since the wide scatter of columnsabove transfer plate could be accommodatedeasily as shown in Figure 7.

8.051

D

D

D

n

n

n Co

L Co

8.05 8.05 8.05 8.05 8.05

1 1 1 1j .. WT- urn -i.. u_,. / u ,. r ,,--TTJ_,.T _ _ , - L

h/ b'Dl'" X°X •-'°X' --IDX x'c

Xy'p X'JX' /-+X' X X'-iX ..••'''tL "•' •*' -'' -'' l'' Ji'

3 ,-' .-'C --'u .-•'qf= '-' ip jp ,-J .-'a .-' ,.-t,-'' I-'' -'I— If--''

3 ,--''' XV+X' X' X* .--'''+X'^X' X'c

^/'•\.,--'>,--''',r----''|j^x-'Tx-'^x-'^

x*' XV* >•''' x'V*' /*/*.'* /: _,-•'' ^--''DI.--'' ,.--'a _./'' _,.-'b _,.-'' _.-|b _, ' _.-•':

TRANFER PLATED

umns below

umns above

8.05 t

1

a —«

a —CD

a — .

a —cor^

a —CM

Figure 7: Transfer plate indicating the location ofcolumns above and below

4.3.2 Solution with transfer girders

This is a more complicated arrangement whichneeds a grid of beams as shown in Figure 8. Thedesigner must propose a suitable layout ofbeams with at least one beam below eachcolumn that has to be transferred.

8.05 . 8.05 . 8.05 . 8.05 . 8.05 . 8.05 . 8.05

Figure 8: Transfer girders indicating the complexarrangement that may be needed

5. Case study for 30 storey apartmentbuilding

In order to perform a detailed study, thebuilding of Figures 3 to 6 was selected as thebuilding under consideration.

5.1 Layout

The layout below the transfer plate is shown inFigure 5, the corresponding column layout isshown in Figure 6.

The layout above the transfer plate and thecorresponding column layout are shown inFigures 3 and 4, respectively. A transfer plate ofthickness 1.0 m was used as the transfermember. Concrete of grade 40 was used in themodel

5.2 Transfer plate with SAP2000 model

When transfer plates are modelled using shellelements, the connectivity of columns/wallsabove the transfer plate to column below is animportant consideration. Therefore, it isadvisable to create the finite element mesh withplate elements manually, ensuring that plateelements would be of acceptable proportions,avoiding situations such as excessivelyelongated members. This can be considered as acoarse mesh as shown in Figure 9.

Once the coarse mesh is created, SAP2000 offera facility to mesh it further as shown in Figure10 so that it would result in a finer mesh thatmay be able to provide results of higheraccuracy, although it is likely to consume morecomputer power or take longer for analysis.

5.2.1 Without mesh

Figure 9: The coarse mesh created for thetransfer plate (Model A)

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Figure 10: Fine mesh created bysub-dividing the coarse mesh (Model B)

5.3 The mesh type

First of all, a finite element must be selected tocorrectly represent the behaviour of the transferplate. Either solid elements or shell elements canbe selected (See Figure 11). Solid elements arevery tedious to work with and will not be usedat this stage of the research. However, there arecertain issues when using shell elements. Theheights of the stories above and below thetransfer level must be increased to maintain theoverall height of the structure. Another concernof using shell elements for this situation is thatof shearing deformations.

Figure 11: Using solid elements and shell elements

Shearing deformations tend to be importantwhen the thickness of the element is greater thanabout one-tenth to one-fifth of the span. Theycan also be quite significant in the vicinity ofbending-stress concentrations, such as nearsudden changes in thickness or supportconditions, and near holes or re-entrant corners.These, shearing deformations can be taken intoaccount as follows:

When defining the shell element properties,there is an option of selecting either a thick-plateor a thin-plate. This governs the way thicknessis formulated (This determines whether or nottransverse shearing deformations are included

in the plate-bending behaviour of a plate or shellelement).

• The thick-plate (Mindlin/Reissner)formulation, includes the effects oftransverse shear deformation

• The thin-plate (Kirchhoff) formulationneglects transverse shearing deformation

Thus, the thick-plate formulation can be used torepresent the transfer plate. However, theaccuracy of the thick-plate formulation is moresensitive to large aspect ratios and meshdistortion than is the thin-plate formulation.(SAP2000 Analysis reference, 2002) This factorhas an important role to play and will bediscussed under the section on meshing of shellelements.

Creating a model of the transfer plate is nomajor task if an advanced finite elementpackage such as SAP2000 is at hand. However,there are certain aspects that the designer mustbe aware of in this situation. All shell elementsmust be properly connected to adjoiningelements to have proper connectivity. If thisfundamental law of finite element theory isviolated, a continuous stress flow as shown inFigures 9, 10 and 12 will not be obtained. It isalso important to have the local axis of allelements in the same global orientation as anydeviation from this will imply difficulty wheninterpreting the output of shell elements. As formodelling the transfer plate itself, once thedifferent column grids are in place they can beconnected with the shell elements maintainingconnectivity and keeping the orientation.

Figure 12: Continuous stress flow in shell elements

5.4 Meshing for better results

In any finite element model, a finer mesh mayprovide a more accurate result. This isparticularly important in this situation as

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concentrations of stress are most likely aroundthe columns that are transferred. Furthermore,as explained earlier, the use of the thick-plateformulation implies that the shell elements aremore sensitive to large aspect ratios. If a coarsemesh is to be used, elements with large aspectratios are most likely particularly in the vicinityof the point loads (columns that are transferred).

This implies that the shell stresses output maybe slightly inaccurate in locations where it ismost critical. Thus, some form of meshrefinement is necessary particularly near thepoint loads. The behaviour of a fine mesh and acoarse mesh is shown in Figure 13.

Figure 13: The behaviour of a fine meshand a coarse mesh

5.5 Wind loads

Figure 14: Wind load application

Wind load calculations were carried outaccording to AS1170.2-1989, for 3 second gustvelocity of 33.0 m/s (Macks et al, 1979)considering a building in zone 3 of Sri Lanka.The dynamic wind pressure was applied on themodels and maximum deflections wereobtained using the gust factor. This enabled thecalculation of wind induced acceleration.Calculations for the wind load are included inappendix A.

5.6 Results of analysis

From the results in Table 4, it can be seen thatthere is no major differences between using afiner mesh and a coarse mesh. It can also be seenthat lateral deflections are fairly low at rooflevel. However the maximum verticaldeflections may not correctly reflect the overallpicture, with the average value being about 10 -11 mm. The real advantage of a finer mesh willbe with the accuracy of the forces and momentsin the transfer plate.

Figure 15: Model C (Without transfer plate)

When comparing results from model A andmodel C, the effectiveness of the thick plate inreducing lateral deflections is clear. It can also beseen that lateral deflection of the plate itself isclose to zero. It should be noted that windinduced accelerations have been reduced due tothis outrigger action of the thick plate. Peoplewho are sensitive to movements would be ableto feel acceleration above 0.05 m/s2. Majority of

Figure 16: Fundamental vibration modeT = 2.24s (Model A)

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Table 4 - Analysis results*

Parameter

Mesh coarseness

Fundamental period of vibration

At roof level

Transfer plate

Maximum lateral deflection

Average lateral deflection

Max vertical deflection

Average vertical deflection

Maximum lateral deflection

Average lateral deflection

Along wind acceleration

Model A

coarse

2.24s26.15mm

22.48mm

23.52mm

10.20mm

2.42mm

2.19mm

0.1058m/ s2

Model B

fine

2.24s

26.27mm

22.56mm

24.04mm

11.01mm

2.47mm

2.25rnm

0.1063 m/s2

Model C

L 2.24s

29.57mm

28.27mm

7.81mm'

5.91mm "

2.93mm*

2.68mm"

0.1197 m/s2

* - These deflection values are obtained at the same level as the transfer levelt - Vertical deflections are for 1.4Gk+1.6Qk load case and lateral deflections are for 1 OGk+1.4Wk load case.

the people will feel accelerations of about 0.1 m/s2 (Smith & Coull, 1991). This means, even thesmall reduction in acceleration that can beachieved in apartment buildings would beimportant because people will be living in them.

When closely inspecting the fundamentalvibration mode in Figure 16, it seems that theapartment above the transfer plate contributesmuch more than the structure below the transferplate as lateral deflections of the transfer plateand below are at a minimum. Effectively, theapartment structure behaves as if it is supportedon a rigid foundation, at the level of the transferplate.

6. Conclusions

It can be concluded that the transfer plates canbe used effectively as outriggers in apartmentbuildings. A proper finite element model willyield the required design parameters for thetransfer plate itself and parameters regardingthe lateral behaviour of the structure. Thisreduction in lateral deformation to almost zeroat the transfer plate level could lead to areduction in lateral deflection at the top of thebuilding. This can reduce the peak windacceleration values. This could be extremelyvaluable in apartment buildings that aredesigned with a greater height due to the needfor accommodating car parking needs. Theprovision of acceptable level of occupantcomfort is one of the primary responsibilities ofthe structural design engineer.

References

1. AS1170. 2-1989, "Minimum design loads onstructures- Part 2: Wind loads", StandardsAustralia, New South Wales.

2. Building Regulations (1999), City of ColomboDevelopment Plan, Volume II, Colombo MunicipalCouncil.

3. Neufert, Architects' data, Blackwell ScientificPublications 1980, p. 252

4. Macks, K. }., Murray, F. J., Wittenoom, R. A.(1979), "Technical Assistance to Sri Lanka on cycloneresistance construction", Vol. 2, Part 5, AustralianDevelopment assistance Bureau, Department ofHousing and Construction, Australia.

5. Mills, Planning Architects Hand Book, 10th ed.,Butterworths, 1985.

6. SAP2000 Analysis reference, Computers andStructures, Inc., Berkeley, Califonia, USA, VersionS.O.July 2002.

7. Smith, B. S., Coull, A.,(1991), "Tall BuildingStructures", John Wiley, USA, pp. 537.

8. Smith, P. R., (1991), "The movement of peopleand goods", In Handbook of ArchitecturalTechnology, edited by J. Cowan, VAN NostrandReinhold, New York, 1991, pp. 423-440.

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Appendix

Dynamic wind load calculation (According toAS 1170.2 -1989)

Calculation of Gust factor (G)

Basic wind speed (V) = 33.0 m/sHeight of the building (h) = 104.2 mNatural period of vibration = 2.33 sof the buildingEstimated natural frequency(na) = 0.43 Hz

Clause 4.4.2.

VJ 2x0.166r = —=— = = (j.ooi

Mt 1.0gv=3.7

gf = ,/21oge(3600na)

= V21oge (3600x0.43) = 3.83

Vh=VM(h/cat)MsM tM i

= 33.0x0.72x1.0x1.0x1.0 = 23.76 m/s

S = l-

1 +3.5x0.43x104.2

23764x0.43x30.(

23.76

= 0.0413

,(2 + N2)5/6

[104. 6 I0'25

i - =1798.3910 J

N = 0.43X1798.3923.76

E 0.47X32.55 1

(2 + 32.5S2)576

w = 7 X °'332 X = 0 2514 4

G = 1 + 0.332 (s.72 x 0.728x(l + 0.254)2

3.832x 0.0413x0.0461/2

0.046 J

G = 2.426

Calculation of wind loads on the building

C for windward wall = 0.8

Cp<e for leeward wall = -0.483

Specimen calculation for Z= 6.6 m

= 33.0x0.4x1.0x1.0x1.0 = 13.2 m/s

qz = 0.6 V2 x 10"3 = 0.6 x 13.22 x 10~3 = 0.105 kPa

Fz = [(0.8) -(-0.483)]x0.105x (3.3xl) = 0.44 kN/m

Lateral deflection at roof level (A) wasobtained from the model considering thefollowing load combination:

1.0gk +1.4GWk =1.0gk + 1. 4 x 2.426 x Average loads

Where G = gust factor =2.426

Along wind acceleration (aa)

ISPaa=(2nnJ2gfr I— A

For model A

aa = (2nx0.43)2 x 3.83x0.332 J

= 0.1058 m/s

0.0413x0.0460.01

For model Baa = 0.1063 m/s

For model Caa = 0.1197 m/s

B =

1 +V36h2+64b2

,V36xl04.22+64x30.02

1798.39

= 0.728

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Table A.I: Wind loads at each floor level (in kN/m)

Level

G1234567891011121314151617181920212223242526272829

Floor to floorheight(m)

3.33.33.33.84.13.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63,63.63,63.63.63.63.63.6

Z(m)

03.36.69.913.717.821.42528.632.235.839.44346.650.253.857.46164.668.271.875.47982.686.289.893.497100.6104.2

Effectiveheight (m)

1.653.33.33.553.553.73.853.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.63.6

M^

0.380.40.440.480.510.530.540.560.580.590.60.610.620.630.640.640.650.660.670.670.680.690.690.70.70.710.720.720.73

vz

12.5413.214.5215.8416.8317.4917.8218.4819.1419.4719.820.1320.4620.7921.1221.1221.4521.7822.1122.1122.4422.7722.7723.123.123.4323.7623.7624.09

<7z(kPa)

0.0940.1050.1260.1510.1700.1840.1910.2050.2200.2270.2350.2430.2510.2590.2680.2680.2760.2850.2930.2930.3020.3110.3110.3200.3200.3290.3390.3390.348

Fz (kN/m)

0.400.440.570.690.810.910.880.951.021.051.091.121.161.201.241.241.271.321.351.351.401.441.441.481.481.521.571.571.61

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