Helideck? That's known territory!
1. A helideck of 60' square shouldn't weigh more than 60 tonnes.
(Hope it is in steel)
Even under worst (full) seismic conditions, the horizontal shear
would be less than 20% i.e., 12 tonnes. For this magnitude of
vertical and lateral loads, the members of the structure would
be governed entirely by K.l/r considerations. The minimum size of
members meeting the kl/r requirements would have more than
adequate strength. In this case, the inverted K-bracing will also
work without any problem. (See IJR's comments). For this
condition, my preferred framing pattern would be as below.
===============
l / \ l
l / \ l 50'
l / \ l
l / \l
l---------------l
l / \ l
l / \ l
l / \ l 50'
l / \l
l---------------l
40'
The deck beams would be WF sections, and plating. The columns and
diagonals would be either tubes 16" dia or square hollow sections 16"
side. Isolated footings at corners to take care of vertical and horizontal
loads and moments. Cantilevers would reduce section sizes. If it
complicates access planning (stairs, elevator?) you may have the legs
at 60' spacing, at the four corners of the helideck.
2. Now, to the other queries.
(a) Regarding permissible bracing patterns for offshore structures,
particularly in areas subject to critical seismic loading conditions,
refer American Petroleum Institute's Recommended Practice for
Planning, Designing and Constructing Fixed Offshore Platforms,
API RP 2A. In the present instance, these limitations need not apply
since the element design would not be governed by strength.
(b) The inverted K bracing was a standard practice in many areas
for offshore structures also a couple of decades ago, but no more.
The reason it is not recommended in seismic conditions is because:
under lateral loads, one member is in compression and one in tension.
The member sizes are equal. The buckling load is smaller than the
tension capacity. Under seismic load, the compression member buckles
first, and if the tension member takes a higher load, it results in
bending of the horizontal member, which normally wouldn't have a
large reserve capacity in bending. Thus the collapse load of the total
structure is not much greater than the buckling load of the individual
member. In other words, it doesn't have much reserve strength.
For an effective seismic design, there should be alternate
load paths to carry additional loads after the failure of some individual
elements. A redundant structure is preferred to a determinate structure.
(This could also be the reason why tension only bracings are not
encouraged). The concept of Rw makes sense only if there is
redundancy and additional load carrying capacity in the structure.
For offshore structures, Rw is about 0.5! In fact, the structure is checked
for collapse under a rare intense earthquake, which is (about) twice as
severe as the design earthquake. Local yielding, individual member
buckling are allowed, but total collapse of the structure is not allowed. I
personally feel that onshore structures should be stronger and offshore
structures could be weaker!
3. Regarding the query of breaks: Even if the structure were totally
enclosed, the total wind loads on such a structure would be
18 m X 30 m X 200 Kg/m^2 (avg) = 108 tonnes, to be resisted by
two sets of braces. Not a big deal. If there is no structural
requirement otherwise, a clad steel structure would be faster, cheaper.
Shear walls would be advantageous if you had many levels and a lot
of dead and live loads, and seismic design requirements (Multistoreyed
Warehouse).
Nice discussion!
M. Hariharan
