Continous for Live but simple for dead load
Continous for Live but simple for dead load
(OP)
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.
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.






RE: Continous for Live but simple for dead load
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RE: Continous for Live but simple for dead load
Mike McCann, PE, SE
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
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
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
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.
RE: Continous for Live but simple for dead load
English is not my main language
RE: Continous for Live but simple for dead load
Mike McCann, PE, SE
RE: Continous for Live but simple for dead load
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.
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
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/
RE: Continous for Live but simple for dead load
"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?
RE: Continous for Live but simple for dead load
http://www.rms.nsw.gov.au/doingbusinesswithus/desi...
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/
RE: Continous for Live but simple for dead load
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.
RE: Continous for Live but simple for dead load
?
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/
RE: Continous for Live but simple for dead load
But is still dont understand the title expression, since it seems that the girders are always analyzed as simle spaning beam.
RE: Continous for Live but simple for dead load
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.
RE: Continous for Live but simple for dead load
RE: Continous for Live but simple for dead load
can you post an egsample, or recommend maybe a book that describes this procedure...
RE: Continous for Live but simple for dead load
The deck is fully replaceable. The girders do carry the deck prior to being composite. Composite action is only required to carry the live loads and superimposed dead loads (barriers, asphalt etc.)
Mar
I do not have an example handy, sorry.
RE: Continous for Live but simple for dead load
I consider replacing a composite deck to be a much more complex operation than replacing a non-composite deck, thus my comment previously.