Deflection according to eurocode 2

[mmmghorab]

I'm reviewing a bridge formwork design prepared by a supplier in house software which used Eurocode. The used timber beam v-20 2.9m length supported on stringers (wales)then, towers (legs spacing 1.5m). The timber beam span now is 1.5m with double overhangs 0.7m each side.
Moreover, the clear distance between towers is 1.575 (two overhangs 0.7*2 plus 0.175m gab between).
The deflection of timber beam is shown is compared to L/250, but the value didn't include the creep deflection or shear deflection.
I'm asking about:
1. Is it mandatory to calculate the creep deflection using Kdef?
2. When I checked the EN 1995-1-1, I've found a range of allowable deflection [for w(inst)= L/300-L/500], what values should be considered?
3. Is there any recommendations regarding the length of the overhang to the span?

Note: the bridge deck height is 2.3m, span is 40m, and will be poured into two stages, the bot. slab with webs, then the top slab. and the area under reviewing is under the end diaphragm.

 

[BigH]

Would be better to post this in the Structural Engineering and Other Technical Topics forum.

 

[RHTPE]

First, I am not sure what a V-20 timber beam is. I am assuming that it is a wood I-beam similar to an H-20, i.e. 20 cm in depth, timber chords with a cross-laminated or plywood web. These members are generally proprietary items and their performance criteria is established by empirical testing. My comments are based on using H-20s.

1. Should creep deflection be of concern? Creep is a long-term phenomina. Formwork in particular is usually designed for a 7-day load duration. Once the concrete has acquired its own strength continued deflection of the formwork is simply not going to occur.

2. Permissible deflection of formwork is a criteria established by whatever code might apply. It is really an esthetic issue in that formwork deflection will be mirrored in the finished concrete. It appears to me that EN 1995-1-1 is focused on permanent wood structures and not temporary works. The criteria for each differs and is influenced by performance over its useful life. This can be a judgement call.

3. Length of overhang relative to interior span matters more in the formwork's performance as it gets loaded. The application of load to a formwork member does not occur instantly - it is applied as the concrete is placed. Some of the load (the concrete) remains and some of the load is transient (the placement crew's live load). At some point in time the cantilever will be loaded while the interior span is not. If the far support is secured against uplift, and the performance of the cantilever is acceptable for the short period of time it alone is loaded, then there is no problem. Much (but not all) of the initial deflection of the cantilever will be mitigated as the interior span receives load. Some formwork designers tend to examine the fully loaded condition and not the time-dependent application of load. Again, this can be a judgement call based upon the magnitude of deflection, the method of concrete placement, and the method of installing the members supporting the sheeting.

The geometric constraints as you have described them seem to be typical for European applications. The performance of the wood members supporting the sheeting will depend on how well the placing crew understands the the formwork construction. Placing concrete first on the interior spans and then onto the cantilevers will most likely yield the best result. But this may not be garanteed. Insuring that all wood members are fully secured against uplift will also help in their performance.

Personally, I think the amount of cantilever is a bit excessive. The wood beams under full load will experience negative moment over their entire length. Deflection will be the greatest at the ends of the cantilevers. The weight of the concrete over the 0.175m gap must be added to the wood members. I would adjust the interior span to equalize the deflection at the ends and at mid-span. A 1.7m central span with equal cantilevers would provide for equal deflection at both mid-span and at the ends of the cantilevers. BUT, that said, does it work with the equipment that the supplier has available?

Much to consider. I will follow this conversation and offer additional insight as needed.

Ralph
Structures Consulting
Northeast USA