Pedestrian Bridge b/w two 10 stories tower
Pedestrian Bridge b/w two 10 stories tower
(OP)
two towers of 10 stories are 30ft apart, and there is a need of pedestrian bridge on every story level between the two towers. Building is in Zone-III. The principle design engineer thinks that it cannot be bridged with a steel structure pedestrian bridge as the buildings are expected to sway upto 4 inches with different mode shapes, Also the second mode shape when both buildings sway towards each other the bridge will come apart.
My question is that is he right ? and is there anyway that we can fit in a steel girder bridge between two towers or not ? how will the joints be detailed to cater building's movement in different mode shapes. ?
My question is that is he right ? and is there anyway that we can fit in a steel girder bridge between two towers or not ? how will the joints be detailed to cater building's movement in different mode shapes. ?






RE: Pedestrian Bridge b/w two 10 stories tower
of course you could always tie the towers together with thebridges, which would change their mode shapes, and re-do the analysis.
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
You can certainly detail a sliding connection at one end to allow that much movement. Other than some crazy joint like on the Petronas Towers, I don't see any other way to do it.
One question is serviceability during frequent wind events. For example, if the wind blows 30 mph (arbitrary number), how much will the connection have to slide? Enough that people will notice and freak out?
Be careful interpreting the mode shapes. Models are FAR from perfect at predicting these. For example, I've seen people come up with modes that seemed to indicate that the bldgs would move in sync. There's no way I would rely on that much precision. For one thing, the eigenvalue analysis is linear and there are usually numerous violations of the underlying assumptions. For example, the structure will almost certainly have some non-proportional damping so the mode shapes will be complex to some degree.
RE: Pedestrian Bridge b/w two 10 stories tower
Not sure how else this can be done.
If your uncomfortable with a sliding joint, you might want to consider a rocker joint at the moving end. Frequently used on bridges to provide movement. Though 8" of movement will be quite a bit, and take a very deep rocker to accomadate. Fortunately, with only a 30' span, your reaction loads should be relatively small.
RE: Pedestrian Bridge b/w two 10 stories tower
tell me! how about having a sliding bearing plate on both ends rather one fixed and one with sliding bearing plate ! and please help find a typical detail of sliding bearing plate for such joint?
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
You'd also want to allow a lot more movement than what is calculated- could get exciting if you drop that top bridge on one end.
I assume these would increase the wind load quite a bit as well.
RE: Pedestrian Bridge b/w two 10 stories tower
Never, but never question engineer's judgement
RE: Pedestrian Bridge b/w two 10 stories tower
you Could make the bridges with sliding joints at both ends, but i think that'd be pretty creepy to walk on !
you Could provide vertical slide at one end and horizontal slide at the other ... but i think it'd be better to fix at one end and slide at the other. you Might build a stub bridge on the building with the sliding end, basically enclosing the cantilevered bridge; this would constrain the bridge in the vertical direction which may or may not be a problem but it may be a functional design.
RE: Pedestrian Bridge b/w two 10 stories tower
I would consider anchoring to one and sliding on the other too, but I would raise the structure above the floor line and extend the ends into the buildings if possible, with special consideration ofthe availaqble headroom, obviously. Tying to the rimbeam, to me, is transmitting too much of the load to only one member, possibly an exterior frame member that is already optimized in design. Extending the bridge to the interior gives the option of tying to more members, but does take up more interior space.
The 8 to 10" maximum movement at one end can be a safety hazard for anyone trying to use the bridge - special design consideration here.
Just thinking out loud here - may not work for your case.
Mike McCann
McCann Engineering
RE: Pedestrian Bridge b/w two 10 stories tower
Check out
h
for some typical seismic joints. You may need to contact them for other specialized joints.
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
after reading your reply at night i thought over it, and i came up with some more questions in my head
-- what about the lateral movement? how will a sliding bearing plate cater for that ?
-- what about rotation in upward/downward direction?
thanks
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
Gotta detail it to slide that way too.
"what about rotation in upward/downward direction? "
Rotation from what? Bending of the bridge beams? Should be very small for 30'. I don't see how you can get measurable rotation from deformation of the bldgs, but you should check it out with the model.
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
Nevertheless, I have a better idea - and cheaper too. Install a two center poles between the two towers, and one adjustable length rope swing. Tarzan could be the swing operator. It would be cheap because Tarzan is non-union.
Mike McCann
McCann Engineering
RE: Pedestrian Bridge b/w two 10 stories tower
wow! why dint i think of that :)
RE: Pedestrian Bridge b/w two 10 stories tower
will this work ?
RE: Pedestrian Bridge b/w two 10 stories tower
For lateral stability, you might consider horizontal trusses in the floor framing plan to transmitt the horizontal load on the bridge back to the building with 'hard' connection end.
I definately would not try to tie the two buildings together rigidly. The lateral forces in 10 story buildings are pretty large, and I'm not sure you will ever be able to make a pedestrain walkway bridge handle these kind of forces.
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
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RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
I'm not saying to leave it unrestrained. Let it slide longitudinally and guide it (maybe with pins?) in the transverse direction. It seems to me you need to maximize the flexibility. I'd have to think about its tendency to "crawl" longitudinally and if that would make a difference.
RE: Pedestrian Bridge b/w two 10 stories tower
On second though, perhaps this isn't such a bad idea if something like a pot bearing is used.
RE: Pedestrian Bridge b/w two 10 stories tower
The idea still might work fine if you have no cladding, but it might be harder to convince the architect to let you have diagonals or even verticals for a vierendeel truss.
RE: Pedestrian Bridge b/w two 10 stories tower
cross rope ties to keep cantilevers in lateral stability!
and yes the bridge is cladded.
by the way! the bridge is 8' wide.
RE: Pedestrian Bridge b/w two 10 stories tower
Another question - if they are not occupied yet, I understand that they are constructed, but what is their level of interior finishing?
Mike McCann
McCann Engineering
RE: Pedestrian Bridge b/w two 10 stories tower
Why not build a separate braced tower to provide the access, with just sliding covers at the floor joints?
RE: Pedestrian Bridge b/w two 10 stories tower
whilst i like the idea of a separate tower supporting the bridges (saves loading the buildings), i wouldn't've thought it was too difficult to reinforce the structure or these bridges, some axial load strcuture, react8ing the end moment as a couple, shearing the load into the floors, some vertical beams reacting the vertical load (derr), just possibly a cable support taking some of the bridge weight into the building structure ...
many ways to skin cats ...
RE: Pedestrian Bridge b/w two 10 stories tower
instead of making them 15' each, make them say 3' to minimise moments transfered to the already designed and constructed frame.
then span a simply supported bridge between the small cantlievers. The box frame for this would be smaller than the box frame of the cantilevers by at least the amount of movement required.
At one end bolt the bridge to the cantlever to allow it to slide in one direction and at the other end bolt it allowing it to slide in the other direction.
Any comments?
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
Never, but never question engineer's judgement
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
If it ties the bldgs together then he has to figure out how much force that generates. I suspect that it would be enormous for some load cases. It also wouldn't be easy to calc this load. Tie them together and try a response spectrum or response history analysis?!
RE: Pedestrian Bridge b/w two 10 stories tower
i think the idea is that the tower is free stnading, supporting the bridges and their loads. the bridges would not be tied into the buildings.
i think it's an elegant solution, but i also think the idea hasn't been picked up by the OP, so i guess he's not interested in it.
RE: Pedestrian Bridge b/w two 10 stories tower
I am by no mean qualified to do something this massive because all I do is residential. I am just throwing some idea outside the box.
Never, but never question engineer's judgement
RE: Pedestrian Bridge b/w two 10 stories tower
RE: Pedestrian Bridge b/w two 10 stories tower
This building is actually a hospital, and just below this bridge is the main entrance. The architect is not happy with the idea of any type of hinderance in entrance or its path,
however i am going to discuss it again with the principle engineer and architect.
u know! the idea of a web tappered cantilever from both ends appealed to the architect and he thinks he can do some fancy stuff with it also. Now its just a matter of some analysis.
RE: Pedestrian Bridge b/w two 10 stories tower
not sure where the idea about dividing the bridge in two and anchoring each 1/2 on either building came from ... sounds tricky or awkward to join together in the middle ?
are the floor levels on both buildings the same ?? that would be very fortunate (and unlikely)