Lateral Bracing for 100-Foot Tall Free Standing Steel Structure 3

[jamie2000]

I am beginning a concept design for a 100-foot tall free standing steel structure and I need some help to determine what is the most efficient bracing system. The structure is 60 feet by 60 feet in plan. There are no intermediate levels, only a top level. The top will be a slab supported on steel joists and girders. The area that the project is located in is a moderate earthquake zone.

Thanks in advance for your help.

 

[Imagineer]

If it is in the US, you cannot use tension-only bracing elements in earthquake zones.


Imagineer

 

[Hariharan]

Without knowing the weight of the top platform including equipment,
I would primarily consider K*l/r as the first starting point.
You should have an X-bracing. Considering The long length, the
bracing could be a lattice member made of 4 angles, laced together
to form a single structural element. Somewhat like the members in
Eiffel tower. The columns, incidentally, could also be of similar construction.
You will need to develop the joint design for proper load transfer
and easy erection.

Hope this helps, Good luck.

M. Hariharan

 

[breaks]

I agree that X-Bracing is a good place to start.

You also will most likely have to add concrete shear walls at the bottom of the structure. The steel by itself will not be able to handle the shear caused by wind loading, which will be substantial given the height of the structure. The cross bracing should help provide a load path to the shear walls. Also, the shear walls (combined with the top slab) will stiffen the structure as a whole and help control deflection.

I have seen this type of system used in a few warehouse-type structures in low earthquake zones. Good luck.

 

[IJR]

Breaks, Imagineer

I also use X-braces but usually prefer single diagonal per bay (zig zag brace) simply because that cuts down the number of connections to make. That makes it cheaper.

Imagineer: I am interested in your comment on prohibitive use of tension only bracing in seismic zones. Do you want to give us a reference for that.

In the area I am we are allowed to use tension only bracing though we are required to limit their slenderness to 250, which usually makes them a bit heavier.

If I use compression braces the limit is 100. Heavier but less amount and less connections

We also use inverted K braces, easier to connect, lesser buckling length and may also provide support for platform beams, thus cutting beam costs, as well as providing area for doors etc.

 

[Hariharan]

Inverted K-braces are not advisable (Prohibited in offshore structures)
under seismic conditions because, if the compression member buckles,
there is no alternate load path for shear and the horizontal member
would be severely overstressed and could possibly lead to collapse. If
you need to use inverted K bracing, you should perform a dynamic
analysis under full earthquake loading (no reduction factor) to ensure
that the structure doesn't collapse.

If diagonal X bracings are provided, is a separate shear wall required?
I don't expect much of a problem with wind loading if the members are
proportioned properly.

Out of curiosity, what does this platform 100' above support? That might
give some clue to a possible solution.

M. Hariharan

 

[JAE]

IJR - the reference to avoiding tension only braces comes from AISC in their Seismic Specification. They require that, along a line of bracing, at most 70% of the load may be taken by tension members with the remaining taken by compression members. This also can be reversed, with a max. 70% taken by compression and the remaining taken by tension.

 

[IJR]

Thanks JAE,thanks Hariharan.

JAE: That was very helpful. I never knew that.

Hariharan: I am now confused and would like to read further on this prohibition of inverted K-braces in seismic zones.

******here is why I am confused******
I understand that eccentric bracing of the form similar to inverted K were highly recommended (See Popov etc.), to the point that codes increased Rw values significantly. Wont the compression side buckle in such systems too?.

I will appreciate more clarification and references.

 

[jamie2000]

Hariharan,

The structure will be supporting a helicopter landing pad. All of the framing will be exposed.

Jamie2000

 

[breaks]

Hariharan - I envisioned a large warehouse-type structure with purlins and siding. In this case (this is from personal experience on one warehouse-like project), the wind load would be very high and shear walls would probably be needed (depending on the system employed, of course). Wouldn't you agree? Since the framing is exposed, I would suspect that only bracing will be more than adequate.

As far as the K-Bracing/Seismic information, I'm enthralled. Didn't know that.

 

[Hariharan]

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

 

[IJR]

Thanks Hariharan for keeping the faith.

2 days ago I also came across an article that discusses Redundancy/Reliability aspect of seismic design. I got to understand your point further. Bracings must be carefully designed.

Can I add a comment: For the helideck and offshore structures which go under "Non building structures", my eccentric-bracing-boosted ductility should not be taken advantage of, except for ease of connections, because codes tend to force normal ductility design for such structures, ie near elastic. I guess the point is to make sure such a structure will not undergo large deformation which will eventually introduce large P-delta effects and cause overall buckling.

So Jamie2000: Use Hariharans system above, or X bracings(watch your Kl/r), ignore my zig-zag for your structure and make sure you have very low drifts.


Thanks all.

 

[Hariharan]

IJR,

In this particular case, the vertical and lateral loads will be so small
that the drift, P-delta effects etc are insignificant. The X bracing
will minimise the lateral movement in any case.

Hariharan

 

[Hariharan]

Sorry, that should read lateral bracing, not only X bracing

Hariharan

 

[IJR]

I agree, Hariharan.

I was just making sure that my eccentric bracing will not go into use there.

Keep in touch.

IJR