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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 design 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 overturning
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.
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
concept 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 building.
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 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.
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 singularly 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.
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.
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