Giant Precast Silo for Storing Fertilizers Built in Mexican Port

Giant Precast Silo for
Storing Fertilizers
Built in Mexican Port
,j.
••
.;.
.i.
Jose Ma. Rioboo M.
Director General
Riobbo, S. A.
Consulting Engineers
Mexico City. Mexico
(Also, Professor and Head,
Structural Department,
School of Engineering,
Universidad Nacional
Autdnoma de Mexico)
♦=4
♦*r
huge silo for storing fertilizer
A (DAP,NPK)
was needed to fulfill
an increasingly important role at the
new Mexican port, Lazaro Cardenas
(Las Truchas), located on the Mexican
T
194.7
west coast facing the Pacific Ocean.
The general concept of the silo is
based upon two triangular section bays,.
224 ft (68.30 m) wide, 918.4 ft (280 m)
long, and 91.9 ft (28.00 m) high.
9.89.
Fig. 1a. Scheme showing main channel beams, purlins, columns and root cover.
108
The structure consists of a set of
three-hinged arches, each one formed
by two precast prestressed channel
beams, 146.6 ft (44.70 m) long, set 28.7
ft (8.75 m) from each other. These main
beams support precast prestressed concrete purlins; the roofing comprises asbestos structural sheets (see Fig. la).
Due to the prevailing conditions of the
site and the unusual length of the main
beams, a precast prestressed alternative
was chosen.
The project was completed on
schedule and within budget under very
difficult conditions, Fig. lb shows the
silo nearing completion.
This article will cover the structural
design, the production of the precast
elements, and the erection aspects of
the silo structure. Included also will be
a discussion of the construction
schedule and the cost of the project.
Synopsis
A huge precast silo for storing
fertilizers was built in Las
Truchas, a Mexican port located
on the Pacific Ocean. The structure has two triangular bays, each
one 224 ft (68.30 m) wide and
918.4 ft (280 m) long. The silo
structure covers a total area of
411,548 sq ft (38,248 m 2). The
DESIGN
precast prestressed concrete
main beams of this silo are channel sections, 146.6 ft (44.70 m)
long, and have a maximum depth
of 5.9 ft (1.80 m). This article
highlights the design, production,
and erection features of the project_ Also discussed are the construction schedule and economics
of the job.
Two precast prestressed concrete
elements were designed for this silo,
the main beams that formed the arches,
and the purlins that supported the asbestos sheets; however, the main
beams are the elements that made this
project such an interesting job.
The following material strengths
were used for the precast elements:
concrete strength, 5000 psi (34.5 MPa);
reinforcing bars, 60 ksi (414 MPa); and
prestressing steel, 270 ksi (1860 MPa).
Fig. 1b. Precast silo nearing completion in Las Truchas.
PCI JOURNALJJuIy-August 1982
109
3.28'
7.09"
7.09'
5.90°
FROM 3
TO 590'
3^fi
.94°
Fig, 2. Cross section of main channel beams, showing prestressing
strands and mild steel reinforcement.
CONCRETE
MAIN BEAMS
—1
STEEL
ADJUSTMENT
NEOPRE
PLATE
Fig. 3. Joint of main channel beams and foundation. Note neoprene
plate and "I" beams for adjustments.
110
Fig. 4. Joint between main channel beams. Note that two 1 1/s-in. (38 mm) diameter
screws tightened connections between both beams, cushioned by neoprene plate.
MAi
BDD
f►`C
l^
Fig. 5. Joint between main channel beams and cast-in-place columns.
PCI JOURNAWuly-August 1982
111
f 5.7
15.7
X1
'H
7.9 •`
STRANDS
Fig. 6. Cross section of purlins and
bracing. Note that five straight strands
were used.
N
Fig. 7. Joint between main channel beams and purlin or bracing.
Main Beams
The main beams are channel sections
with a length of 146.6 ft (44.70 m). The
depth varies from 5.9 ft (1.80 in) at the
middle of the beams to 3 ft (0.91 m) at
the supports. The width of the beams is
3.3 ft (1.00 m) (Fig. 2).
These beams were prestressed with
32 straight strands, in. (12.7 mm) in
diameter, located at the bottom of the
channel section. Some of them were
unbonded at the ends.
Both ends of these beams are solid
(concrete was cast between both legs)
112
and are shaped to provide a well designed hinge. At the top, there is a
joint between every pair of beams; at
the bottom, the beams are hinged to
the foundation.
These beams were designed to take
into account not only dead and live
loads but also seismic and wind loads.
The main beams were loaded in two
stages, the first one for live and dead
loads. During this initial stage, the
beams were assumed to be simply supported, i.e., connected at the foundation
and at the top (by the adjacent beam).
At the second stage, a third support, located in the same plane as the retaining
walls, was provided.
This third support, located 19.5 ft
(5.95 m) from the bottom end of the
beam, was needed to augment the relatively weak strength of the channel
cross section under compressive stresses, which are produced by negative
moments caused by wind suction. Note
that now the three supports make the
structure continuous,
At the foundation, there are several
post-tensioned tendons designed to
take the horizontal forces provided by
the hinges. The beams rest on neoprene plates, s/a in. (17 mm) thick. Horizontally, there are two steel "I" sections with the steel plates needed to
adjust the beams to the right position as
shown in Fig. 3. Later, the empty space
is filled with concrete.
Between every pair of main beams,
there is a vertical neoprene plate,
which has four horizontal screws 1' in.
(38 mm) in diameter. These screws
fasten the beams to the plate, thus
vertically joining the tops of every pair
of main beams (Fig. 4).
To join the main beams and the column, concrete was cast into the space
between both legs; afterwards, the two
elements were tightened with a vertical
screw (Fig. 5).
Purlins
The purlins of this silo are also precast prestressed concrete elements.
They are angular in shape, 15.7 in. (0.4
m) wide, 7.9 in. (0.2 m) thick, and 28.7
ft (8.75 m) long, with five straight
strands, 44z in. (12.7 mm) in diameter
(Fig. 6). These purlins are located 22 ft
(6.70 m) from each other, over the main
beams.
To provide an auxiliary support to the
purlins, the main beams have small
concrete boxes on their top surfaces.
The angles placed in the purlins were
welded to steel plates anchored to the
main beams (Fig. 7).
PCI JOURNALJJuly-August 1982
Wind and Seismic Bracing
Diagonal purlins forming horizontal
trusses were furnished one-third of the
length from each end of the silo. They
are provided to resist wind and seismic
forces and to brace the structure. These
elements are prestressed and precast
also, following the same procedure as
the purlins described above. The diagonal purlins also have five straight
'/a-in. (12.7 mm) diameter strands. Their
length is 32.8 ft (10.00 m). These purlins have a steel angle at each end,
welded to the steel plates embedded in
the main beams.
PRECASTING
It took about 136 working days to
precast the prestressed elements of this
silo. It should, however, be mentioned
that while the elements were being
cast, the silo was also being erected,
The precasting plant had the capability
to fabricate, at the same time, all the
elements needed for an arch.
Fig. B. Precast plant showing casting ends
of main channel beams. Note proximity to
silo under construction.
113
a ,.
Fig. 9. Equipment used in casting of prefabricated elements.
To cast the main beams, two steel
molds were designed with sufficient
flexibility to allow the element to be
removed easily. Due to the huge contact surface between the mold and the
elements, and the weight of the beams,
it would otherwise have been almost
impossible to take them out of the mold
(Fig. 8).
While the main beams were being
cast, the purlins and bracings for this
silo were cast in a nearby prestressing
bed. These beds were 206.7 ft (63 m)
long; therefore, it was possible to cast
14 elements at the same time. Because
the purlins and bracings have the same
section and prestressing force, it was
possible to cast them in the same steel
molds.
The equipment used in this plant
(Fig. 9) included.
—Two hydraulic jacks with the
capability to prestress a single strand
at a time;
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—A steam boiler to cure the concrete and accelerate its strength gain;
—A concrete mixer having a capacity of 15 cu yds per hr;
—Some carrying frames (portal
cranes) sliding on rails;
—Equipment needed for the quality
control laboratory;
—A tank for supplying water; and
— Silos for storing cement and aggregates.
The casting procedure employed in
this plant was fairly conventional. Essentially, it consists of getting the
molds ready, placing the strands and
the reinforcing steel, stressing the tendons, casting the concrete, and vibrating the fresh concrete.
Because of the unusual weight,
length and depth of the main beams, it
was necessary to cast these elements
close to the site of the job in order to
avoid the cost and difficulties of transportation.
Fig. 10. Erection of the first arch.
Fig. 11, Erection of main channel beams.
PCI JOURNAUJuIy-August 1982
115
ERECTION
At the floor, coinciding with every
arch, four post-tensioned tendons, 12 ^¢
7 mm, were inserted to carr
y the horizontal forces produced at the bottom
ends of the beams. Afterwards, the two
columns of every arch were cast in
place near the bottom ends of the main
beams. Finally, the asbestos sheets
were placed in position (Figs. 15 and
16).
To erect the main beams of this silo,
ordinary construction procedures were
not possible. The weight and length of
the beams and the care needed while
positioning them made this erection
unusual.
The erection of the main beams was
carried out with the aid of three
wheeled cranes. With this equipment,
it was possible to erect an arch every 2
days.
SCHEDULE
First, two of the cranes lifted one of
the elements and placed it in position;
The design of this silo began in Dethen, one of the two cranes helped the cember 1978. Two months later the
third one place the second beam. While foundation work was started. The castthe two beams were held in this posi- ing and erection of the precast elements
tion, concrete was cast in the hinges at were done from May to October 1979.
the bottom ends of the beams. The ver- This particular silo was completed and
tical neoprene plate was put in place in put into service by the middle of 1980.
the upper hinge of the arches; then the Since then, additional silos have been
purlins and bracing were placed to constructed in the same area.
stiffen the structure (Figs. 10, 11, 12,
The general consensus during the
13, and 14).
last couple of years is that the silos have
:^.r { ^i fir.+
Fig. 12. Erection progress of silo frame
116
Fig. 13. Array of channel beams with partial bracing in place
Fig. 14. Skeletal view of silo frame.
POI JOURNAL/July-August 1982
117
Fig. 15. Front view of silo frame.
Fig. 16. Closeup of asbestos sheets and cast-in-place columns.
118
been performing well structurally and
they have also been operating with full
satisfaction.
COST
The total cost of building these silos
(including the area recently built) was
about $18 million (in U.S. currency).
The entire area of the project was
1,151,737 sq ft (106,789 m 2). Hence, the
unit cost of the project amounted to
$15.63 per sq ft.
CONCLUDING REMARKS
This project was completed on
schedule and within budget under very
difficult conditions. This success was
possible because of the close cooperation between owner, designer, and
general contractor together with teamwork provided by the precaster and
erection crews.
PCI JOURNAUJuIy-August 1982
The experience gained so far has
shown that the precast prestressed system adopted fulfilled the owner's expectations and that this construction
method (suitably modified) can be used
economically to build similar industrial
structures.
CREDITS
Project Management: Rioboo S.A.
General Contractor: Direccion y Coordinacidn de Obra, S. A. (DICORSA).
Precast Contractor: Tecnicas Internacionales de Construcci6n, S. A.
(TICONSA).
Erection Contractor: Montajes y Construcciones, S.A. (MICSA).
Structural Designer: Riobcio, S.A.
Owner: Fertilizantes Mexicanos, S.A.
(Unidad Lazaro Cardenas, Las
Truchas) Mexico.
119