Falsework Support

[Zambo]

We are to construct a falsework to support a segmental bridge structure prior to post-tensioning. The segments will total about 500 tonne and then there is the load from a 20m high falsework. The plan area of the falsework will be around 200m2, but we are still considering using a tower system rather than spreading the load over the full area.

We have some earlier broehole results from the feasibility study which shows that the soil is a sandy clay with N values around 5. We will now proceed to carry out further testing to decide on the construction method. We are planning some soil improvement, probably just by replacing the upper layers with a better material and then we will precast concrete slabs and prestress them together (several spans to do these slabs will be reused)

My question is, as we are considering short term loading (around 20 days with the full load) are plate bearing tests useful? Apart from SPT tests, are there any other suggestions?

Should mention that where I am working we must specify the testing and just tell the geothechnical company what to do - sound advice not available.

Zambo

 

[VAD]

Hello Zambo:

A few comments. It would be better to discusss this problem with the geotech Engineer who provided the report for the bridge structure. Since there is a plan to remove the upper layers, what are the N values of the upper layer and how deep is this layer. From the information provided one can assume that the soil with depth has a constant N value of 5, but this may not be the case. Can you provide the distribution of N with depth. This will help.

You need to have a discussion with your company about the approach being taken on this matter. This is a project for both geotech and structural to sit and discuss the issue and then decide on the plan of action.

If you plan to use a tower rather than spread the load then the geotech would be able to discuss and advise on the necessary tests, if any further is required above what you already have at this time.

Just a wild thought, can you support the Tower on some ties and rails (tracks). This could allow the load to be spread on the foundation. I am guessing here as I do not have a clear picture of the problem as yet.

Regards

 

[BigH]

Be careful - check out California website - they have a falsework manual. On a nearby project - they've had two girders "drop" - make sure you are sure about your support - if you don't use the tower system, ensure that you have a metre of granular pad compacted over your loose sand - make sure you have the falsework design checked and rechecked, make sure that no elements can be removed without proper and authorized clearance of Resident engineer and design engineer.

 

[Zambo]

The falsework design will be checked and I have no problem with this. BigH - I had a problem before with locking pins being stolen from a falsework, on that occassion we had a delay in construction (no money) so had to weld all removeable pins.

We are considering the use of a granular pad (600mm as a first try). We have these old borehole logs which aren't detailed enough and we don't trust entirely anyway. Apart from that we are working in the area and know the soil is poor. On top of the granular pad will be the precast concrete slabs.

What I would like advice on is the most appropriate soils tests to be carried out so that I can get a geotech company to come to site. I am interested to know if plate bearing tests would be useful as the loading is short term, around 20 days.

Zambo

 

[BigH]

Timely thread - had a collapse yesterday (22Feb04) in West Bengal on a nearby project. Happened during the pouring of the long span concrete girder. Somethiing like 50 hurt, 20 seriously - don't know of deaths. Contractor had already poured 6 spans successfully. Different from others I noted that happened (1) just as pre-stressing finished and (2) some 14 days after pour. I can e-mail photo(s) after I scan them. bohica@indiatimes.com . So - all I can say - is be careful. Familiarity breeds "run of the mill" in many cases and . . .

 

[VoyageofDiscovery]

Hi Zambo,

Has the designer of the bridge allowed for post-tensioning through the sections for construction? If this is the case it will be cheaper to balance the cantilevers from the pier, out to the midspan, with a final closing pour, than to build a 20m high falsework system. That is an enormous height which will also attract alot of wind. I have only used these falsework systems for cast-in-place construction.

Typically for these types of structures, the method of construction is a very important part of the design, so check with the designer first unless you already have the drawings.


HTH
Regards

VOD

 

[Zambo]

BigH interested to see photos.

VOD, the design is complete and the construction underway. Two techniques are being used, erection by launching gantry and erection on falsework. It is not possible at this stage to change to balanced cantilever, but in any case it is not clear cut that that method is cheaper.

I am still interested for views on the best soil investigation method for under a falsework. I am sure I only need an accurate estimate of the soil bearing capacity, but not sure whether SPT or plate bearing tests (or another method) will give me the most appropriate results considering the short term loading.

Zambo

 

[Focht3]

Forget plate bearing - unless the loaded area will be the same size as the footings...

CPT (or other in situ method) is better than SPT; but you have to use what's available to you.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[VoyageofDiscovery]

Hi Zambo,

You original post mentioned "We are to construct a falsework to support a segmental bridge structure prior to post-tensioning." I did not realize construction is underway. That precludes what I proposed. I was also going to mention the launching gantry. I see now that you have already got two methods underway, hope it all works out well.

Regards

VOD

 

[Zambo]

Well still not much improvement with this. Our local partner decided that a dynamic cone penetration test using 4.5kg hammer dropped 300mm would be a good test. But now how to find the allowable bearing capacity. The testing company gave a table but no mention of hammer weight on the table, so cannot be sure the graph is correct for the test carried out. Am I right in thinking that the blow count should be averaged over a depth equal to the width of the footing? Is the blow count taken every 300mm?

If my assumptions are right we have a concrete pad 5m width and a average blow count to a depth of 5m of 100 per 300mm penetration. The next 10m tested shows increasing blow counts. Anyone like to give an approximate bearing capacity?

Zambo