Continous for Live but simple for dead load 1

[mar2805]

Hi folks!
I was wondering if anyone here is into prestressed concrete bridge design maybe?
I recently attended a presentation on a residential partialy-precast floor system wich is made up from precast "girders" (not prestressed) wich contain main bending reinforcement for positive span moment.
"Girders" are asambled at site, spaning from support to support (walls) and then after, a concrete "slab"v (topping) is poured between the girders and above them.
The whole system is to be analyzed as a simple span beam, only postive span moments.
Because of the partial continuity that comes from the slab thats cast at the site, there was some conversation on designing for negative moments wich had to be designed at the supports.
"Continous for Live but simple for dead load" term was mentioned.
Can some clarify a bit whats does this mean.
Heres the sketch of the system.

 

[JAE]

I think bridge engineers do this all the time. In the past when I was designing bridges (yuck) we would do that. In fact some software programs have that written into them (check out LEAP or STLBRIDGE).



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[msquared48]

It means that continuity is not established to support the live load until the deck is cured. The prestressed girders simply support the dead load of the deck, and their dead load, until that curing occurs.

Mike McCann, PE, SE

 

[mar2805]

SO after the concrete deck develops its full strenght it attracts negative moments at the support (coming from only live load on the deck)?

 

[paddingtongreen]

In my experience, over time, the concrete in the span creeps, increasing the midspan deflection and the slope at the supports, causing cracks over the beam; that is unless it is specifically reinforced not to. Once the cracks appear, it is back to simple span.

We came up on us when an incensed client found cracks in a 2 year old building that had remained un-occupied. If occupied, these cracks are covered by flooring and carpet etc.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[hokie66]

I think this type system is more often used in buildings than in bridges. The precast, typically with some intermediate props, supports the construction load, but after the whole assembly is composite, continuous action is achieved, provided of course that appropriate top reinforcement is provided. The ultimate negative moments come from the entire design load, not just the dead load.

 

[dhengr]

Note that Hokie talks about “ some intermediate props” under the precast. So, his negative moments and negative steel will/should involve some or all of the dead loads after the system becomes composite and the props are removed.

 

[hokie66]

Yes, dhengr is correct, because those steel trusses in the precast are not typically sized to support the dead load over the full span.

 

[dhengr]

Hokie:
I wasn’t trying to steal your thunder, but I couldn’t resist. People/E-Tips members (engineers?) are reading these forums so quickly these days, not for content or the/your deeper meaning, that this might have gone right over their heads, in their haste. If it is longer than a tweet, they just don’t have the time or the inclination to absorb it or think about it. They are too busy doing important stuff.

 

[mar2805]

What are "intermediate props"?
English is not my main language :-(

 

[msquared48]

Shoring.

Mike McCann, PE, SE

 

[mar2805]

Yes you guys are correct.
Those small precast "girders" are assambled at site, from support to support, but during the asembly they are always supported by shoring! Every 5 feet!
When fixed in place, those clay infills are asembled between the "girders" and then the whole thing is poured with concrete.
Shoring removed after 14 days.
This is quite different from bridge precast concrete girders that dont need shoring.
So in bridge situations, your deck thats cast on the girders is not designed for negative moments?
Found some picture of the situation.

 

[hokie66]

You can't generalize about bridges. The philosophy varies from place to place. I think most precast girder bridges depend on the girders themselves to carry all loading on simple spans, without composite decks. But I may be wrong.

 

[IDS]

The most common detail for bridges, at least in Australia, is to provide a link slab at supports, i.e. the top slab is continuous over the length of the bridge, but the precast girders are not connected with a moment connection.

In cases where the girders are made continuous over the supports then this would be designed for any loads occurring after the connection was complete, including live load, superimposed dead load, differential temperature, differential settlement, shrinkage and creep.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[mar2805]

@IDS
"The most common detail for bridges, at least in Australia, is to provide a link slab at supports, i.e. the top slab is continuous over the length of the bridge, but the precast girders are not connected with a moment connection"
IDS I think thats the philosophy for continouslive-simpledead.
Do you have any detail of this design?
Slabs are then analyzed for negative moment above the support?

 

[IDS]

mar2805 - There are some typical details here:

http://www.rms.nsw.gov.au/doingbusinesswithus/designdocuments/bridge_drawings.html

Note that the top slab is de-bonded from the top of the girder. It is intended to provide longitudinal connectivity to avoid the need for expansion joints at the end of every span. The bending resistance is very small compared with the precast girders, which are of the order of 1.5 m deep, and the girders are designed as simply supported for all loads, unless there is a full depth moment connection, which is unusual.



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[mar2805]

IDS
see picture attached.
Is it correct?

This is the only detaio that doesnt have and diaphragma between the girders.
Ive seen a lot of details were the girder bars were anchored in diaphragmas.

 

[IDS]

see picture attached.
Is it correct?

This is the only detaio that doesnt have and diaphragma between the girders.
Ive seen a lot of details were the girder bars were anchored in diaphragmas.

?

There are three link slab drawings at the link, and they all show a gap between the longitudinal girders.

Yes, the transverse reinforcement is continued over the supports.

Why wouldn't it be?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[mar2805]

Understood!Thanx.
But is still dont understand the title expression, since it seems that the girders are always analyzed as simle spaning beam.

 

[gwynn]

[quote}You can't generalize about bridges. The philosophy varies from place to place. I think most precast girder bridges depend on the girders themselves to carry all loading on simple spans, without composite decks. But I may be wrong.[/quote]

You are incorrect, both in the assumption that precast girder bridges do not rely on composite action and that all precast girder bridges are designed as simple spans. Bridges constructed with precast concrete girders generally rely on composite action with the CIP deck. While there may be places where this is not the case, in Canada it is the standard and I would assume it is most other places as well (it would not make sense to do otherwise in most cases).

Many of the mutlispan bridges I have seen constructed recently with precast girders are designed exactly as the OP has suggested in the original post. They are designed as simple span for the dead loads imposed by the girder self weight and concrete deck. Pier diaphragms are poured at the same time as the deck. Once the diaphragms have cured they transmit the compression flange forces across the pier while reinforcing in the deck takes up the tension forces, allowing them to be designed as continous for superimposed dead loads and live load.