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Design of Long Span Steel Structures and Hangars [With Eave
Bracing Concept]
Amit Bharana, President, ERA BUILDSYS Ltd.
The majority of industrial buildings are made out of steel for
obvious reasons of high
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strength/weight ratio. A study, on the Efficient Design of Large
span hangars/structures, ensuring
lighter foundations is presented.
Introduction
Amit Bharana
Structure with Span larger than 40 m can be regarded as long
span structures and need to be
carefully designed keeping a balance of all the aspects like its
weight, deflections (sway) and
foundation forces. There are many combinations of designing
large spans, like conventional truss
& RCC column combination, truss & steel columns,
Pre-engineered building (PEB) etc.
These days with the concept of PEB, the major advantage we get
is the use of high strength steel
plates (usually Fe 350), lighter but high strength cold form
purlins, and 550 Mpa Galvalume
profiled sheets. The use of PEB not only reduces the weight of
the structure because high tensile
steel grades are used but also ensures quality control of the
structure. In the following study, we
have designed a hangar using this modern concept of PEB.
Design Philosphy
The design under discussion is a 42 meter clear span hangar for
aircrafts maintenance. We have
designed this Hangar in 3D on STAAD software, for proper
simulation of the load distribution
uniformly in three co-ordinates system i.e. X, Y and Z. All the
Basic loads i.e. Dead, Live, Wind,
Temperature, seismic etc have been taken into consideration for
designing of the frames. The
structure has been designed under enclosed as well as open
condition for application of wind
loads, because of the opening & closing of the large sized
Hangar Door.
The basic philosophy of rigid frame design is by adopting FIXED
or PINNED column base conditions. A fixed column base is always a
sturdy frame and helps in controlling allowable
deflection (side sway) in the frames. Steel designers always
prefer fixed base to pinned base
frames. On the contrary, for foundation designers the design of
foundations becomes a nightmare
particularly in large span buildings. In fixed base design, the
frame is rigid, but transfers heavy
moments to the foundations. On weak soil, designing foundations
becomes tedious task.
Likewise for pinned support, the frame does not transfer any
moment to the foundation and only
vertical & horizontal reactions affect the design of
foundation. It looks simple but in case of
large spans, controlling deflections of frame in pinned base
condition is a challenging task.
To control this deflection, the simplest way is to increase the
Geometrical properties/sectional
sizes of frame, but it is not advisable as it adds to the
tonnage of the whole building, adding not
only to the seismic forces but also adding to the cost
subsequently. We need a solution wherein
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the sway of the frame can be controlled and the section sizes
are also not increased.
The best way we could find is to BRACE the frame to control the
excess deflection. In the present case we have provided bracing at
eave level (braced eave) on both sides of the structure
along the length for this purpose. Span of this Eave bracing is
taken approximately L / 10 of each
side. We can observe in the following example that eave bracing
is of a great help in controlling
Horizontal deflections and leading to lighter foundation
design.
Example
To conclude above, as example hangar has done with following
parameters having both side
hangar doors with following details:
Span = 42 m
Length = 76 m
Bay Spacing = 7.6 m
Clear Ht. = 10 m
Wind Speed = 50 m/s
Roof Slope = 1 in 10
Alternate 1 (Fixed Base Design)
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In fixed base, due of uniform moment distribution at base as
well as eave, the straight column of
section size Web = 1275 X 8 mm and Flange = 375 X 16 mm have
been used. By using these
dimensions of frame, the deflection is allowed in control.
Please refer Table 1.
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As regards foundation forces, the moment (Mz) is approximately
165 MT and vertical force (Fy)
is 41 MT at the base of the column, which is being transferred
to foundation. Please refer forces
generated at base of frame in Table 2.
For above forces a tentative size of Isolated foundation is
worked out as under -
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Eccentricity (e)
e = (Mz/Vy) = (165 / 41) = 4.03
The length of the pedestrian (L/6) should be greater than
eccentricity (e).
i.e. L/6 > e
L > 6e
L > 6 X 4.03 = 24.18 m 25 m (At Single Side)
Thus, it may be observed that the open foundation for this type
of frame will be highly
impractical & uneconomical and only solution is to provide
pile foundation, which will again be
time consuming & expansive.
Alternative 2 (Pinned Base design)
As second option, the same structure has designed with Pinned
Base condition, now we can have
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tapered columns where moment (Mz) at the base will be ZERO. Only
lateral (Fx) and vertical
(Fy) forces will be acting. Please refer forces generated at
base of frame in Table 3.
Due to zero moment we could design very light foundation
(Sub-structure) for the same
structure.
It may be observed that the foundation forces are very limited
and a small isolated foundation
can be design for the columns.
But at the same time, in pinned base, the deflections of the
Portals in lateral as well as
longitudinal direction will become critical parameter. And in
case no special provision is made to
control this deflection, the frame will have a considerable
sway. Therefore for control of
deflection, the provision of Eave Bracing (braced eave as
highlighted) longitudinally on both
sides of the structure has been made.
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Span of this Eave bracing should be approximately span / 10 of
each side. For 42 m span the
width of eave bracing used 42/10 m = 4.2 m 4 m at each side (see
below 3 D model)
For comparison between the same sizes of frames with &
without using the eave bracing, the
deflection resulted in the framed are given in Table 4 and
5.
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As per codal provisions, deflection allowed in columns is
(H/150) that is 66 mm. In case of
braced eave structure, deflection value was under control with
60 mm in longitudinal direction,
however at the same time, the value was much more than allowable
deflection in case the eave
bracing is not provided i.e. 7566 mm which is very high.
Thus Eave Bbracing Concept is very beneficial in large span
structures with pinned base concept
and since no moment is transferred to base level, the design of
foundation is also very
economical.
NBMCW January 201
