Strength of different profile shapes

[shunter75]

Hi all,

Just a quick question. I am in debate with someone I work with regarding the strength of steel profiles. I generally design primary steel using W-flanges (IPE, UB's etc. depending on what part of the world) Recently a colleague challenged this by saying that the box shape (SHS, RHS) is stronger and more suitable for offshore design, to which I don't agree. (his arguments are is stronger and less sharp edges..what ever that means???) We are both not structural engineers, but have had many years experience in the offshore industry and to just say that it is stronger, to me seems to be a very simplistic view. The question I have is what is the most common use of SHS also what is generally stronger, I know this depends on the application but lets say HS89x64x8 against an IPE100. The IPE has less weight and therefore is better for offshore buildings where weight is an issue. and lastly what is availablily of SHS/RHS in high strength grades i.e. AH-36A etc.

Any help would be greatly appreciated.

 

[Lion06]

I think it really depends on the application. If you're talking simply about strong axis bending......... you can't beat a wide flange for efficiency - it puts most of the material at the extreme top and bottom where it is needed for bending.

If you're talking about for torsion, well then you want the closed section.

 

[ToadJones]

Agree with EIT.
Situations vary way too much to use a coverall statement like "HSS is better".
In some situations HSS will be more efficient and in others, WF's will be.
If that weren't true, mills would be pumping out nothing but HSS.
Cost is another issue.

 

[steellion]

For beams, no question, you want an I-Shape.

For members subject to torsion or bi-axial bending, the tube will probably be the better choice.

For columns, it depends. I typically use tubes for lightly-loaded one or two story columns, and wide-flanges otherwise. In the States, you pay a 20-40% premium on HSS shapes over the wide-flange.

One last thing, for corrosive environments, a tube shape will corrode slower than a wide-flange due to reduced exposed surface area.

So, as SEIT said, it depends on the application.

 

[shunter75]

Hi guys

Thanks for the quick response..that was amazing! Yeah I know it does depend on the application. Generally what I'm building is of recatagular construction i.e. not crazy architectual designs. Torsion can be an issue however as the design is generally square everything is braced with itself. The Corrision issues are mainly covered by the high specfications on surface treatment, so not a real big issue. I guess the potential problems are the availabilty of high grade (AH-36A etc.) SHS in various countries and the weight/strength ratio between differnt profiles, as being offshore the lighter the better (although not weak and light) I think this is best achived by clever design. As I normally use the W-profile I also think that it is easier to use various connection details not available when using SHS/RHS. Also if I need to use gusset plates under areas of hevay load this too is much easier than SHS/RHS.

 

[Ussuri]

For offshore use tubulars are generally used, for a number of reasons which may include:

They generate lower hydrodynamic loads;
Can be used to provide buoyancy;
Do not have crevises which can trap water;
Wave loading can be applied about any axis;
Torsional capacity;

For offshore use on topsides all the structural shapes are used, it is really only when wave action is the predominant loading that tubulars are favoured (Not to say that open sections are note used form time to time so does depend on application).

 

[paddingtongreen]

I'm surprised that you use tubes in an ocean environment, unless they are sealed, unlikely at best, they can take on sea water and rot from the inside where it is not visible untill too late.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rb1957]

wide flanges are also not good for compression. square/rectangular tube is easier to connect together
but I-beam webs are more forgiving for misalignment
"less sharp edges" ... is that a safety concern ?? maybe indirectly pointing to a closed section ??

 

[steellion]

rb, I'm confused by your comment that "wide flanges are also not good for compression". Wide flanges are used all the time in compression: Once buckling is resolved, compressive strength is based simply on area.

Standard shear connections are also much easier with I-shapes than tubular shapes. You need to be more concerned with local effects that the connecting member has on a tube wall than a W-shape.

 

[rb1957]

i guess your section aren't subject to crippling.

"You need to be more concerned with local effects" ... that's what i meant "I-beam webs are more forgiving for misalignment".

 

[hokie66]

paddingtongreen's point is well taken, but closed sections used in marine environments are protected both inside and out, either by hot dip galvanizing or less frequently, by filling with concrete.

 

[csd72]

There is an important point that is not mentioned above - I beams are stronger in bending when well braced in their weak axis whereas tubes are stroger for long unbraced spans.

I beams are near useless in torsion.

 

[Ussuri]

I'm surprised that you use tubes in an ocean environment, unless they are sealed, unlikely at best, they can take on sea water and rot from the inside where it is not visible untill too late.

Sometimes they are sealed, but then the buoyancy can cause problems. In which case they are flooded. In order to prevent corrosion a cathodic protection system is included. Flooded members are allowed to flood through holes fitted with plugs that allow water in very slowly. Once the water is in it tends to stay there. Inside the tubulars you can include biocide which kills organisms looking to eat the steel and also oxygen scavengers which remove the oxygen. You are left with reasonably inert water inside