Wide Span Portal frame.

[civeng80]

Been asked to design a 60m wide clear span portal frame (eaves height 7.5m). The maximum Ive ever done so far is about 35m clear span. Has anyone designed such a wide span portal using standard available sections? If so designed by plastic or elastic analysis? Any further comments ideas would be appreciated.

 

[kslee1000]

Not a simple task, but can be done. Is the roof to be flat, or tappered? What is the use of this building? Need more details.

I remains in linear-elastic thinking. Others may have better view on plastic design, which could be beneficial for long span structures.

 

[kslee1000]

By the way, I afraid the "standard shapes" is not readily available for this job, you may need build-up shapes.

 

[asixth]

You are going to need big sections and some decent moment splices. It may be worth while tapering the beams or even using castellated section.

 

[abusementpark]

Plastic Design could be beneficial to you, especially if you can develop fixity at the bases. If it is architecturally allowable, I would try to put some knee braces at the top corners. I have found those to be helpful on wide, short frames.

 

[csd72]

Truss the end columns to increase the end moments and reduce deflection.

 

[civeng80]

The building will be used to house fertilizer. Welded beams are readily available and these come at about 1200mm deep sections pretty heavy. Im thinking of haunching at the knee and at the ridge of course. The bay spacings I will put at 9.0m centres. Im more concerned about the knee connection. All comments would be greatly appreciated.

Also any good software for designing these particular types of structures?

 

[kslee1000]

If you have access to pre-engineered building supplier's website, you may gain some ideas on the most economical tappered-hunched frame for your task.

In general, I vision a outward taper column (interior straight), and a hunch beam with the largest dimension at the beam-column joint, and gradually reduces depth to the middle. If lateral load is not of great concern, could it be a 3 hinged structure? You may want to throw some numbers to find out the advantages and disadvantages vs a rigid frame.

The hunch area would consist stiffeners and huge web panels. Pay attention to buckling and warping. You can model the hunch using finite element program to make sure the stresses are confortably within allowable. I am not aware of any commericial program can handle this type of analysis. Try to see can you get some advices from pre-engineered manufacturer, they deal with this type of construction in a daily basis, but not to this size.

 

[nicam]

no problem! I've designed 55m portals with standard welded beam sections.
(building is still standing after 1 year!!)

 

[civeng80]

kslee thanks

What would be the advantage of a 3 hinged portal ? And what would the ridge connection be like as a pin?

nicam thanks !

Did you design plastic or Limit state design? Also did you exceed deflection limits?

 

[kslee1000]

By provide a hinge in the mid span, it becomes a static determinate frame, which can be calculated by hand. I have tables to minimize your trouble in determining reactions for varies types of loading conditions, I will upload on monday.

 

[civeng80]

Again thanks kslee

Is this the only reason for having 3 hinges? (i.e. to make the structure statically determinate) Also wouldn't the weight be greater with 3 hinges than the normal 2 hinges making it uneconomical?

Also how would the hinge be designed at the ridge? And wouldn't this cause erection problems?

Just a few quick problems I can think of.

 

[csd72]

I have never designed a 3 hinged arch but my engineering judgement tells me it would not be a good idea in this case.

Moments at the eaves would be much greater and deflection at mid span will be much greater. As these two are probably your critical constraints I would not think it wise to increase them.

50m is definitely achievable with a portal frame, 70m is pushing it.

Another option is to truss out the rafter making it a much stiffer element and therefore vastly reducing the column size(and therefore useable area).

 

[hokie66]

I concur with csd72's advice. The main reason in the past for using a 3 hinged portal or arch was to make it determinate, but with powerful computing now available, that is unnecessary. A trussed column and rafter portal would likely be more economical, certainly less tonnage. One thing you should investigate though, before going too much further, is the potential for corrosion in the environment of a fertilizer storage building. If corrosion is an issue, trusses may not be as attractive.

 

[nicam]

Limit state.

Deflection? can't remember but I'm pretty sure We didn't exceed the limits!
My boss did the original design, he asked me to verify the design.
I think the portals were less then 9m c/c

I'll check the model again and come back to you.

 

[kslee1000]

civeng80:

I am not pitching for 3 hinged frame. Actually me too, never done the design before, but a few homeworks at colledge days.

It may have some advantage over the weight issue (similar to the reason the pre-fabricated building provides), however, I concur the labor cost (for fabrication) could out weigh the saving in materials, it depends on the region the building to be assembled. I do not anticipate much more problems with hinge design, vertical deflection (roof in tapered arch form), and erection, than any other forms of framing. I kind of think the design of foundation could be made a little easier, because now you are clearly design hinges to take the loads, no other complications (full restrain, partial restrain).

A hinge responses to external applied loads by forms of internal vertical & horizontal reactive forces, rotation is resolved into a pair of shear forces. The simplicity alone was the reason it popped up my mind when I saw this thread.

 

[civeng80]

Yes corrosion is a big problem here. Thats why a trussed portal would not be the first choice. Also purlins are generally out of laminated timber. I feel that the knee connection will require a bit of thought and stiffening more than necessary here. Deflection criteria may be laxed a little I would imagine although I think a big issue is the loading of the fertilizer using front end loaders against the side concrete panels. The panels would be pounded on and the load would be transfered to the steel structure.

 

[hokie66]

civeng80,

Corrosion being an important issue, I would rule out the trussed sections. Look at making a welded knee joint with a tapered haunch on the rafter. That brings more of the moment into the knee. Make your bolted splice a reasonable distance out for transport, say 3000. I assume the wall panels are inside the columns to give a flush surface. I don't know how much fixity you will aim for at the bottom, but a pair of bored piers with a capping beam is a good option for resisting substantial base moment.

 

[kslee1000]

civeng80:

Here is the 3 hinged frame table for your info.