Deflection Check - Culvert

[eelssm]

Hi Everyone,

I finished designing a three sided culvert and wanted to know if anyone had an approach, method, or guide for checking the deflection.

I want to make sure the peak is adequate for no ponding on the structure.

Thanks!

 

[PST09]

eelssm ... bit off topic but i would be interested in seeing your design steps because i have to design one of these shortly. Did you design by hand or do you have any good refrences?

 

[eelssm]

I received the drawings and specs from the state.

Once you have this, I used a software package to design the culvert.

What state are you in?

 

[PST09]

In in the UK so it does not really help! I was just trying to feed the thirst for knoledge and interested the the design method used and how it differs to the way we whould do things over here!!

 

[PST09]

Also it seems I can’t spell or type...apologies

 

[ishvaaag]

Just what could be approximate in a building

You first estimate the deflection with a realistic modulus of deformation of the concrete at 28 days, and separate the deflections corresponding to

DL Dead Loads
SDL Superimposed Dead Loads
LL Live Loads

All these would be then ordinary elastic deflections, only separated by cause (what can be made dedicating separate hypotheses).

Then, the total final deflection will be in the range of...
DeltaDL·2.8+DeltaSDL·1.9+DeltaLL·1

This should be lesser than span/250.

The Active deflection, which is the one able to cause damage in nonattached parts will be in the range of...

DeltaDL·0.9+DeltaSDL·0.9+DeltaLL·1

In buildings, shouldn't be more than span/400 nor 1 cm

This is a quick way to estimate relevant parameters of RC deflection on the otherwise always quite inaccurate subject of RC deflections.

 

[JStructsteel]

do you not have any cover on this culvert? what was your span?

 

[eelssm]

There is almost three feet of soil on the culvert with a span of 36 feet.

 

[JStructsteel]

I guess I dont see how you will get ponding. Is this a flat slab culvert? Most spans that size seem to be a arch or have a crown to them

 

[eelssm]

Yeah, it's completely flat.

I guess I could talk to the precasters and see if they can add a small crown or peak to the culvert?

 

[cvg]

if you are placing fill on the culvert, why wouldn't you grade the fill to drain? This will prevent ponding. Structural design for saturated soil on the top should have already been done and any deflection due to saturated soil should be negligble. I would be a lot more worried about deflections due to highway truck or construction vehicle loading than for water.

 

[Ussuri]

I think this is mainly for hydraulic design, but it might provide other useful references.

http://www.ciria.org/SERVICE/Home/core/orders/product.aspx?catid=5&prodid=304

 

[ishvaaag]

In my previous post above, the stiffness of the members should be based in its state at service level, be it cracked or uncracked. The usual formulation for cracked section's inertia Ieff in ACI of EHE is recommended. When cracked, the code EHE allows the central section be adopted for members fixed or supported at both ends, and the root (fixed) section for cantilevers.

Also, even if lesser LL than 100%LL could lead to more realistic evaluation of the actual deflection (and especially of the active one, able to cause actual damage), a normative check would involve 100%LL

An initial analysis with gross inertia may soon reveal the extent of the cracked zones then leading to adjudication of new stiffnesses in the model.


I have even coarser rules of thumb for RC deflection checks; one that has proven generally satisfactory for actual ordinary building structures in two senses

1. that no notorious problems appear in the building caused by deflection
2. that if you check in more detailed way the deflection you use to get satisfactory compliance of the code

is using gross sections' inertiae and modulus of deformation as directly by default given by RISA 3D (or if you want, Ec at 28 days age from ACI or EHE code) and limit the maximum service level deflection to 7 mm. I have been pushed sometimes by the undersigning architects to twice that limit rule of thumb deflection check in spans just a bit over 9 m and still be able to show code compliance.

AND ... remember this is only a guide on deflection for ordinary buildings, say houses, condominiums, offices, maybe some retail buildings, not THE proper check.