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INCREASING
CAPACITY OF ANTENNA SUPPORT STRUCTURES BACKGROUND
Telecommunication towers
have traditionally been designed for single carrier occupancy, with little or
no allowance for future expansion requirements. There has been a growing
tendency for municipalities or other regulatory bodies to oppose building of
new structures, especially in residential areas. Also it makes more economic
sense to place antennas at a desired height on an existing tower. TOWER REINFORCING CHECKLIST
ü
Do you have all the tower
design details (installation drawings, part drawings, foundation design, soil
reports, materials used)? ü
What is the design
standard the tower was designed to? Was the design adequate then with the
existing and future antenna and transmission line loading? It is important to
establish the structural adequacy of the tower at the time when it was
designed. You don't want to be responsible for reinforcing work for a tower
that was under-designed. Did the tower have any capacity left? Were the member
connections designed to the maximum loads in members or to member capacities?
It is common practice that connections to be designed to member capacities, to
allow for future increases in loads. ü
What is the current
capacity of the tower? National standards are updated every five years or so.
Tower design standards reflect the state of the art knowledge at a particular
time. Older standards might have been more conservative with the wind loading
and member capacities. However, there are components of the towers that will
have their design capacity reduced as a result of past performance of
components in the same class. ü
For example, the
caisson foundations have their capacity reduced almost to 50% with the new
update of the CSA-S37 Canadian standard in 2001. Diagonal members in
cross-braced panels have their capacity considerably reduced from the 1986 to
1994 editions of the CSA-S37 standard to have it increased in the 2001 update. ü
If there is no change
in the loading of the tower, generally there is no need to analyze the tower
when new standards come into force. However any modification in the equipment
loading (antennas, transmission lines, etc.) will require a structural
assessment. Modifications in the equipment loading do not necessarily mean
addition of new equipment. Removal of equipment can have a deleterious effect
on some structures, particularly guyed towers. ü
Is the analysis method
used the most appropriate? There is a variation in the analysis methods
employed by various tower designers and consultants. Some of the simplified
methods, like modeling a tower as cantilever beam or a continuous supported
beam might be more conservative but they might be missing some secondary
effects on the towers. Finite element methods where every tower member is modeled
are preferred. Loading modeling is critical, too. Exact positioning of the
antenna lines will model the loading more accurately as shielding between lines
and structural members is considered. ü
The more sophisticated
the analysis method is the more accurate picture of the actual state of the
tower is. Simplified methods might be adequate for the tower initial design but
when the tower is analyzed in the as-built condition a more sophisticated
analysis is required. ü
Does the model reflect
accurately the as-built condition of the tower? This is very important
especially for loading. Relying only on the original tower drawings might not
be the answer. Tower antenna mapping is required especially for transmission
line positioning on the tower faces or legs. ü
Is there physical room
for the addition of the proposed antennas and transmission lines at the desired
height? If it is a monopole is there room in the existing entry and exit ports
for the additional cables? Do you require new ports? If new ports are required
then a structural analysis of the monopole section with the additional port is
required. ü
What are the critical
components of the structure? What are the weakest points? Some components can
be easily upgraded (replacing a diagonal member of a lattice tower with a
stronger member), while others are very costly to upgrade (foundations in
uplift). ü
What reinforcing
options are available? The following section will describe the most common
reinforcing solutions. For example, bolting or clamping on an existing member
is preferable to welding on site. ü
What is the likely cost
of tower reinforcing? Technically any tower can be reinforced short of building
another structure around it but economically might not be feasible, as it will
cost more than a new structure. ü
What is the timescale
of tower reinforcing? These questions need to be
answered to ensure a successful tower reinforcing. REINFORCEMENT SOLUTIONS
RISKS Some of the structures,
especially guyed towers might experience loads close to capacity even in
absence of wind conditions. While reinforcing towers, some of the members might
need to be taken out and replaced. When member replacement is required then
temporary supports must be installed. Reinforcing work should be carried out in
good weather with little or no wind. Excavating around a
foundation decreases significantly the uplift or pullout resistance. CONCLUSIONS It is more economical to
allow for a multi-carrier occupancy when tower are initially designed. However,
if reinforcing is required, then good Engineering can make the difference
between a money-saving solution, easy to implement and a non-feasible solution. |