transfer structure

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Version 2.3 May 2008

10.0 Transfer Structures

10.1 According to Cl. 5.5 of the Code, transfer structures are horizontal elements

which redistribute vertical loads where there is a discontinuity between the

vertical structural elements above and below.

10.2 In the analysis of transfer structures, consideration should be given to the

followings as per Cl. 5.5 of the Code :

(i) Construction and pouring sequence – the effects of construction

sequence can be important in design of transfer structures due to the

comparatively large stiffness of the transfer structure and sequential built

up of stiffness of structures above the transfer structure as illustrated in

Figure 10.1;

(ii) Temporary and permanent loading conditions – especially important

when it is planned to cast the transfer structures in two shifts and use the

lower shift to support the upper shift as temporary conditions, thus

creating locked-in stresses;

(iii) Varying axial shortening of elements supporting the transfer structures –

which leads to redistribution of loads. The phenomenon is more serious

as the transfer structure usually possesses large flexural stiffness in

comparison with the supporting structural members, behaving somewhat

between (a) flexible floor structures on hard columns; and (b) rigid

structures (like rigid cap) on flexible columns;

(iv) Local effects of shear walls on transfer structures – shear walls will

stiffen up transfer structures considerably and the effects should be taken

into account in more accurate assessment of structural behaviour;

(v) Deflection of the transfer structures – will lead to redistribution of loads

of the superstructure. Care should be taken if the structural model above

the transfer structure is analyzed separately with the assumption that the

supports offered by the transfer structures are rigid. Re-examination of

the load redistribution should be carried out if the deflections of the

transfer structures are found to be significant;

(vi) Lateral shear forces on the transfer structures – though the shear is lateral,

it will nevertheless create out-of-plane loads in the transfer structures

which needs be taken into account;

(vii) Sidesway of the transfer structures under lateral loads and unbalanced

gravity loads should also be taken into account. The effects should be

considered if the transfer structure is analyzed as a 2-D model.



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Stage (1) :

Transfer Structure (T.S.)

just hardened

Stage (2) :

Wet concrete of 1/F just

poured

Stage (3) :

1/F hardened and 2/F wet

concrete just poured

Stage (4) :



Stage (5) :



Stage (6) and onwards

Structure above transfer

structure continues to be

built. Final force induced on

T.S. becomes {Fn} + {Fn-1}

+ {Fn-2} + ........... + {F2 } +

{F1} + {FT}.

G/F

1/F 1/F

3/F

1/F2/F

4/F

3/F

G/F

G/F

2/F

G/F

Figure 10.1 – Diagrammatic illustration of the Effects of Construction Sequence of

loads induced on transfer structure

Stress/force in T.S. : {FT} due

to own weight of T.S.

Stiffness : the T.S only

Stress/force in T.S. : {FT} +{F1}, {F1} being force

induced in transfer

structure due to weight of1/F structure.

Stiffness : the T.S. only.

Stress/force in T.S. being due

to {FT} + {F1} + {F2}, {F2} being force induced in

transfer structure due to

weight of 2/F structure.

Stiffness : the T.S. + 1/F.

Stress/force in T.S. : {FT} +

{F1} + {F2} + {F3}, {F3}

being force induced in T.S.due to weight of 3/F

structure.

Stiffness : T.S. + 1/F + 2/F

Stress/force in T.S. : {FT}

+ {F1} + {F2} + {F3} +

{F4}, {F4} being force

induced in T.S. due toweight of 4/F structure.

Stiffness : T.S. + 1/F + 2/F

+ 3/F

2/F

G/F

1/F



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10.3 Mathematical modeling of transfer structures as 2-D model (by SAFE) :

The general comments in mathematical modeling of transfer structures as 2-D

model to be analyzed by computer methods are listed :

(i) The 2-D model can only be analyzed against out-of-plane loads, i.e.

vertical loads and out-of-plane moments. Lateral loads have to be

analyzed separately;

(ii) It is a basic requirement that the transfer structure must be adequately

stiff so that detrimental effects due to settlements of the columns and

walls being supported on the transfer structure are tolerable. In view of

the relatively large spans by comparing with pile cap, such settlements

should be checked. Effects of construction sequence may be taken into

account in checking;

(iii) The vertical settlement support stiffness should take the length of the

column/wall support down to a level of adequate restraint against further

settlement such as pile cap level. Reference can be made to Appendix H

discussing the method of “Compounding” of vertical stiffness and the

underlying assumption;

(iv) Care should be taken in assigning support stiffness to the transfer

structures. It should be noted that the conventional use of either L EI /4

or L EI /3 have taken the basic assumption of no lateral movements at

the transfer structure level. Correction to allow for sidesway effects is

necessary, especially under unbalanced applied moments such as wind

moment. Fuller discussion and means to assess such effects are discussed

in Appendix H;

(v) Walls which are constructed monolithically with the supporting transfer

structures may help to stiffen up the transfer structures considerably.

However, care should be taken to incorporate such stiffening effect in the

mathematical modeling of the transfer structures which is usually done

by adding a stiff beam in the mathematical model. It is not advisable to

take the full height of the wall in the estimation of the stiffening effect if

it is of many storeys as the stiffness can only be gradually built up in the

storey by storey construction so that the full stiffness can only be

effected in supporting the upper floors. Four or five storeys of walls may

be used for multi-storey buildings. Furthermore, loads induced in these

stiffening structures (the stiff beams) have to be properly catered for

which should be resisted by the wall forming the stiff beams;



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10.4 Modeling of the transfer structure as a 3-dimensional mathematical model can

eliminate most of the shortcomings of 2-dimensional analysis discussed in

section 10.3, including the effects of construction sequence if the software has

provisions for such effects. However, as most of these softwares may not have

the sub-routines for detailed design, the designer may need to “transport” the

3-D model into the 2-D model for detailed design. For such “transportation”,

two approaches can be adopted :

(i) Transport the structure with the calculated displacements by the 3-D software

(after omission of the in-plane displacements) into the 2-D software for

re-analysis and design. Only the displacements of the nodes with external loads

(applied loads and reactions) should be transported. A 2-D structure will be

re-formulated in the 2-D software for re-analysis by which the structure is

re-analyzed by forced displacements (the transported displacements) with

recovery of the external loads (out-of-plane components only) and subsequently

recovery of the internal forces in the structure. Theoretically results of the two

models should be identical if the finite element meshing and the shape functions

adopted in the 2 models are identical. However, as the finite element meshing of

the 2-D model is usually finer than that of the 3-D one, there are differences

incurred between the 2 models, as indicated by the differences in recovery of

nodal forces in the 2-D model. The designer should check consistencies in

reactions acting on the 2 models. If large differences occur, especially when

lesser loads are revealed in the 2-D model, the designer should review his

approach;

2-D model (usually finer meshing) with

nodal forces recovered by forceddisplacement analysis at nodes marked

with

External nodal

force is { F 2 D} { F 3 D} after

re-analysis

External nodal

force is { F 3 D}

3-D model (usually coarser meshing)

with displacements at nodes with

external loads marked with

Figure 10.2 – 3-D model to 2-D with transportation of nodal displacements

transfer structure

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