Prestressed I Beam Bridge

[ahmd8201]

Assalamualaikum dear forum members,

 

I am currently designing a 35m span prestressed precast I beam bridge. The deck is formed from 300 rc insitu slab and the beams are 1.8m deep precast I beams. These span simply supported onto bearings shelf/abutment. Due to a high watertable, a secant pile wall option is used to form the abutments (with a hard/soft pile configuration). There is also a raft to form the underpass and act as a prop to the secant walls. 

 

The secant walls are formed from 1200mm diam. piles and the raft is currently 2m deep. Apparently another engineer (quite senior) advised to have the raft dowelled into the piles, i.e. maybe 3 dowels per pile in the secant and then continued into the raft at an appropriate anchorage.

 

The 35m span deck is supporting a curved road on top and the underpass is supporting 6 lanes of traffic. The whole bridge is designed to AASHTO

 

I have a few issues which are troubling me which I would like to ask for opinions on;

 

 

1) The prestressed I beams at ULS are sagging as they are simply supported. At SLS the ends are hogging; is this due to the prestressing force being too much? The secant pile wall is being designed by geotech. I assume that under any limit states as the deck is supported on bearings, the forces transferred to the secant will only ever be axial forces and shears ie. fx, fy, fz?

 

2) The raft/ secant pile wall connection does not sound particularly convincing to me. The actual secant pile wall is very long and is about 480m. The raft slab therefore has movement joints every 10m or so. I thought that the raft plus secant could either be considered as an inverted portal frame or as a wall with a lateral prop. The senior engineer want to assume this is acting as an inverted portal frame but how can we have fully fixed moment connection with 3 dowel bars! Also if the abutment was actual wall, the raft could be detailed with rebar to be cast as a full moment connection, but with the secant the rebar will be very congested and I do not know an easier way to achieve this full moment connection of the raft with the secant. 

 

Please advise.

[UmarMakhzumi]

W/salaam..

 

Can you please post drawings or snapshots of how this looks like.. You can post your structural framing. It would be easier to reply once the information is at hand.

 

Thanks.

[ahmd8201]

Salam, please find attached drawings of the proposed structure. The raft slab is proposed to have movement joints every 10m. Is the movement joint (dowel anchored on one end of the slab and on a sleeve/debonded on other side) therefore only transmit shear? Also the raft (~35m wide) will be by supported on piles at approx 12m intervals to 35m raft. As the raft is 2m deep and the piles are 1.2m dia. I have assumed the piles will modelled as simply supports. Is this a reasonable assumption?

See attached drawings

[ahmd8201]

Can you also advise what a typical pile raft connection would be like?

[UmarMakhzumi]

W/salaam..

 

Thanks for uploading the photos. Actually, the photos do give some idea, but I can't read the details. If you can upload a pdf copy of drawings, that would help me to see what is being presented. However, I will still try to answer your questions.

 

 

 

Can you also advise what a typical pile raft connection would be like? 

 

The connection depends on pile type. If you are having concrete piles, you can just prolong the dowels from the piles into the raft/ pile cap. However, the length of dowels in the piles "must" satisfy development length requirements or your pile tension capacity shall be capped. If you are using steel piles, you need to weld dowels or nelson studs on top of your pile cap plate and provide enough development into the pile cap/ raft.

 

 

 

Also the raft (~35m wide) will be by supported on piles at approx 12m intervals to 35m raft. As the raft is 2m deep and the piles are 1.2m dia. I have assumed the piles will modelled as simply supports. Is this a reasonable assumption?

Generally speaking, thick rafts (depends on depth of rafts and spacing of supports) are considered as rigid bodies for force distribution. Given that, you are fine assuming a pin support. A rigid raft/ mat foundation will produce only compression and tension so pin assumption would work. However, I will leave it to you to decide if your raft foundation is rigid enough to justify the assumption or not.

 

 

 

The raft slab is proposed to have movement joints every 10m. Is the movement joint (dowel anchored on one end of the slab and on a sleeve/debonded on other side) therefore only transmit shear?

Your photos aren't clear enough that I can read the details about the movement joint. Upload a pdf and then perhaps I can reply.

 

 

 

1) The prestressed I beams at ULS are sagging as they are simply supported. At SLS the ends are hogging; is this due to the prestressing force being too much? The secant pile wall is being designed by geotech. I assume that under any limit states as the deck is supported on bearings, the forces transferred to the secant will only ever be axial forces and shears ie. fx, fy, fz?

2) The raft/ secant pile wall connection does not sound particularly convincing to me. The actual secant pile wall is very long and is about 480m. The raft slab therefore has movement joints every 10m or so. I thought that the raft plus secant could either be considered as an inverted portal frame or as a wall with a lateral prop. The senior engineer want to assume this is acting as an inverted portal frame but how can we have fully fixed moment connection with 3 dowel bars! Also if the abutment was actual wall, the raft could be detailed with rebar to be cast as a full moment connection, but with the secant the rebar will be very congested and I do not know an easier way to achieve this full moment connection of the raft with the secant.

I have seen your photos, but like I said, I can't read the details about how the the deck is supported by Pre-stressed beam, and how the beams are supported to the foundation level. Please upload a pdf version of drawing that shown all the detail.  Also, when you post the pdfs, please also state that what section on the drawing relates to your question. I honestly couldn't find the detail of raft and secant connection and the 3 dowels. Please repost.

 

Thanks.