PACIFIC CONCRETE CONFERENCE

New Zealand 8-11 November 1988

THE APPLICATION OF NZS 3106:1986 TO CIRCULAR PRESTRESSED CONCRETE RESERVOIRS

R G Taylor

Special Projects Office

Works and Development Services Corporation (NZ) Ltd

1.0 SUMMARY

The New Zealand code of practice NZS 3106:1986 (S] "Design of Concrete Structures for the Storage of Liquids" presents a fully prestressed (alternatively called an allowable stress) design approach. The code also permits, as an alternative, a partially prestressed design but does not

provide detailed methodology. In WORKS' experience direct application of the allowable stress approach requires up to 74% more hoop post-tensioning

and considerably more vertical pretensioning than the former New Zealand

provisional code, DZ 3106 Part 2:1978 [6]. With use of a partially

prestressed design, levels of prestress similar to those used in previous

satisfactory designs have been achieved. To overcome the time-consuming

calculations of a partially prestressed design method, a computer

programme using LOTUS 1-2-3 was developed.

2.0 INTRODUCTION

Late 1986, a new New Zealand Code of Practice NZS 3106:1986 (S] was

released. This new code sets out design loads and it provides detail on a

fully prestressed (or allowable stress) approach.

The fully prestressed (or allowable stress) approach of NZS 3106:1986 was applied to a reservoir selected from a range of eight standard reservoirs

previously designed by WORKS to DZ 3106 Part 2:1978. Results indicated

that of the order of 70% more hoop post-tensioning and considerably more vertical pretensioning is required.

Consideration of temperature, swelling and shrinkage loads with no

allowance for cracking is a major factor in the requirement for high

levels of prestress with the allowable stress approach of NZS 3106:1986.

NZS 3106 permitted a reduction factor FT applied to loads in reinforced

concrete reservoirs, which allows for the softening effect due to

cracking. It is understood (pers comm J Vessey) that relatively little

proven information was available to the code writers, on the cracked

stiffness of prestressed sections. Consequently no reduction was recom­

mended in the code for prestressed elements, ie, FT = 1.0.

This paper shows that considerable savings in prestress can be achieved using a partially prestressed approach permitted by NZS 3106:1986.

Experience indicates that it is only necessary to apply the partially prestressed approach to the temperature load case of NZS 3106; economical

design can be achieved by applying the fully prestressed or allowable

stress approach to all other load cases.

453

PARTIALLY PRESTRESSED DESIGN OF CIRCULAR CONCRETE RESERVOIR

Input Data

Number of position on wall 8 Nonstrain related stresses eg, EQ, F, EP

FNO (Outside stress) :\1Pa 2.67 F:---fl (Inside stress) MPa 2.67 (Tension + ve)

t ( section thickness) mm 150 b (section width) mm 1000

fs(p) ( steel stress) MP a 100 A (Area steel) mm2 433 s d ( depth to steel) mm 115 A' (Area comp. steel) mm2 433 d' (Depth to Comp. steel) mm 35 A ps (Area prestress) mm2 1170

f ps (Stress in prestress) MPa 818

VertiHoop index (0/ 1) Strain related stresses eg, T, Esh' EswFSO (Outside stress) :vtPa FSI (Inside stress) MPa (Tension +ve)

n ( :vtodular ratio) C ( creep coefficient) Es (Young's mod steel) MPa

fcuc (After loss prestress) MPa

fdc (Concrete compressive stress) :V1Pa

FACTOR! FACTORA

1

-6.229.20

6.73 1.80

200000 -5.70

-401.41.2

FIG. 3: INPUT PANEL FOR SPREAD SHEET PROGRAMME

Output Data Basic Moments/ Axial

MN (Nonstrain related) kNm 0 MS (Strain related) kNm 28.9 CN (Nonstrain related) kN 400 cs (strain related) kN 223.5

Reduced Moments Applied Computed Capacity RTI (Crack I/gross I) 0.327 RTA (Crack A/gross A) 0.581

Mreduced 9.45 M . dres1ste 21.65

Creduced 530.3 C . dres1ste 530.3

Compare above results to check if adequate Capacity

Max Concrete Stress MPa -10.5X (Compression block) mm 80.1

Calculate Crack Width A l (Area around bar) 5250 w (Crack Width) mm 0.124

FIG. 4: OUTPUT PANEL OF SPREAD SHEET PROGRAMME

457

___ 1_2_m_□ __ �OLUMl'S

r CONCRETE ROOF

• 150mm rT RADl

�S

� . IJ t=========�-w

1-1

FIG. 5 : ST ANDA RD 2500m3 CIRCULAR PRESTRESSED RESERVOIR USED FOR COMPARISON OF CODES AND DESIGN METHODS

1-0 \ P {PREVIOUS DESIGN LEVEL OF

\ \

\ EFFECTIVE POST-TENSIONING)

..J 0·8 <(

o.. 0·6 ::)

::c 0·4 £2

::c 0·2

. �

\ ·,

. {

F + 0.8E + 0.5Sw {NZS 3106 EARTHQUAKE LOAD CASE)

F ,1- 0.5Sw {NZS 3106 FLUID LOAD CASE).

4·0 2·0 0 2·0 4 ·0 6·0 8{) COMPRESSION TENSION (MPa)

FIG. 6 : EARTHQUAKE AND FLUID LOAD CASES, HOOP STR,ESSES IN WALL

rF + T + 0.35Sw INSIDE OF WALL

--- ..... , 1·0 - •�,l {NZS 3106 TEMPERATURE LOAD CASE)

' . '\. ........' 0·8 •• '

\ ' ', P (PREVIOUS LEVEL OF \ •. EFFECTIVE POST-TENSIONING) \ 0·6 '

S \ • \F +T+0.35 w \ •, IOUTSIDE OF \0-4 •• IWALL

� I ,,f

,,,,. ✓0-2 ...... � .,...../.. REQUIRED LEVEL OF EFFECTIVE_____ ___

•• POST-TENSIONING BY NZS 3106__,..--,---.---.--..--.-=r:�--'--r--..,..-.c;...---fl,:._,...-+-�-r--,_- FOR FULLY PRESTRES SEO6·0 6·0 4·0 20 0 2·0 4·0 6-0 8·0 DESIGN COMPRESSION TENSION ( MPa )

FIG. 7 : TEMPERATURE LOAD CASE, HOOP STRESSES IN WALL

459

...J ...J

< O· � C.

::::, O· 1-:c

£:! O· w :r:

P (PREVIOUS LEVEL OF EFFECTIVE PRETENSIONING )

REQUIRED LEVEL OF EFFECTIVE PRETENSIONING FROM NZS 3106

P +sh+ D (NZS 3106 TANK EMPTY LOAD CASE)

0.7 MPa RESIDUAL COMPRESSION

4·0 2·0 0 2 · 0 4· 0 6·0 8·0

COMPRESSION TENSION (MPa) FIG. 8 : TANK EMPTY CASE

"'

VERTICAL STRESSES IN WALL

1-:r:

£2 O·w :c

P (PREVIOUS LEVEL OF EFFECTIVE PRETENSIONING)

REQUIRED LEVEL OF PRETENSIONING FROM NZS 3106

/F + P + 0.8E + 0.5Sw•,_

(NZS 3106 EARTHQUAKE LOAD CASE )0

ALLOWABLE TENSION 3.2 MPa ----ro-.--,-Jll;��....--.....--....---.....---.--.---

4 · 0 2·0 0 2·0 4·0 6·0 8·0

COMPRESSION TENSION (MP a) FIG. 9 : EARTHQUAKE LOAD CASI;:,

VERTICAL STRESSES IN WALL

<(

� C.

:c c:, w :c

4·0 2·0

1·0

0·8

0·6

0·4

0·2

COMPRESSION FIG 10 :

P (PREVIOUS LEVEL OF EFFECTIVE PRETENSIONING)

REQUIRED LEVEL OF PRESTRESS FROM NZS 3106 ALLOWABLE TENSION 3.2 MPa

.

I

a

/'-F+P+T (NZS 3106 TEMPERATURE LOAD CASE)

0 2·0 4·0 6·0 8·0

TENSION (MPa) TEMPERATURE LOAD CASE, VER°TICAL STRESSES IN WALL