Pedestrian Bridge b/w two 10 stories tower 3

[zeeshanahmed]

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. ?

 

[DRC1]

Two have the bridge be able to allow the buildings to move indepenently would be difficult. If you cantilevered the bridge from one building, you may still distort the mode due to reactions in the Z-direction, which as the building tried to deflect, would become x&y Reactions. Having only 30 feet and loking for 4-8 " movement and doing it at every level is I think possibly asking too much. I agree with others that advocate tying the buildings togeter and investigating the resultant.

 

[msquared48]

This is frustrating... It really is a major building addition. Not sa simple solution here.

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

 

[zeeshanahmed]

msquared48
wow! why dint i think of that

 

[zeeshanahmed]

how about two cantilevers from both the towers, 14'-8" each and a deck extension and joint in the center, this way each member becomes the part of each tower. And some seismic sliding joint of the deck in the center ?
will this work ?

 

[lkjh345]

I'm not sure what cantilevering sections off of each building gets you. I would think this would be much more difficult that attaching a bridge to one building and providing a slider or rocker connection at the other.

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.

 

[JLNJ]

Can you let it slide at both ends?

 

[HgTX]

I second the question about lateral movement.

Hg

Eng-Tips policies: faq731-376

 

[271828]

If it slides at both ends, then what provides ANY restraint to lateral movement?! That's not a plan IMO.

 

[JLNJ]

271828

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.

 

[271828]

I think it'll end up stuck up against one side, so will have a lot of movement potential in one direction, but none in the other.

On second though, perhaps this isn't such a bad idea if something like a pot bearing is used.

 

[271828]

Are these bridges enclosed by cladding? I have to think they are because I don't know anybody who would walk on an pedestrian bridge 9-10 floors in the air. If it's enclosed by cladding, then I'm warming up to the idea of cantilevering from one bldg. You could surely have some diagonals and have several story-deep trusses to help cantilever that far. Then just keep it free at the other end. Of course, there are detailing issues with the joint, but those are there regardless. Use your models to figure out hte worst case combniation of movements. Don't count on them to move in sync or exactly how the model predicts. Of course, there's wind issues if cladding exists.

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.

 

[zeeshanahmed]

Please comment on the Idea of two cantilevers from each tower and the sections not joining in the middle (total 8" gap in the center of both the cantilevered sections) and just the Decks extending into each other and 8" of rubber sealent fitted at this mid location as siesmic joint!

cross rope ties to keep cantilevers in lateral stability!

and yes the bridge is cladded.

by the way! the bridge is 8' wide.

 

[msquared48]

One question - are the buildings currently occupied? If not, what is the projected occupancy date?

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

 

[hokie66]

Sorry to come in late on this. I have been fishing. Since the building is already constructed, the idea of supporting all these bridges on the buildings, by cantilevering or otherwise, doesn't appeal to me.

Why not build a separate braced tower to provide the access, with just sliding covers at the floor joints?

 

[rb1957]

how much load on thes bridges at any time ? maybe there's a design code requirement ??

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 ...

 

[patswfc]

i like the idea of the cantilevers, however i would modify it a bit.

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?

 

[rb1957]

i guess you could extane patswfc's idea and make the bridges as pinned at one end, and sliding (longitudally) at the other, but that would require more bridge structure.

 

[COEngineeer]

I still dont understand. You are going to make 9 bridges?? Why not just support these 9 walkway onto the ground and tie the 2 buildings together?

Never, but never question engineer's judgement

 

[slickdeals]

I agree with COEngineer. Instead of trying to work out crazy solutions, it might be more economical to build a middle bulding (a one bay-9 story frame) that is separated (or maybe tied) to one building. The architect might not like it, but he's not the one paying for it.

 

[271828]

There might be a reason that there can't be columns.

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?!