Settlement of Ground Supported Scaffolding

There is about 5mm settlement of a ground supported steel pipe scaffolding for a box girder span of 48 m of a two span continuous box girder 3.5 m deep. The other span of 57 m is supported on steel trusses for the construction is still to be poured. The pipe supports of the first span of 48 m are spaced longitudinally at 0 .75 m resting on a continuous strip foundation of concrete slab of 300 mm thickness which is resting on consolidated marine clay of (assumed) bearing capacity of 5 t/sq.m. The settlement is zero mm at the pier supports varying to a maximum of 5mm at the pipe support at the mid span. While there is no visible distress at the present, I am afraid that the clay may consolidate further producing bigger settlement. It will still take about three weeks before the second span and the connecting diaphragm are poured and the bridge is prestressed.

I would appreciate if someone can advise the method to arrest the settlement and likely distress expected in the case of further settlement of the pipe scaffolding and remedial measures.

 

[Ginger]

I cannot advise about the effects on the bridge structure. You will need to speak with the bridge designer about that matter.

If recovery of the bridge deck profile is required then you will need to install a jacking regime under the deck. This could be done by insrting new supports with jacks between the existing scaffolding poles. Alternatively, remove one tubular support a ta time (if the temp works design will allow) and install the jack at this position. The first solution is the safe and better one.

As for the settlement, you may need to resort to compensation grouting. This is NOT a cheap solution and you will need a specialist contractor to do this properly.

Regards Andy Machon

 

[austim]

Hi, bhavel.

I can tell you a little about the effect on the bridge itself, which may help you to sleep better.

I estimate the likely maximum bending stress in the girder as about 1.3 N/mm^2 (or MPa if you prefer) neglecting any beneficial reduction from creep effects. See below for derivation. It would not be unreasonable to assume that creep would eventually halve this, getting you down to fairly minor stress levels.

If the bulk of your settlement occurred during initial loading, at a time when the concrete had not developed its full stiffness, then the stress could be considerably less.

With in situ concrete supported on falsework as you have described, some settlement would be inevitable anyway, and the bridge designer should have anticipated this.

At least, this may give you some 'ammunition' for your discussion with the designer(s?). Good luck with that.



My 'magic' formula for bending stress (which is very handy for quick approximations) goes like this :

Fb = K * E * (deflection/L) * (Ymax/L)

where K = 12 for beam bent under central point load,
9.6 for uniformly distributed load
8 for constant BM over full span.

Thus for almost any practical loading, you can take K = 10 and get quite close.

Assuming that Ymax (the maximum distance from the neutral axis to top or bottom level of the girder) is about 2.0 m, I get
Fb = 10 * 30000 * (5/48000) * (2000/48000) = 1.3 MPa or so.

 

[Hariharan]

I agree with many points of austim. However, creep is not to be expected
at all so early, and in absence of stress.
I am not sure if you are the designer or the constructor. Formally, the
data should be given to the designer and he/she should evaluate the
consequences and advise on any action to be taken.

The following points should help:

1. The deflection of 5mm is too small for a span of 48 m (L/9600).
2. Much of the loading and settlement would have taken place before
the concrete attained its stiffness. I am presuming that the concreting
would have been done in stages. The girder did not have any stiffness
till the last piece was poured in place. The shape could be considered
the "as built" profile. The stresses are likely to be much less than
the values computed by austim. This is very small, and will not have a
significant effect after prestressing. There would normally a margin in
the design, which could accommodate such stress
3. There would have been some camber in the design. That would have
got reduced by 5mm. No effect foreseen on function.
4. You may, however, have to prove to the owner that the effect is
negligible. The construction sequence can help you determine that
the stresses are indeed negligible.
5. You say the clay is consolidated marine clay of bearing capacity
5 T/M^2. With a factor of safety of 2, the shear strength would be about
20 KPa. The value appears too small. What is the computed bearing
pressure?
6.Most of the settlement should have taken place by now(elastic). Any
further settlement would be due to consolidation of the clay. It may not
be substantial (a few mm more?). How long is it since this first girder
was concreted? (Now the girder would resist any further settlement!)

Good Luck

M. Hariharan