×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pedestrian Bridge b/w two 10 stories tower
3

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. ?

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
The Building is already constructed.

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
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?

RE: Pedestrian Bridge b/w two 10 stories tower

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?

RE: Pedestrian Bridge b/w two 10 stories tower

Rope bridges would be cool :)

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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.  

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
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 ???

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
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

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

"... 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.

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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.  bigsmile

Mike McCann
McCann Engineering

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
msquared48
wow! why dint i think of that :)

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
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 ?

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

Can you let it slide at both ends?

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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.  

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
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.

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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?

RE: Pedestrian Bridge b/w two 10 stories tower

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 ...

RE: Pedestrian Bridge b/w two 10 stories tower

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?

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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

RE: Pedestrian Bridge b/w two 10 stories tower

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.

RE: Pedestrian Bridge b/w two 10 stories tower

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?!

RE: Pedestrian Bridge b/w two 10 stories tower

271828,
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

you can make the the 9 free standing and still make it look like they are 9 seperate bridges if you putt some air gap between level.

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

rb, you might be right.  There might be some reason that he can't have any columns, however.  Perhaps there's a street and/or sidewalk below.

RE: Pedestrian Bridge b/w two 10 stories tower

(OP)
the idea of the seperate frame was on the table before this post !
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

oh geez, we're not going to let an architect design something ??

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)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources