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Title: Transactions of the American Society of Civil Engineers, Vol. LXX, Dec. 1910 - A Concrete Water Tower, Paper No. 1173
Author: Kempkey, A.
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "Transactions of the American Society of Civil Engineers, Vol. LXX, Dec. 1910 - A Concrete Water Tower, Paper No. 1173" ***
AMERICAN SOCIETY OF CIVIL ENGINEERS
INSTITUTED 1852
TRANSACTIONS
Paper No. 1173
A CONCRETE WATER TOWER.[A]
BY A. KEMPKEY, JR., JUN. AM. SOC. C. E.[B]
WITH DISCUSSION BY MESSRS. MAURICE C. COUCHOT, L. J. MENSCH,
A. H. MARKWART, AND A. KEMPKEY, JR.
The City of Victoria is situated on the southern end of Vancouver
Island, in the Province of British Columbia, Canada, and is the capital
of the Province.
In common with all cities of the extreme West, its growth has been very
rapid within the last few years. The population of the city proper,
together with that of the municipality of Oak Bay, immediately adjacent,
is now about 35,000.
The Victoria water-works are owned by the city and operated under the
direction of a Water Commissioner appointed by the City Council. By
special agreement, water is supplied to Oak Bay in bulk, this
municipality having its own distributing system.
The rapid increase in population, together with the fact that in recent
years very little had been done toward increasing the water supply,
resulted in the necessity for remodeling the entire system, and there
are very few cities where this would involve as many complex problems or
a greater variety of work.
Water is drawn from Elk Lake, situated about five miles north of the
city; thence it flows by gravity to the pumping station about four miles
distant, and from there is pumped directly to the consumers.
The remodeling of the system, as recently completed, provided for:
1.--Increasing the capacity of Elk Lake by a system of levees.
2.--Increasing the capacity of the main to the pumping station by
replacing about two miles of the old 16-in., wrought-iron, riveted pipe
with 24-in. riveted steel pipe.
3.--Increasing the capacity of the pumping station by the installation
of a 4,500,000-gal. pumping engine of the close-connected,
cross-compound, Corliss, crank-and-fly-wheel type.
4.--The construction of a 20,000,000-gal. concrete-lined distributing
reservoir in the city.
5.--The entire remodeling of the distributing system, necessitating the
laying of about 1/2 mile each of 18-in. and 27-in. pipe, and about 1
mile of 24-in. riveted steel pipe; also about 3,000 tons of cast-iron
pipe, varying in size from 4 to 12 in.
6.--The provision for a high-level service by means of an elevated tank
of approximately 100,000 gal. capacity, water being supplied to the tank
by two electrically-driven triplex pumps, each having a capacity of
100,000 gal. per 24 hours, against a dynamic head of 150 ft., and
arranged to start and stop automatically with a variation of 3 ft. in
the elevation of the water in the tank. These pumps are located about
one mile from the tower, and are controlled by a float-operated
auto-start, in the base of the tower.
A description of the elevated tank, which is novel in design, with the
reasons for adopting the type of structure used, the method of
construction, and the detailed cost, form the basis of this paper.
The tower is on the top of the highest hill in the city, in the heart of
the most exclusive residential district, beautiful homes clustering
about its base. The necessity for architectural treatment of the
structure is thus seen to be of prime importance. In fact, the
opposition of the local residents to the ordinary type of elevated tank,
that is, latticed columns supporting a tank with a hemispherical bottom
and a conical roof, rendered its use impossible, although tenders were
invited on such a structure.
It is believed that under the conditions of location, three types of
structure should be considered: First, an all-steel structure, the
ornamentation being produced by casing in with brick or concrete;
second, a brick-and-steel, or a concrete-and-steel, structure, such as
the one actually erected; third, a typical reinforced concrete
structure.
Considering only that portion below the tank, the amount of material
required to case in a structure of the first type would be substantially
the same as that used to support the tank in a structure of the second
type. Consequently, the steel substructure, for all practical purposes,
would represent a dead loss, and, therefore, the economy of this type is
open to serious question.
A tender was received for a reinforced concrete structure identical in
outward appearance with the one built, but, owing to the natural
conservatism of the local residents regarding this type of construction,
it was not acceptable.
The tower, as built, consists of a hollow cylinder of plain concrete,
109 ft. high, and having an inside diameter of 22 ft. The walls are 10
in. thick for the first 70 ft. and 6 in. thick for the remaining 39 ft.,
and are ornamented with six pilasters (70 ft. high, 3 ft. wide, and 7
in. thick), a 4-ft. belt, then twelve pilasters (12 ft. high, 18 in.
wide, and 7 in. thick), a cornice, and a parapet wall.
A steel tank of the ordinary type is embedded in the upper 40 ft. of
this cylinder. To form the bottom of this tank, a plain concrete dome is
thrown across the cylinder at a point about 70 ft. from the base, the
thrust of this dome being taken up by two steel rings, 1/2 in. by 14 in.
and 3/8 in. by 18 in., bedded into the walls of the tower, the latter
ring being riveted to the lower course of the tank.
The tank is covered with a roof of reinforced concrete, 4 in. thick,
conical in shape, and reinforced with 1/2-in. twisted steel bars. The
design of the structure is clearly shown in Fig. 1.
The tower is built on out-cropping, solid rock. This rock was roughly
stepped, and a concrete sub-base built. This sub-base consists of a
hollow ring, with an inside diameter of 20 ft., the walls being 5 ft.
thick. It is about 2 ft. high on one side and 7 ft. high on the other,
and forms a level base on which the tower is built. The forms for this
sub-base consist of vertical lagging and circumferential ribs. The
lagging is of double-dressed, 2 by 3-in. segments, and the ribs are of 2
by 12-in. segments, 6 ft. long, lapping past one another and securely
spiked together to form complete or partial circles. These ribs are 2
ft. from center to center.
[Illustration: FIG. 1.--(Full page image)
WATER TOWER VICTORIA, B.C. WATER-WORKS]
Similar construction was used to form the taper base of the tower
proper, except, of course, that the radii of the segments forming the
successive ribs decreased with the height of the rib. Tapered lagging
was used, being made by double dressing 2 by 6-in. pieces to 1-3/4 by
5-13/16 in., and ripping on a diagonal, thus making two staves, 3 in.
wide at one end and 2-3/4 in. wide at the other. This tapered lagging
was used again on the 4-ft. belt and cornice forms, the taper being
turned alternately up and down.
[Illustration: FIG. 2.--FORMS FOR WATER TOWER VICTORIA, B.C.]
The interior diameter being uniform up to the bottom of the dome,
collapsible forms were used from the beginning. These forms were
constructed in six large sections, 6 ft. high, with one small key
section with wedge piece to facilitate stripping, as shown in Fig. 2.
There were three tiers of these, bolted end to end horizontally and to
each other vertically.
Above the taper base and except in the 4-ft. belt and cornice,
collapsible forms were used on the outside also. There were six sections
extending from column to column and six column sections, all bolted
together circumferentially and constructed as shown in Fig. 2. Three
tiers of these were also bolted together both vertically and
horizontally.
Having filled the top tier, the mode of operation was as follows:
All horizontal bolts in the lower inside and outside forms were removed,
as was also the small key section on the inside; this left each section
suspended to the corresponding one immediately above it by the vertical
bolts before mentioned. It is thus seen that in each case the center
tier performed the double duty of holding the upper tier, which was full
of green concrete, and the sections of the lower tier, until they were
hoisted up and again placed in position to be filled.
These lower forms were then hoisted by hand--four-part tackles being
used--and placed in position on the top forms, their bottom edges being
carefully set flush with the top edge of the form already in position,
and then bolted to it. On the outside, the column forms, and on the
inside, the wedge and key sections were set last. A 3-lb. plumb-bob on a
fine line was suspended from the inner scaffold and carefully centered
over a point set in the rock at the base. This line was in the exact
center of the tower, and the tops of all the forms, after each shift,
were carefully set from it by measurement, thus keeping the structure
plumb.
The first 23 in. of the barrel of the tower was moulded with special
outside forms, constructed so as to form the bases of the large
pilasters. After eleven applications of the 6-ft. forms, these 23-in.
sections were reversed to form the capitals, thus making these
pilasters, 69 ft. 10 in. over all.
The forms of the 4-ft. belt and beading were made in twelve sections of
simple segments and vertical lagging, as shown in Fig. 2.
Two sets of the outside forms were split longitudinally, as shown in
Fig. 2, and used to form the small pilasters. The first set was put in
place, filled, and the concrete allowed to harden. The bolts were
loosened and the forms raised 5-1/2 in. vertically, again bolted up, and
the second set was placed in position, bringing the top of the second
set up to the bottom of the cornice. The bases and capitals of the small
pilasters were moulded on afterward.
The cornice forms are clearly shown in Fig. 2. The small boxes
separating the dentils are made of light stuff, and tacked into the
cornice forms so that, in stripping, they would remain in place and
could be taken out separately, in order to prevent breaking off the
corners of the dentils. A number of outside and inside sections were
sawed in half horizontally in order to provide forms for the parapet
wall.
The inside diameter of the tank is 8 in. greater than the inside
diameter of the base. Two sets of inside forms were split longitudinally
and opened out, as shown in Fig. 2, and another small section was added
to complete the circle. The remaining set was left in place to support
the dome forms.
The dome forms were made in twelve sections, bolted together to
facilitate stripping. All ribs and segments were cut to size on the
ground, put together in place, and then covered with lagging and two-ply
tar paper. The lagging on the lower sharp curve was formed of a double
thickness of 3/8-in. spruce, the remainder being 1 by 4-in. pine, sized
to a uniform thickness of 7/8 in. Fig. 3 shows the construction of these
forms and the method of putting on the lagging.
The roof forms were made in eight sections and bolted together to
facilitate stripping. All ribs and segments were cut to size on the
ground, put together in place, and covered with 1 by 4-in. lagging,
dressed to a uniform thickness of 7/8 in., and two-ply tar paper. Fig. 3
shows the construction of these forms. The segments being put in
horizontally instead of square with the lagging, gave circles instead of
parabolas, making them much easier to lay out, and giving a form which
was amply stiff.
The question of using an inside scaffold only was carefully considered,
but owing to the considerable amount of ornamentation on the outside,
necessitating a large number of individual forms, it was not thought
that any economy would result.
Fig. 4 and Figs. 1 and 2, Plate XXIII, show clearly the construction of
the scaffolding.
[Illustration: PLATE XXIII, FIG. 1.--SCAFFOLDING FOR WATER TOWER.]
[Illustration: PLATE XXIII, FIG. 2.--COMPLETED WATER TOWER.]
All concrete was mixed wet, in a motor-driven, Smith mixer, and handled
off the outside scaffold, being sent up in wheel-barrows on the ordinary
contractor's hoist and placed in the forms through an iron chute having
a hopper mouth. This chute was built in three sections bolted together,
either one, two, or three sections being used, depending on the distance
of the forms below the deck. When the top of the forms reached the
elevation of any deck, the concrete was put in through the chute from
the deck above. The chute was light and easily shifted by the
wheel-barrow men, assisted by the man placing the concrete, during the
interval between successive wheel-barrows.
[Illustration: FIG. 3.--FORMS FOR WATER TOWER VICTORIA, B.C.]
The concrete, except that for the roof and parapet, was composed of sand
and broken rock, the run of the crusher being used. That for the roof
and parapet was composed of sand and gravel. The only reason for using
gravel for the concrete of the roof was the ease with which it could be
obtained in small quantities, the supply of broken rock having been used
up, and this being the last concrete work to be done.
The concrete used was as follows: 1:3:6 for the sub-base and taper base;
1:3:5 for the barrel of the tower and tank casing; and 1:2:4 for the
dome and roof. The dome was put in at one time, there being no joint,
the same being true of the roof. Vancouver Portland cement,
manufactured on the island about 15 miles from the city, was used
throughout the work.
Before filling, the inside of the tank was given a plaster coat,
consisting of 1 part cement to 1-3/4 parts of fine sand. This proved to
be insufficient to prevent leakage, the water seeping through the dome
and appearing on the outside of the structure along the line of the
bottom of the rings. Three more coats were then applied over the entire
tank, and two additional ones over the dome and about 8 ft. up on the
sides, and, except for one or two small spots which show just a sign of
moisture, the tank is perfectly tight.
The barrel of the tower was carried up to a height of 66 ft. A special
set of inside forms, about 2 ft. high, extending to the springing line
of the dome, was then put in, and the dome forms were set up on it. The
idea was that this 2-ft. form could be knocked out piece by piece and
the weight of the dome form taken on wedges to the last 6-ft. form,
these wedges being gradually slackened down in order to allow the dome
form to settle clear of the dome. As a matter of fact, this was done,
but the dome forms, being very tight, did not settle, and had to be
pried off a section at a time. A similar method was used for slacking
down the roof forms, with similar results.
After the dome forms had been put in, the concrete was carried up
approximately to the elevation of the bottom of the rings. Small neat
cement pads were then put in and accurately leveled, and on these the
steel rings were placed, and the steel tank was erected.
In order to insure a perfectly round tank, each course was erected
against wooden templates accurately centered and fastened to the inside
scaffold. The tank is the ordinary type of light steel, the lower course
being 3/16-in., the next, No. 8 B. w. gauge, the next, No. 10 B. w.
gauge, and the remaining four, No. 12 B. w. gauge.
Work on the foundation was started on August 15th, 1908, and the tower
was not completed until April 1st, 1909. Much time was lost waiting for
the delivery of the steel, and also owing to a period of very cold
weather which caused entire cessation of work for about one month.
The tower as completed presents a striking appearance. In order to
obliterate rings due to the successive application of the forms and to
cover the efflorescence so common to concrete structures, the outside
was given two coats of neat cement wash applied with ordinary
calcimining brushes, and, up to the present time, this seems to have
been very effective in accomplishing the desired result.
[Illustration: FIG. 4.--(Full page image)
SCAFFOLD FOR WATER TOWER]
Irregularities due to forms are unnoticeable at a distance of 200 or 300
ft., and the grouting gave a very uniform color.
The application of two coats of cement wash cost, for labor, $97.68, and
for material, $15.18, or $1.32 per 100 sq. ft., labor being at the rate
of $2.25 per 8 hours and cement costing $2.53 per bbl. delivered on the
work.
The tower was designed by Arthur L. Adams, M. Am. Soc. C. E., under
whose direction the plans for all the work of remodeling the water-works
system were prepared and executed. The forms, scaffolding, etc., were
designed by the writer, who was also in immediate charge of the
erection.
Tenders received for the construction of the tower covered an extremely
wide range, and indicated at once the utter lack of knowledge on the
part of the bidders of the cost of a structure of this kind. Inasmuch as
none of them had had previous experience in this class of construction,
the engineer deemed it the part of wisdom and economy to retain the
construction under his immediate supervision, and, therefore, the work
was done by days' labor.
Table 1 gives the cost of the structure. The total herein given will not
coincide with the total cost as shown by the city's books, for the
reason that various items not properly chargeable to the structure
itself have been omitted, the principal ones of which are the cost of
the site, the laying of about 600 ft. of sewer pipe to connect with the
overflow, and considerable expense incident to the construction of a
wagon road to the tower.
The rates of wages paid, all being on a basis of an 8-hour day, were as
follows:
Common labor $2.25 and $2.50
Carpenter 4.00
Carpenter's helper 2.75
Boiler-maker 3.50
Holders on 2.50
Boiler-maker foreman 5.00
Plasterers 6.00
Plasterers' helpers 3.00
The cost of material was as follows:
Cement, per barrel $2.53
Sand, per yard 1.47
Rock, per yard 0.80
Lumber, per 1,000 ft. b. m. 14.00 and 16.00
All these prices are for material delivered on the work.
An examination of the cost data, as given, will show that for the most
part the unit costs are very high. This is due chiefly to the continued
interruption of the work, during its later stages, owing to bad weather,
particularly in the case of the erection of the steel tank. The material
cost in this case was also exceedingly high.
In the case of the concreting, inability to purchase a hoist and motor
and the high cost of renting the same, together with the delays
mentioned, added greatly to the unit cost.
When it is considered that the cost of plastering covers that of four
coats over the entire inside of the tank and three more over about
one-third of it, it does not appear so high, especially in view of the
high rate of wages paid.
The cost per yard for concrete alone was $25.126, and this is probably
about 25% in excess of the cost of the same class of work executed under
more favorable conditions as to location, weather conditions, etc.
TABLE 1.--COST OF HIGH-LEVEL TOWER, VICTORIA WATER-WORKS. (412 cu. yd.)
=============================================================================
| TOTAL COST. | UNIT COST.
---------------------+---------+--------+----------+---------------+---------
| Rate | | | |
| per | Amount.| Complete.| Labor. |Material.
| hour. | | | |
---------------------+---------+--------+----------+---------------+---------
Preliminary Work: | | | | |
Labor, Carpenter |$0.50 |$11.00 | | |
Labor | 0.344 | 64.94 | | |
" | 0.281 | 249.67 | $325.61| $0.790 |
Material | | 133.62 | 133.62| | $0.324
| | | | |
Forms: | | | | |
Buildings, shifting | | | | |
and stripping: | | | | |
Labor, Carpenter | 0.50 |1,832.99| | |
Labor | 0.344 | 80.85| | |
" | 0.281 | 563.84| 2,477.68| 6.014 |
| | | | |
Material: | | | | |
Lumber | | 583.49| | |
Hardware | | 325.51| | |
Miscellaneous | | 13.90| 922.90| | 2.240
| | | | |
Scaffold: | | | | |
Erecting and | | | | |
tearing down: | | | | |
Labor, Carpenter | 0.50 | 693.00| | |
Labor | 0.344 | 350.59| | |
" | 0.281 | 117.27| 1,160.86| 2.818 |
| | | | |
Material: | | | | |
Lumber | | 487.77| | |
Hardware | | 202.79| 690.56| | 1.676
| | | | |
Concreting: | | | | |
Labor | 0.50 | 142.00| | |
" | 0.344 | 11.00| | |
" | 0.281 | 947.81| 1,100.81| 2.672 |
Material: | | | | |
Rock | | 317.30| | |
Sand | | 385.72| | |
Cement | |1,581.97| | |
Motor and Hoist: | | | | |
Rental | | 406.56| | |
Power | | 83.53| 2,735.08| | 6.638
| | | | |
Plastering | | | | |
(3,000 sq. ft.): | | | | |
Labor, Plasterers | 0.75 | 116.50| | |
Labor | 0.46-7/8| 15.00| | |
" | 0.37-1/2| 198.52| | |
" | 0.281 | 105.66| 435.68| 14.52 |
| | | | per sq. ft. |
Material: | | | | |
Sand | | 8.64| | |
Cement | | 66.10| | |
Alum and Potash | | 16.00| 90.74| 3.25 |
| | | | per sq. ft. |
| | | | |
Cement Wash | | | | |
(8,560 sq. ft.): | | | | |
Labor | 0.48-3/4| 50.00| | |
" | 0.281 | 47.68| 97.68|1.14 per |
| | | | 100 sq ft. |
Material: | | | | |
Cement | | 15.18| 15.18| 0.18 " " " " |
| | | | |
Windows, doors, | | | | |
and scuttle: | | | | |
Labor | 0.50 | 49.00| 49.00| |
Material: | | | | |
1 door, | | | | |
7 windows, etc. | | 47.26| 47.26 | |
| | | | |
Equipment: | | | | |
40% of $461.46 | | 184.58| 184.58| 0.448 |
| | | | |
Superintendence | | | 1,241.45| 1.506 |
| | | | |
Steel Tank: | | | | |
Labor, Carpenter |$0.50 | $124.24| | |
Helper | 0.344 | 2.75| | |
Boiler-makers | | 382.57| | |
Holders on | | 147.33| | |
Labor | | 40.61| | |
Foreman | 0.625 | 186.25| $883.75|$0.0441 per lb.|
| | | | |
Material: | | | | |
Tank, rivets, etc.| | | | |
(20,000 lb.) | | | 1,740.69| | $0.0875
| | | | |
Iron-work: | | | | |
Spiral stairway, | | | | |
inlet, and overflow| | | | |
pipes, ventilator, | | | | |
reinforcing steel, | | | | |
etc.: | | | | |
Labor, Machinists | 0.50 | 89.50| | |
Helper | 0.344 | 240.16| | |
Labor | 0.281 | 100.79| 430.45| |
| | | | |
Material | |1,814.71| 1,814.71| |
---------------------+---------+--------+----------+---------------+---------
Total | | |$16,578.29| |
=============================================================================
DISCUSSION
MAURICE C. COUCHOT, M. AM. SOC. C. E. (by letter).--It appears to the
writer that in the design of this structure two features are open to
criticism. The first is that such a high structure was built of plain
concrete without any reinforcement. Even if the computation of stresses
did not show the necessity for steel reinforcement, some should have
been embedded in the work. As a matter of fact, the writer believes
that, with the present knowledge of the benefit of reinforced concrete,
a structure such as this should not be built without it. This applies
mainly to the tower below the tank.
The second feature, which is still more important, refers to the
insertion of a shell of smooth steel plate to take the stresses due to
the hydrostatic pressure, and also to insure against leakage in the
walls of the tank. The 6-in. shell of plain concrete outside the steel
shell, and the 3-in. shell inside, do not work together, and are
practically of no value as walls, but are simply outside and inside
linings. Although the designer provided lugs to insure the adhesion of
the concrete to the plate, such precaution, in the writer's opinion,
will not prevent the separation of the concrete from the smooth steel
plate, and, at some future time, the water will reach and corrode the
steel. It would have been better to have reinforced the wall of the tank
with rods, as is generally done. The full thickness would have been
available, and less plastering would have been required. Furthermore,
the adhesion of concrete to a smooth steel plate is of doubtful value,
for, in reinforced concrete, it is not the adhesion which does the work,
but the gripping of the steel by the concrete in the process of setting.
L. J. MENSCH, M. AM. SOC. C. E. (by letter).--This water-tower is
probably the sightliest structure of its kind in North America; still,
it does not look like a water-tower, and, from an architectural point of
view, the crown portion is faulty, because it makes the tank appear to
be much less in depth than it really is.
The cost of this structure far exceeds that of similar tanks in the
United States. The stand-pipe at Attleboro, 50 ft. in diameter and 100
ft. high, cost about $25,000. Several years ago the writer proposed to
build an elevated tank, 60 ft. in diameter and 40 ft. deep, the bottom
of which was to be 50 ft. above the ground, for $21,000.
Among other elevated tanks known to the writer is one having a capacity
of 100,000 gal., the bottom being 60 ft. above the ground.[C] The total
quantities of material required for this tank are given as 4,480 cu. ft.
of concrete, 23,200 lb. of reinforcing steel, and 27,600 ft., b. m., of
form lumber and staging. Calculating at the abnormally high unit prices
of 40 cents per cu. ft. for concrete, 4 cents per lb. for steel, and $50
per 1,000 ft., b. m., for lumber, the cost of the concrete would be
$1,792, the steel, $928, and the form lumber and staging, $1,380. Adding
to this the cost of a spiral staircase, at the high figure of $7 per
linear foot in height, the total cost of this structure would be $4,598.
The factor of safety used in this structure was four, but some engineers
who are not familiar with concrete construction may require a higher
factor. By doubling the quantities of concrete and steel, which would
mean a tensile stress in the steel of only 8,000 lb. per sq. in., and a
compressive stress in the concrete of only 225 lb. per sq. in., the cost
of the tank would be only $7,318, as compared with the $16,578 mentioned
in the paper. This enormous discrepancy between a good design and an
amateur design, and between day-labor work and contract work should be a
lesson which consulting engineers and managers of large corporations,
who prefer their own designs and day-labor work, should take to heart.
A. H. MARKWART, ASSOC. M. AM. SOC. C. E. (by letter).--It is the
writer's opinion that the steel tank enclosed within the concrete of the
upper cylinder, to take up the hoop tension and presumably to provide a
water-tight tower, will not fulfill this latter requirement. If a
plastered surface on the dome-shaped bottom provided the necessary
imperviousness, it would seem that plastered walls would have proved
satisfactory.
Apparently, the sheet-metal tank is intended to exclude the possibility
of exterior leakage, but it occurs to the writer that it will fail to be
efficient in this particular, because, under pressure, the water will
force itself under the steel tank and the dome thrust rings and out to
the exterior of the tower just below the tank, thus showing that
insurance against leakage is actually provided by the plastered interior
surfaces and not by the sheet-metal tank, and, for this reason, ordinary
deformed rod reinforcement, in the writer's opinion, would have proved
cheaper and better, and more in line with other parts of the
reinforcement.
Mr. Kempkey states:
"Before filling, the inside of the tank was given a plaster coat,
consisting of 1 part cement to 1-3/4 parts of fine sand. This
proved to be insufficient to prevent leakage, the water seeping
through the dome and appearing on the outside of the structure
along the line of the bottom of the rings. Three more coats were
then applied over the entire tank, and two additional ones over the
dome and about 8 ft. up on the sides, and, except for one or two
small spots which show just a sign of moisture, the tank is
perfectly tight."
This substantiates the writer's contention that water-tightness was
actually obtained by a liberal use of cement plaster, which would also
have been true had the reinforcement been rods.
As a further comment, it might be stated that a water-tight concrete for
the tank could have been obtained by adding from 8 to 10% of hydrated
lime to the 1:2:4 mixture. This seems advisable in all cases where a
water-tight concrete is necessary. The interior plastering could then
have been done as a further precaution.
A. KEMPKEY, JR., JUN. AM. SOC. C. E. (by letter).--Mr. Couchot's
statement, that the 3-in. inside and outside sheets forming the tank
casing do not act together, is quite true, and it was not expected that
they would, other than to protect the steel and form an ornamental
covering for it.
There is certainly adhesion between concrete and steel, even though the
steel be in the form of a thin shell, and in a structure of this kind
where the steel is designed, with a low unit stress, to take all the
strain, and where the load is at all times quiescent, it is difficult to
see how this bond can be destroyed; the writer feels no concern on this
score.
Mr. Markwart's statement, that the steel tank enclosed within the
concrete of the upper cylinder, presumably to provide a water-tight
tower, will not fulfill this latter requirement, is not true, as shown
by the statement in the paper that the only leakage which occurred was
that which passed under the tank, the entire remaining portion being
absolutely tight. The amount of leakage, while insignificant, was, until
remedied, sufficient to spot the outside of the tower, making it
unsightly; and this, in the writer's opinion, is just what would have
happened had the tank been constructed in the ordinary manner, with
deformed bars, except that it would have extended over more or less of
the entire surface, instead of being localized, as was actually the
case, and would have required more instead of less plastering. It is
also doubtful whether the addition of hydrated lime would have produced
a tight tank, in the sense that this structure was required to be tight.
In the paper the writer endeavored to bring out the fact that this is
one of the few instances where the æsthetic design of a structure of
this sort is of prime importance, and cost a secondary consideration.
There is, therefore, no use in comparing its cost with that of a
structure in no way its equal in this respect and the use of which would
not have been permitted any more than the use of the ordinary type of
steel structure, even though the estimated cost were 75% less.
Mr. Mensch has been pleased to term this design amateurish, presumably
because of the conservative character of the stresses used and because
of its cost; at the same time, he sets up the design to which he makes
reference as a good one simply because of its cheapness. He will find
the "enormous discrepancy," to which he calls attention, accounted for
by the fact that the "good design" would not have been tolerated because
of its appearance and because of the fact that the excessively high
unit stresses, of which Mr. Mensch is an exponent, did not commend
themselves either to the designer, in common with most engineers, or to
Victorian taste; while the design used has proven eminently satisfactory
to a more than usually conservative and discriminating community.
Mr. Mensch's statement of unit costs, even though applied to a much
plainer structure, is not calculated to inspire confidence in the
soundness of his deductions in any one familiar with Victoria
conditions.
FOOTNOTES:
[Footnote A: Presented at the meeting of March 16th, 1910.]
[Footnote B: Now Assoc. M. Am. Soc. C. E.]
[Footnote C: "The Reinforced Concrete Pocket Book," p. 124.]
*** End of this LibraryBlog Digital Book "Transactions of the American Society of Civil Engineers, Vol. LXX, Dec. 1910 - A Concrete Water Tower, Paper No. 1173" ***
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