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. ?

 

[rb1957]

i would've thought that you could anchor the bridge on oe tower, and have a sliding support on the other.

of course you could always tie the towers together with thebridges, which would change their mode shapes, and re-do the analysis.

 

[zeeshanahmed]

The Building is already constructed.

 

[slickdeals]

I second the idea of fixing the bridge to one building and provide slide bearing connection on the other. You can detail the bridge to move and provide the required expansion joint at one of the building/bridge interfaces.

 

[zeeshanahmed]

when two towers are moving away ! thats like 4" + 4" = 8". eight inch slide brearing plate connection /??

 

[271828]

That's one of the most screwed up problems I've heard of!!

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.

 

[lkjh345]

I agree with the others. One end of the bridge is anchored to one of the towers, the other is allowed to moved.

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.

 

[zeeshanahmed]

thanks 271828

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?

 

[cvg]

how about a vertical rocker on one end and horizontal slider on the other? how will fixed on one end work when there is a vertical component to the movement, won't it deflect and / bend?

 

[PMR06]

Rope bridges would be cool

 

[JStephen]

Seems to me you could have problems not just with the large movements but with continual small movements; would tend to wear out things or start making noise when moving.

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.

 

[COEngineeer]

If you need to have an access @ each level, why do you need to build bridges? I would just do "addition" and tie the 2 buildings together.

Never, but never question engineer's judgement

 

[rb1957]

i thought tieing the biuldings together might make them stiffer, tho' it would put sizeable load in the bridges (and in the buildings near the bridges).

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.

 

[msquared48]

Could get real tricky here.

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

 

[jike]

I agree with fastening the bridge to one building and letting it move at the other end.

Check out

http://www.seismicjoints.com/images/PowellCat04.pdf

for some typical seismic joints. You may need to contact them for other specialized joints.

 

[PMR06]

Maybe you could get some ideas from airport jetway designers. They slide many feet, rotate, etc?

 

[zeeshanahmed]

how about a very stiff fixed connection at both the ends so that there is no movement at both ends ! and heavy connections here at ends. so they dont move at all ???

 

[zeeshanahmed]

271828

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

 

[LittleWheels]

I agree with COEngineer. A connection for all 10 levels sounds like a building addition, not a bridge.

 

[271828]

"... what about the lateral movement? how will a sliding bearing plate cater for that ? "

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.