|
We
provide consultation sevices to help our client determine what services
are needed to properly install, rig and use the necessary window
washing equipment . We do this by examining the building,
reviewing its roof structure, determining the buildings needs and
providing the client with the necessary Engineering drawings,
specifications, relevant calculations & where necessary a building
rigging plan (OPOS) which demonstrates how the building is to accessed
with the necessary equipment (powered platform, bosun chair, ect.)
. Each equipment component is briefly described for the
enlightenment of the user browsing this site.
Portable
equipment
The Davit
On building structures, the term davit has
come to mean any of a variety
of arched, curved, or dog leg shaped member, used singularly or in
pairs,
typically constructed of aluminum. Davits
support a platform or manual chair at
its outer end and is
anchored at its lower end by a single, “moment” carrying
connection. This over turning moment
connection, whether
it be a socket, base pipe sleeve, pedestal, or base, transfers the over
turning
load at the end of the davit arm to the parapet or roof’s structural
frame. The davit and its roof attachment
device are required to have a minimum overturning safety factor of four.
A pedestal is simply a steel
pipe or square tube, approximately six inches in diameter, with a flat
plat
welded over the top of the tube. This
top plate is approximately twelve inches square by ¾ of an inch to on
inch
thick. The reader must bear in mind that
all sizes are approximations due to the custom de sign nature of the
industry. The pedestal, if attached to a
steel roof structure, is welded to a roof member whose load carrying
capacity
has been verified by the engineer. If
the pedestal is to be anchored to a concrete roof, it typically has a
bottom
plate welded to the tube with four holes drilled through it to
accommodate
anchor bolts that are cast in the roof pour.
A davit base usually refers
to
a fixture that is welded or bolted to the pedestal’s top plate. The differentiating nature of the base is
that it contains holes drilled horizontally through the base’s metal
parts so
as to permit the davit to be pinned to the pedestal.
Two parallel pins are needed to create the
required overturning resistance.
A pipe sleeve is a pedestal
without a top plate. Its attachment
methods are identical. As a sleeve, the
davit mast is inserted directly into the pipe for vertical and
over turning
support. The pipe sleeve method should
only be used with two piece davits when the arm can be safely inserted
thru the
mast or safely attached after the mast unit is secured in the sleeve. A rain cap is often supplied with the sleeve
to prevent water from collecting in the pip sleeve.
The socket, either portable
(with wheels), or fixed, is an assembly that attaches, at its lower
end, to the
pedestal and is open at its upper end. The
upper portion is usually a pipe or
circular tube whose diameter is
just greater than that of the davit mast. The
mast is inserted into the socket for its
attachment to the
building. Most sockets are of the
tilt-up variety, allowing the davit to be tilted upwards from the
horizontal to
the vertical, working position.
All davits are either rotatable or fixed with
respect to their vertical
axis. Rotatable davits
have arms that are either fixed to the mast (the
vertical member) or are pinned to the mast and free to rotate with
respect to
the mast. The later is often referred to
as a “top rotation” davit. When the arm
is free to swing with respect to the mast, operators in the stage can
freely
swing the platform through the masts while in the stage.
A transportable davit is one
that is relocated from one position to a new position around the
building’s
roof, from roof level to roof level, or to an entirely new building.
A single piece davit combines
the arm and mast in the common, inverted “L” shape.
The two
piece davit separates the arm from the mast for ease of relocating
the
davit around the roof. This con cept
permits easy access through tight areas and reduces the weight to be
carried by
one man.
Tilt-up davits permit the
operator to position the davit over its base or socket, insert one pin
(for
attachment), then lift the arm and mast to their upright position then
set the
second pin. Davits that are erected in
their complete “L” shape are required by Federal Labor Codes to be of
the
tip-up variety. By pinning the davit to
the base before tip-up, the operator is sure of a firm foundation to
push
against and thus sure that the davit will not fall over the parapet. Note that a great majority of davit
operations occur on or adjacent to the building’s parapet.
If the davit is simply inserted vertically
into a pipe sleeve, it is quite possible for the user’s hands to slip
from the
davit mast, allowing the entire assemble to fall. Since
the davit’s repositioning is all to
often attempted with rigging lines still attached and with the arm
outboard, a
slip could mean a davit falling the height of the buil ding. Recently published safety standards
restricted the weight a man must lift in an effort to erect or move a
davit. Davits exceeding the weight
limit, or those davits with long, cumbersome arms, should be equipped
with a davit jack or hoist. As
a mechanical tool, often a light duty
manual hoist or sometimes a screw jack, the davit jack is an
inexpensive,
usually portable, device that save the “backs” of operators who must
repeatedly
tip up and lower heavy davits.
High profile davits are tall
enough to allow a powered platform to be raised over the parapet then
swung
over the side of the building in a roof rigged operation.
A low
profile davit will just clear the top of the parapet and supports a
ground
rigged platform operation.
The roof carriage is similar to a rolling
davit carriage except the
counterweighted carriage usually rolls on non-pneumatic rubber tires
instead of
rails. The carriage may be mechanically
propelled or pushed manually to each drop position around the periphery
of the
roof. Once positioned, the carriage’s
tie-down arm, similar in design to that of the roof car, is pinned and
electrically interlocked to the roof tie - down lug.
The assembly’s pair of davit shaped arms
often do not tilt down or rotate because the entire carriage assembly
can be
rolled away from the parapet, out of view, eliminating the need for
previously
described davit features.
The Outrigger
The outrigger, often referred to as a “look
out” by window cleaners,
functions much like a low profile davit, its beam just clearing the top
of the
roof parapet, except the outrigger is a beam with a fulcrum and a rear
tie-down
anchor. Used in pairs for supporting a
long platform or sin gularly when suspending a cage or chair from its
outboard
end, the outrigger may be tied down to a roof anchor.
All counterweighted outriggers, typically
used for construction applications, must be tied to an independent roof
anchor
and shall have lateral stability to prevent roll over in case of an
accidental
lateral load.
The Drop Thru Sleeve
Buildings featuring projections out from the
vertical face, such as
cornices, ledges, “eyebrows” or fins, create difficulty for the stage
to
maintain contact with the building. These
sleeves act as the top side of the
sleeve, prohibiting the wire
and its fitting to fall through. Drop
thru sleeves, with a four to one safety fa ctor, must be designed by an
engineer
capable of analyzing the sleeve, its attachment to the ledge, and
identifying
its overall effect on the cornice. The
drop thru concept only works for powered, ground rigged equipment.
Monorail and “Halo Track”
An alternative to the drop thru sleeve is a
continuous monorail track
around the building, tucked under the ledge or cornice.
Its circular appearance around the top of the
building structure has earned the nick name of halo track.
A pair of monorail trolleys ride the track,
hanging from the rail’s outside face, yet not interfering with the
support
tubes.
The track and trolley system may support a
single manned cage, a powered
platform, or occasionally, a non-powered chair. If
a long stage is used, one trolley for each
stage end is engaged on
the monorail.
Parapet Clamps and Hooks
Buildings without direct roof attachment
capability, used with davits or
outriggers, often rely on structural parapets to support the load of
the
platform or manned chair. As a steel
fixture, fitting over the parapet cap, a parapet clamp or parapet hook
is
carried form building to building.
Roof Anchors
In California,
roof
tie-back anchors are required on all buildings exceeding 36 feet in
height for
“unscheduled” maintenance. Most
high-rise roof tops around the country have them for safety line
tie-offs. In either case, the anchors are
structurally
designed to safely carry a line load of 5,400 pounds in any direction. The load requirement recently reduced to a
5,000 pound capacity, incorporates the requirement for dynamic loading
of a 250
pound man free falling six feet before being quickly de-accelerated in
a
stopping condition, multiplied by a four to one safety factor. The design clause, “in any direction” is
required due to the unpredictable nature of a cleaner’s set up. An anchor that is adjacent to one drop may
actually be used as a safety line tie-off for another drop on the other
side of
the building.
Powered Platforms
The Stage
A powered platform, “swing stage” or ‘stage”,
is a work platform, whose
frame is typically constructed of aluminum. The
stage is vertically propelled using
electrically powered motor(s),
either traction hoist or drum type, on board. Compressed
air powered hoists are occasionally
used in lieu of
electrical power. The platform,
suspended by at least two independent wire ropes of 5/16 inch diameter
or
greater, may be as short as three feet for a single man cage, or as
long as
forty feet, carrying three men plus tools, equipment and supplies.
Whether “ground rigged” or “roof rigged”, the
stage and its electrical
cord are suspended from one of several types of roof top supports,
including:
davits; rolling roof carriage; outriggers; “drop thru” sleeves;
monorail track;
parapet clamps or parapet hooks. Each of
these supporting devices are elaborated upon in subsequent paragraphs.
There are two basic construction styles for
work platforms, a “ladder
board” with handrails or a truss type that incorporates the handrail
within the
truss design. The “ladder board” variety
utilizes an “off-the-shelf” unit constructed of “I” beam rails held
together
with tubular rungs molded through the beam’s web, then covered with a
perforated decking. Bracket clips are
bolted to the webs at multiple positions along the stage’s length to
support
handrail stanchions.
The truss type stage is an all welded frame
with vertical and diagonal
members forming a triangular shaped truss. The
truss’ top cord becomes the handrail’s top
rail and the truss’ lower
cord is used to support cross members that span the stage’s width. A non-slip flooring covers the cross members
to complete the walk area. Design
conditions elaborated upon above for ladder stages should also be
considered in
the design of a truss stage.
Metal mesh or screening is used to enclose the
sides of the stage for
both types of construction, ladder or truss. The
screening helps keep tools or materials
form falling out of the
stage, a safety requirement on permanent equipment design, in
consideration for
the general public below who may be at risk.
Due to a building’s geometry or its roof
structural grid, one elevation
may require different length stages on the same roof.
It is impractical of an owner to purchase
numerous stages or for a window cleaning contractor to bring different
length
stages to the same building. The custom
designed and fabricated stage is therefore often made to be modular,
utilizing
stage sections that couple together in various lengths as needed for
any of the
building’s roof top areas. To accomplish
this, the stage is constructed in modular lengths from as short as two
feet to
as long as twenty feet. The stage’s end
sections contain the power units and the center sections pin together
to create
various lengths, like adding leaves to a dining room table. Both the truss type and the ladder board
variety of stages can incorporate the modular design.
Hoist Motors
There are two basic varieties of powered
hoists used to lift and lower a
powered work platform, the drum hoist or traction hoist.
All hoists used in the industry are “man
rated”, requiring a primary and secondary brake and an “over speed”
protection
device.
The drum hoist, powered by electricity,
collects wire on a traditional
flanged drum located under or just behind the stage work area. The support wire rope has its end clamped off
to the flange for protection against the rope accidentally spooling off
the
drum.
Traction hoists, usually powered by
electricity but occasionally powered
by compressed air, incorporated mechanical rope gripping, spring loaded
paws or
roller chain that encircle a sheave inside the hoist’s housing. The wire rope, fitted with a “bullet” end, is
fed into the mechanism, passing between the sheave’s hub and the spring
loaded
chain or paws.
With their “power feeding” of the wire rope,
traction hoists are great
for ground rigging. One interesting
feature of a traction hoist is that the greater the load on the wire
rope, the
greater the grip exerted on the rope by the roller chain.
|
