pacific concrete conference
TRANSCRIPT
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