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RE: Difference between rigid and flexible culvert

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While there is a tendency to simply classify conduits according to material type as rigid or flexible, it's not really quite that simple. The behavior of the pipe as flexible or rigid is a function of the pipe stiffness relative to the stiffness of the surrounding soil. If the pipe is stiffer than the soil, then the pipe is rigid. If the soil is stiffer than the pipe, then the pipe is flexible. 
 
For a lot of ordinary applications, the classification is easy and intuitive. Corrugated and plastic pipe is generally flexible, as is steel and ductile iron, at least in larger diameters, and concrete and AC pipe are generally rigid. However in small diameters, ductile iron pipe in conventional thicknesses is fairly stiff and will tend to act more like rigid pipe. Hybrids like concrete cylinder pipe are sometimes classified as "semi-rigid."
 
If you define the pipe rigidity as kp = 96(EI/D^3)/D, where D is the diameter in inches, E is the modulus of elasticity in psi, and I is the moment of inertia of the wall section. kp is measured in lbs/in^3 and is the deflection of a ring under a uniform horizontal loading applied over the diameter of the pipe.
 
If you compare this to ks, the subgrade modulus, which is measured in the same units, you will have some idea of the relative stiffness. If kp/ks is greater than 1.0, then the pipe is probably rigid, if not, the pipe is probably flexible. I don't pretend that this is an exact method, but most of the time the ratio is either much greater or much less than 1.0 for real pipes, and you can rapidly sort out how to approach the design.
 
A flexible pipe has very low pipe stiffness and as a result deforms more and can only transfer load through axial load in the plane of the wall. The soil is doing most of the work. A rigid pipe has high pipe stiffness and as a result deforms very little although the surrounding soil may be trying to settle. The pipe does most of the work, mobilizes flexural resistance in the pipe wall, and actually experiences higher load than a flexible pipe because if cannot "travel" with the settling soil around it. Flexible pipes usually fail by buckling, while rigid pipes usually fail by exceeding the strength of the wall.
-----Original Message-----
From: Eleanor G. Guarin [mailto:eguarin(--nospam--at)escaengineers.com]
Sent: Monday, August 05, 2002 9:35 AM
To: seaint(--nospam--at)seaint.org
Subject: Difference between rigid and flexible culvert

 
Im currently designing manholes and handholes which i know that is almost same when designing box culverts. In LRFD design, there is a load factor design of 1.0 for rigid culvert and 1.5 for flexible culvert, I just want to know what's the difference between them. Could you give me example of flexible culverts?
Please help, thanks.
 
Ellen


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