RCC slab+ Composite Deck--> How will it behave 2

[brick2020]

Hi all.

I have a 2 identical long building 200mx48m connected by a steel bridge at the center.
Please see the attached PDF for clarity. I have couple of questions:

1.) The bridge is being designed integral to the two building, i.e. with rigid supports? Is this ok? How will the bridge behave when subjected to seismic forces? In general is the approach of a rigid connection feasible...or should the bridge between the buildins be design as a simply supported structure?

2.) The composite deck is assumed to be rigidly connected with the RCC slab by providing horizontal bars at their interface? Will this mean that for a particular floor plate, the composite deck+RCC will act like a single rigid diaphragm? Is this ok? Wont the differential stiffness of the two floor system prevent such a behavior? any thoughts on this.

Other points: No expansion joints as temperature analysis is being done, also shrinkage strips will be provided during construction.

Any views will be great as it will help me understand the behavior of the structure better... my main concern is the assumption that composite deck + RCC slab will act as a single rigid diaphragm.

regards
rags

 

[EngineeringEric]

From a lateral standpoint I believe this to be a problem. The two building's motion will not always match (different frequency, wind loading, etc.) and this will create a possible variation in the "bridge span" and direction. If both buildings move +/-3" in an EQ (in line with each other and the bridge) then the span will change in span by +/-6", the supports on at least one side would have to account for this. This would also happen when the building move perp. to the bridge span and there would be a change in span angle relative to the buildings.

As for the composite deck plus the RRC deck, I don't know of any issues but in a quick design i would probably only design it for one of the two, and assume a belt & suspender result. But maybe this is not feasible...

 

[TXStructural]

Even if the two buildings are essentially identical in all aspects, the risks associated with slight variations in an seismic event would still be large. This will occur as a ground wave hits one building sooner than the other. You would also end up with trouble if there is torsion due to plan irregularities or the layout of lateral-resisting elements (which doesn't seem to be the case.) If the buildings are not identical in stiffness and mass, then the odds against them moving in unison, which is what would be required, are very high.

It is possible that if the bridge is sufficiently stiff against bending in both axis, it could tie the buildings together, structurally speaking. This would be very difficult to achieve.

If you create a vertical "pivot" at the ends of the walk, such as a stair, the two buildings could essentially disconnect without loss of stability. I would always plan for the ends to tear free, and would design for the bridge to work in single bending for factored self-weight. Ideally, with a slip plane but meant to remain captive during an event.

The use of composite deck in exposed applications is never good, since it is almost certain that the deck will rust and delaminate from the slab (or simply rust away.) I would imagine you intend for this to be enclosed, but just wanted to mention it, in case there is any potential for exposure to moisture. When considering reinforcement, rebar or mesh are not expensive in a single instance (or few instances), so don't concern yourself with the added cost to make it as strong as you like. Since this would be on steel structure, I would most likely use tubes to ensure sufficient stiffness. Consider a horizontal and vertical "boxed" truss.

 

[hokie66]

Agree with everything TX said about the link. Tieing everything together is a recipe for heartache.

As to a 200 metre long building without movement joints, neither "temperature analysis" or shrinkage strips will completely make the volume change issues disappear. Suggest you reconsider.

 

[brick2020]

Thank you all for sharing your inputs.

what i gather is it is better to keep the bridge independent of the two buildings.. would it be logical to have the bridge as simple supported on either ends and design it as a box truss (@TX). ( but calculate the displacement during seismic and provide accordingly) ( a study on petronas bridge in Malaysia.. the bridge has been designed to have some movement...which is the same logic i am looking at)

The bridge is at 3rd level, it is a 11 storey structure. The center portion is 9 level of composite deck system.

The other question was: the RCC & Composite deck interface.. will it act like a homogeneous system and will my floor plate have a single diaphragm action? or due to the 2-different stiffness.. am i to consider 2-RCC diaphragm and 1 composite diaphragm. Because based on the diaphragm stiffness my lateral forces will get distributed accordingly..
so its 1 vs 3? what should be the approach?

can anyone plz share any articles on such a interface system?


thanks in advance.

 

[Ron]

Agree with Tx on the comments, but I think you(brick2020) are interpreting their comments differently than I.

There is nothing wrong with connecting two buildings with a pedestrian bridge...it's done a lot. The point here is that the buildings will move independently and "the tail won't wag the dog" in this case...the bridge will not stop the movement of either. Correspondingly, the bridge should not transfer or magnify the movement of one building to another. Having said that, you only need to accommodate the movements of the three relative to each other. You do not have to support the bridge on independent foundations or isolate completely from the buildings. The buildings can support the bridge, you just have to make sure the connections accommodate that movement.

As your sketch shows, the bridge sits on each of the buildings. A roller connection for one or both sides could be used, but you'll obviously have a lot of details to work through, particularly waterproofing.

 

[brick2020]

thanks for the inputs.

@Ron - i agree that the connection must accommodate the movement of the three.

(the issue has cropped up because, my senior partner wants the bridge to be integral while i would like to see some play in the supports and let the bridge be independent of the building)

My concern is: when you design the bridge integral to the buildings, Like connected through rigid supports.
If the buildings were to move out of sync and towards each other.. the bridge will be subjected to compressive forces. In that case how do i detail it? Should i design the supports to withstand these forces? Wont the bridge sections be uneconomical to withstand those compressive forces?

or am i missing something.

I did some r&d and found that most of the prdestrian bridges, do allow for some movement of the supports or are detailed with other means. The case of petronas bridge.. where the movement is allowed in the middle as the long knee brace beneath it is supported on a roller like support at floors below.

And what i have gathered from all the responses above, the bridge should be on roller like supports and it should not act as a transfer medium(diaphragm).

 

[TLycan]

Brick2020
I've designed a whole two steel floors hanging between two high rise buildings on the fifteenth or so floor. the steel floors are rested on an Elastomeric bearing. a space between the two floors and the R.C building is left. the spacing depend upon the movement of the R.C building and the two floors, each analyzed separately. attached is a file of these floors

 

[brick2020]

@TLycan - Thank you for sharing the file.
I agree this is the most logical way of detailing the bridge between two RCC structures.