VSL Technical Report.. - Free

and 148). It rests on an octagonal substructure.
The silo comprises 4 buttresses for anchoring the
post-tensioning tendons. Its capacity is approx.
7000 m3. The silo wall was built by the slipforming
method (Fig. 149). On top of the silo there is a
steel structure for the charging equipment.
Post-tensioning
The silo wall is post-tensioned with 160 VSL
cables EE 5-7 Dyform, each 24.70 m in length.
Each cable extends around one half of the
circumference. The distance of the cables from
the outer face is 70 mm and the cable spacing
varies from 450 to 1000 mm.
Figure 150: Buttress with anchorages VSL type B
3.2.7. Coal silos, Gillette, Wy., USA
Owner North Antelope Coal Co.,
St. Louis, Mo.
Engineer SMH Engineering Inc.,
Lakewood, Col.
Contractor The Nicholson Co., Marietta,
Ohio
Posttensioning
VSL Corporation, Dallas, Tx.
Years of
construction 1982-1983
Introduction
At the North Antelope coal mine three silos each
of 20 000 tonnes capacity were built between
September 1982 and June 1983. Beneath the
silos there are rail tracks on which the trains move
forward slowly during loading with coal.
Details of the structures
Each silo has an internal diameter of 21.34 m and
a wall thickness of 350 mm. The height of the wall
is 60.15 m and there are 4 buttresses each 1.98
m wide for anchoring the tendons. The roof is of
steel girders, resting in recesses in the silo wall,
and of a profiled metal sheet covered with
concrete which projects slightly beyond the silo
wall. The roof is not connected with the wall, but
the wall is fixed in the substructure.
Post-tensioning
The silos are post-tensioned with cables 5-4, 5-5
and 5-6. The largest units, cables 5-6, are located
in the central region of the wall. All the tendons
extend around onehalf of the circumference. For
the anchorages, VSL type B (Fig. 150) was
chosen. The distance from the outer face of the
silo wall to the axes of the cables is 89 mm and
the cable spacings vary from 560 mm minimum to
1520 mm maximum.
During construction of the wall by the slipforming
method, only the empty ducts were placed. After
the wall had been completed, the strands were
pulled by hand from the dispenser set up on the
roof, were pushed into the duct and cut to length.
When all the strands of a cable had been
installed, the anchorages were fitted (Fig. 151).
The cables were first stressed to 20% and
immediately afterwards to 100% of the required
force. At this stage the concrete strength had to be
at least 24 N/mm 2 .
Figure 151: One of the silos during the post-tensioning operations
46