precast structural design updated

8/21/2019 Precast Structural Design Updated

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1. Project Description

The building is a 12-storey office block in a mix commercial development comprising carparks,

shopping malls and service apartments.

A typical floor of the building measuring 24 m x 2 m !ith " m building grids in both directions

is sho!n in #igure 4.1. The design floor-to-floor height is $.% m. &taircases, lift cores and

other building services such as toilets, A'(, )*+ risers are located at each end of the floor

!hich are to be cast in-situ.

2. Design Information

a. Codes of Practice

& %$ esign /oading for uilding

0 % The &tructural (se of 0oncrete

0$, 0hapter 3 ind /oad

b. Materials

Concrete 0$5 for topping, !alls and all other in-situ !orks

04 for precast beam

05 for precast columns and hollo! core slabs

Steel

c. Dead loads

0oncrete density 6 24

artitions, finishes and services 6 1."

rick!alls 6 $.5

d. Liveloads

7ffices, staircases, corridors 6 24

/ift lobby, A'( rooms 6 1."

Toilet 6 $.5

3. Structural System

recast construction is adopted from the second storey up!ards to take advantage of the

regular building grids and simple structural layout. The areas from grids A to 0 and from 8 to

9 are, ho!ever, cast in-situ due to drops, floor openings and !ater-tightness considerations.

eside acting as load bearing !alls, staircase !ells and lift cores also function as stabilising

cores for the superstructure. The !alls are $55 mm thick, cast in-situ and are tied monolithically

at every floor.

The precast components consist of hollo! core slabs, beams, columns and staircase flights.

a. Hollow core slabs

The design of hollo! core slabs :21 mm thick; is based on class 2 prestressed concrete

structure !ith minimum 2 hours fire rating. The hollo! core slabs are cast !ith 05 concrete.

+ach unit :1.2 m nominal !idth; is designed as simply supported !ith nominal155 mm seati

at the support.

<esultant stresses are checked at serviceability and at prestress transfer. esign of the slab

is carried out by the specialist supplier.

b. Precast beams

$ mm deep full precast beams are used in the office area. The beams, !hich are unpropped

during construction, are seated directly onto column corbels and are designed as simply

supported structures at the final stage. To limit cracking of the topping concrete at the supports,

site placed reinforcement is provided as sho!n in the typical details in #igure 4.2.

c. Precast columns

fy 6 25 =>mm² mild steel reinforcement

fy 6 4%5 =>mm² high yield steel reinforcement

fy 6 4" =>mm² high yield steel reinforcement

k=>m³

k=>m²

k=>m²

k=>m²

k=>m²

k=>m²



The columns are 55 mm x 55 mm and are cast 2-storey in height !ith base plate connection

at every alternate floor. They are designed as pin-ended at the ultimate limit state.

The base plate connection is designed !ith moment capacity to enable the columns to

behave as a 2-storey high cantilever. This is to facilitate floor installation !orks !hich are to

be carried out t!o floors in advance of a finally tied floor at any one time during the construction

of the office block. The use of base plate connection !ill eliminate heavy column props and

result in a safer and neater construction site. A nominal 5 mm gap is detailed in the design of

the column-to-column connection in order to provide sufficient tolerances for the insertion of

in-situ reinforcement at the beam support regions. The gap !ill be filled !ith

05 non-shrink grout.

+ach column is cast !ith reinforced concrete corbels in the direction of the precast beams.

The corbels are provided !ith T2 do!el bars !hich are used to prevent toppling of the

precast beams !hen the hollo! core slabs are laid. The depth of the corbel is designed to be

concealed visually !ithin the final ceiling space.

At the final state, all columns are considered braced in both directions.

d. Floor diapragm action and structural integrity

All precast components are bound by a % mm thick concrete topping !hich is reinforced !ith

a layer of steel fabric. The steel fabric serves as structural floor ties in order to satisfy the

integrity ties re?uirement under the building robustness design considerations.

The final floor structure !ill behave as a rigid diaphragm !hich transmits hori@ontal loads to

the stabilising cores at each end of the floor.

!. Design of Precast Components

a. Design of Hollow Core Slab

=et length of hollo! core slab 6 "555-55-155

6 455

6 5.5%24

6 1.%

Bmposed 6 1.

// 6 $

#rom #igure 2. of 0hapter 2, 21 mm thick hollo! core slab is ade?uate. Actual design

of prestressing reinforcement !ill be furnished by the producer of the slabs.

Support reinforcement

Although the slabs are designed as non-composite and simply supported, additional loose

/oading C / 6 Topping :% mm thick;

"555

25 5

155 155

255



reinforcement should be placed in the topping concrete over the support in order to minimise

surface cracking and limit the crack !idths. The additional support reinforcement may be

calculated as belo! C

/oading C / :imposed; 6 1.%

// 6 $

(ltimate load 6 1.4 1. D 1.% $.5

6 .2

&upport moment 6

6

6 $2.

h 6 21D%

6 2"5

@ 6 5."h

6 224

As 6 )>:5."fy@;

6 $$.5 15E%>:5."

6 $%".1

5.5"$?l²

5.5"$ X .2 X .4²

"se #$% & 2%% c'c ()s * 3+3 mm² 'm,



55

% 5 5

4T25 :B6$555; %

F5 0orrugated pipebe grouted

4T25 :B6$555;

% 5 5

55



T15G255 :B62455;% mesh

% 5 5

$ 2 5

2 1

% t o

p p i n g

21 T

55>"55beam

"55

15

55 1555

T15G255 %

% 5 5

1 2

55>"55x$ deep prec

155 seating

25 455

15



!. Design of Precast -eams

%5

T15G%55 c>c

255

2 5 5

% mesh

2mm Hap

% mesh T15G255 c>c

5.$x&lab span

% 5 5

455

$55

"555



$. Design oadings

a. -ending Moment

? 6 / :beam s>!; 6 :5." x 5.$2 D 5.21 x 5.; x 24

'ollo! core slab :8ointed !t; 6 2."x.

Topping 6 5.5%x24x"

Bmposed 6 1.I"

Total

// 6 $.5x"

Design ultimate load 6 1.4x%."D1.%x24

2. imit State Design

6 11".5$x.$E2>"

h 6 $ mm

d 6 $- mm

b 6 55 mm

6

6 .$x15E-4

Js 6 .$x15E-4x4

As 6 4"%

)t mid&span M/ * 0l² '1

)D>:bd2fcu; "%.2 15E%>:55 4%5E2 4;

#rom #igure 2.2% in 0hapter 2, ρs>fcu

mm²

55

55x55 0 0olumn

55

"55

1515

$

$ 2 5

2 1

l6$55



Proide #32 ()s * 452+ mm2,

JsK>fcu 6 min. 4.4x15E-4

JsK 6 5.51"

AsK 6 4%5 mm²

Proide 3#$5 ()s * 5%3 mm²) in top face as sown below

6.$x15E-4

65.1%1



b. 6ertical Sear

Total shear 3 6 11".5$ x .$>2

6 4$5." k=

v 6 4$5." x15E$> :55x4%5;

6 1."$

)inimum steel to be provided at the beam end to prevent bond slip failure isC

As 6 3>5."fy 6 4$5."x15E$>:5."x4%5;

6 111

6 5.4L

#rom #igure 2.2, in 0hapter 2, vc 6 5.

'ence Asvfsv 6 :1."- 5.; 55>:5." 4%5;

6 1.

Proide #$%& 4c'c for $.! m bot ends. (%.2l,

( )sfs * 2.%+ 7 $.5%8 9:,

=>mm²

mm²

=>mm²

$T1%

4T15

T$2



At 1.4 m from beam end, 3 6 4$5."- 11".5 x 1.4 k=

6 2%.% k=

v 6 2%.% 15E$ > :55 4%5;

6 1.1

Asv>&v 6 :1.1%- 5.; 55 B :5." 4%5;

6 5.1

Proide #$%&2%% lin;s ()s's, * %.1 7 %.$, for te remaining middle span.

c. Interface Hori<ontal Sear

Although the topping is considered non-structural, it is prudent to check that there should be

no separation of the topping from the precast beam at the interface. At mid-span, compression

generated in the topping concrete isC

0ompression #orce 6 5.4fcu be t

6 5.4 $5 55 % 15E-$

6 4$"." k=

0ontact idth 6 55 mm

0ontact /ength 6 5.le

6 5.x$55

6 $.% m

Average interface hori@ontal shear stress,

vh 6 4$"." x 15E$>:55 x $%5;

vh 6 5.24

peak vh 6 2x5.24

6 5.4"

Hence te topping layer sould not separate at te interface.

=>mm

=>mm

=>mm²

=>mm²

=>mm²

wic is less tan %.44 ='mm² in Part $8 #able 4.48 CP54 for as&cast surface witout lin;s.



d. Support >einforcement

Although the beams are designed as simply supported structures, it is advisable to place

additional reinforcement over the support to prevent excessive cracking !hen the beams

rotate at the support under service load. The additional steel may be calculated as follo!s C

/oading C / :imposed; 1.x" 6 14

// $x" 6 24

1.4x14D1.%x24 6 "

&upport moment 6 5.5"?lM

6 5.5" " .$M

6 24.$

h 6 %55

@ 6 5."h

As 6 )>:5."fy@;

6 24.$ 15E%>:5."

6 12"%

Proide !#2% ()s* $24 mm2 , oer te support as sown in Figure !.2.

e. Deflection

At mid-span, )D 6 "%.1

6 "%.2 15E%> :55

6 .4$

fs 6 :>";fy :As re?>As p

6 :>"; 4%5 :4"%>

6 2"5.2

#rom art 1, Table $.11, 0% the modification factor for tension reinforcement6 5. and for

compression reinforcement 6 1.5"

)inimum d 6

6 45.%mm N 4%5 mm

(ltimate loading

)D>bd²

$55 I :25 x 5. x 1.



f. ?nd -earing

i. ?ffectie -earing @idt

ermissible ultimate bearing stress from art 1, clause .2.$.4, 0 %

6 5.4fcu

6

6 1"

To even out surface irregularities at the beam and corbel surfaces, provide 155 x 45 x " mm

thick neoprene pad at the beam support.

0ontact pressure 6 4$5." 15$>:155 4

6 .$

6 .N1"

ii. Sear Friction Steel

5.4 X 4

=>mm

=>mm

=>mm

4 -

255

12

25

' 6 % 5 5

155x45x" thk



'ori@ontal shear friction steel is calculated as As 6 3>5."fy assuming conservatively 6 1.5

As 6 4$5." 15E$>:5."

6 15%

rovide $T1% loops at 5 c>c at the beam ends as sho!n. :As6125mmM;

Cec; minimum bar bending radius8 r

Tension force per bar 6 5." x 4%5 x 251 x 15

6 "5.44

Actual tension force per bar #bt 6 "5.4 15%>125

6 1.1

ab 6 5

F 6 1%

)inimum r 6 #btO1 D 2:F>ab;P >2fcu

6 1.x15E$O1 D2:1%>5

6 "1.%%

say r 6 %F

6 %

c. Design of Precast Column

A typical 2-storey high precast column is sho!n in #igure 4. belo!C

mm²

$xT1% (-loop G 5 c>c

55

r6%F

T1% /oop

25

$ 5

% 5 5

1

%

t h k

5 thk non-shrink groutolt from lo!er 0 column



$. Design oadings

/C beams 6 :5." x 5.$2 D 5.21 x 5.; x 24 x .$ 6 %$.% k=

h.c.s. 8ointed !eight; 6 2." x . x " 6 1$.4 k=

topping 6 5.5% 24 " " 6 ."4 k=

imposed dead load 61. x " x " 6 112 k=

self !eight 6 5. x 5. x 24 x $.% 6 $5.24 k=

corbel 6 5.2%2 x 5.x 5. x 24 x 2 6 1. k=

4"5. k=

//C 6$ x " x " 6 12 k=

/C 6 4"5. 12 6 % k=

// :5L reduction; 6 112 k=

(ltimate axial load 6 1 k=

2. Column Design

All columns are braced and pinned at the floor.

About minor axis Be 6

6 1.5x$.%

6 $.%

le>b 6 $%55>55

#rom roof to 2nd storey :12 floors;, total column load :above 1st storey;C

6 12.5 X 12 X 5.

6 1.4 %".4 D 1.% 112.5

ßlo

% 5 5

corbel

2 storey height single length 00olumn

55

1255

25 2555

5 5

5 5

T2 do!el



6 .2

6 .2N1

Columns are considered as sort columns.

#rom art 1, e?uation $", 0 %,

= 6 5.4fcuAc D 5.Ascfy

= 6 1 k=

fcu 6 4

Asc 6

Asc 6 "4%1

0heck bearing capacity of column section in contact !ith base plate

The ultimate bearing stress at interface !ith steel plate6 5."fcu :art 1, clause .2.$.4;

late si@e 6 45 x %5 mm

'ence direct bearing capacity 6

6 1155 k=

&teel reinforcement need not be provided from the steel plate into the column section.

3 Column to column Aoint

a. 6ertical Aoint strengt

8oint bet!een column to column is 5 mm thick and is embedded 1 mm belo! the floorKs

final structural level :#igure 4.2;. ased on the lo!er column section, the ultimate bearing

stress for bedded bearing6 5.%fcu :clause .2.$.4;.

Assuming non-shrink grout strength fcu 6 5

Total bearing strength 6

6 1555 k=

b. -ase plate connection

The steel base plate connection at alternate floors is used to provide moment capacity for the

stability of t!o-storey high cantilever column at the installation stage. The base plate connection

is designed to allo! for precast components to be installed t!o storeys ahead of a

completed floor. The columns are unpropped in order to achieve fast installation and minimise

site obstruction by props. The design of the base plate connection needs to consider different

load combinations so as to arrive at the most critical stresses. Bn accordance !ith the 0ode,

the follo!ing load combinations are consideredC

0ase 1 6 full dead and construction live loads

6 1.4/ D 1.%//

0ase 2 6 full dead D construction live D!ind :or notional load;

6 1.2 :/ D // D /; or

6 1.2 :/ D //; D =otional load

0ase $ 6 full dead D!ind :or notional load;

6 1.4 :/ D /; or 6 1.4 :/; D =otional load

The design !ind load is considered less critical and its effects are mainly felt from the exposed

=>mm²:1.5 X 15E$ -5.4 X 5 X 55 X 55; > :5. X 4%5; =>mm²

=>mm²

Proide $2#32 ()sc * +54% mm² , 2.1B,

5." X 5 X 45 X %5

=>mm²

5.% 5 55 55



faces of the beam and column perpendicular to the !ind direction. This !ill be less than the

notional load of 1.L dead load.

The base plate connection is be checked in both the maQor and minor column axes and

taking into account the column slenderness effect.

y inspection, the most critical stresses !ill take place at the edge columns next to the in-situ

floor as there !ill be minimum axial load !ith maximum overturning moment and hence

maximum tensile forces in the bolts during the temporary stage. The transfer of floor loading

to the edge column is sho!n in #igure 4.".

A final check on the connection should be made to ensure that continuous vertical ties are

provided through the column Qoint and throughout the building height.

)ial column load per floor

/C eams 6 $1."4 k=

'ollo! core slab 8ointed !eight; 2." x . x 4 6 "%. k=

Topping 6 4.2 k=

1%". k=

// :construction; 1. x 4 x " 6 4" k=

=otional ori<ontal load

er floor ' 6 1. x 1%".>155 6 2.2" k=

:acting at the top of th

ending moment at point $ in #igure 4." 6 2. x :$.51 D %.%1;

6 24.5" k=

:5." 5.$2 D 5.21 5.; 24 .$>2

5.5% 24 4 "



mm

k=>m²

k=>m²

k=>m²



5

k=>m>m

mm

mm

%5 224;

k=>m²

k=>m²

k=>m²

mm²>mm



)esh

2

s :4nos; to



k '.0. &lab

$ deep precast

)esh

% t

o p p i n g

21 Thk '.0. slab

st beam



% 5 5

%



6 ".24 k=>m

6 21.%" k=>m

6 12.4" k=>m

6 14.24 k=>m

6 .12 k=>m

6 24 k=>m

6 11" k=>m

6 "%.1 k=m

6 4%5 mm

6 5.1%1

6 5.52$

$



k=>mM

k=>mM

k=>mM

k=>m>m

mm

4%5 5."x%55;

mmM>mm

k=-m

%5E2;

ov;

2;

=>mmM

79

";



5;

79



4%5;

-$

k=

k=

mm

mm

:+?. 5 art 1;

;P>:2x4x1%;

mm

mm

mm



$ % 5 5

$ % 5 5

1



˃ 1 k=

˃ 1 kN 79

15E-$

15E-$



e corbel;

precast structural design updated

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