column design

[bnrg]

Hello all,
I am designing a column that is under combined bending and compression loads. Using Roarks (5th ed)para 11.5 for this. The material that I will be using needs to be non conductive electrically so a fiberglass or plastic pipe will be the likely choice. My question pertains to the failure criteria that I should be using. Roark provides 2, one for steel and the other for aluminum. They seem to vary by the ratio of the modulus but I am not sure if it is reasonable to revise the 'Fe' value by the modulus of the material that I wind up using. If this is ok, what modulus would be appropriate-tensile, flexural...?
Just to get a place to start, I used steel for the column and wound up with an 'L/r' of about 34.
Thanks,
Bob

 

[ishvaaag]

Since I am not familiar with the particulars of Structural Design of Plastics I would refer to one reliable source. google when asked about structural design of plastics soon gives some information about one

Structural Plastics Design Manual, ASCE Manuals and Reports on Engineering Practice No. 63, American Society of Civil Engineers, New York, 1984.

 

[jheidt2543]

A call to the material manufacturer would also be in order. They can tell you allowable design values and direct you to code authorities and design references.

 

[Taro]

There isn't much in the way of code provisions or design standards for FRP because it's a relatively new product. jheidt2543's advice to contact manufacturers is a good one. Most FRP structural shape manufacturers' literature includes load tables and design guides.

 

[SweetNlow]

You might want to define more closely your criteria. Size, length,and load Non-conductive materials include concrete and wood.

 

[Bbird]

bnrg,

For fibreglass material one must be absolutely clear about the stress direction as there are at least hoop tensile, hoop flexure and axial tensile modulii to choose from.

The fibreglass material relies on glass fibre filament for the tesnile strength but there are many different ways of winding the filament by different manufacturers or proprietary systems. Typical no manufacturer can offer continuous filament embedded axially in resin and my reaction to a column application would favour a double helix constrcution as it should perform better. The double helix winds the pipe wall forward and backward at an angle. One component of the glass filament in the axial direction provides the very strength needed for resisting bending in the column (other complimentary component being in hoop direction). AWWA M45 gives a good description at the front on different proprietary systems.

In bending an axially loaded circular pipe I would imagine the longitudinal tensile modulus is the most relevant. This property is obtainable by cutting a uniform cross section strip of filreglass pipe wall longitudinally, measure its area and pull it through a standard tensile testing rig. Although ASTM D3753 (described below) prefers flexural test by bending as a beam to ASTM D790. In practice getting a perfectly flat specimen out of a pipe wall can be challenging and many tensile properties I come across is by tensile pulling described as above or using ASTM D638 in which a dumbell shape specimen is prepared. Austrailian AS/NZS 3572.23:1997 also pulls the specimen to prove its longitudinal strength. For 6" diameter or less the complete pipe can be tested as per ASTM D2106.

For column application I would recommend to have a look at ASTM 3753 relating to glass-fiber-reinforced polyester manholes and wellwells and ASTM D3517 relating to GRP pressure pipes.

As a sanity check the longitudinal tensile modulus is usually the lowest among the three and can be 50% of the hoop tensile. Hoop flexure should lie somewhere in the middle. This is because the winding angle is usually between 90 to 70 degrees, resulting a GRP pipe strong in hoop direction. You have to remember the standard connection in GRP pipes by push-fit gasket joint and the tensile strength is often a minor requirement.

Good luck