Max Span of Portal Frame Rafter? 3

[civengzak]

Hello valuable eng-tippers,

I'm to design a 40m width, 115 m long and 10m heigh ( 12.25 to roof pitch)portal frame for a factory . I have experience with standard length portal frames using Eurocodes. So I'd really appreciate if you can help me on...
1- Can I use 20m long rafter? Whats the max allowed length of rafter/beam?
2- Whats the thermal expansion limit for steel structures? For concrete strucutres 30m max. Ambient temperature is 48 C (middle east).
3- Due to the length, would it be okay to give centred bracing in the middle and not at both ends?

Huge thanks in advance!

 

[msquared48]

20 meter or 66 foot long "rafters" seem a bit much. These are normally set more in the range of 20 to 30 feet - 6 to 9 meters.

Mike McCann
MMC Engineering

 

[hokie66]

Depends purely on availability and transport. No point in asking here, as it varies a lot from country to country. Where I am, transporting 18 or 20 metre long length is common, but takes some planning.

 

[civeng80]

20m for transport is asking a bit much.
But you can splice midway, so that problem can be solved.
I would still brace both ends. But if you stiffen the purlin caps or place other members it is possible to have mid bracing.
Thermal expansion - I would put an expansion joint in the roofing. This would serve for tansporting the roof sheets as well as movements due to temperature effects.

 

[civengzak]

Thanks to everyone for your replies.

So I'll design as
1) Two 10m rafters spliced between the column and apex
2) Brace both sides (at begining and end), giving expansion joints for purlins and side rails at 50m

 

[civengzak]

Regarding the spliced connection at 10m, the rafter would have max bending moment and it's not advisable to splice at this point?

Ta!

 

[kieran1]

Better to splice at 6-7m from the stanchion. Need to consider the erection of this structure very carefully. Temp supports will be required.

Kieran

 

[hokie66]

kieran1 is correct in principle, but I would place the rafter splice at about 2.5 to 3 metres from the knee. Fabricate the knee joint in the shop, with the lower part of the rafter and column as one piece. If the rafter to the ridge is still too long for transport, make the ridge splice in the shop, so now you have a total of 5 pieces to make the frame.

The bracing in the side walls could be just one bay at about mid point of each section, so 4 bracing bays in total. You will need a bay of roof bracing on each end to form a truss to distribute the end wall loads to the side wall bracing. If the roof bracing is not adjacent to the end walls, provide struts between the wind columns and the truss.

As civeng80 said, provide a movement joint in the roofing about half way down the slope. Assuming your roofing is a concealed fastener type, this will require a step at the joint, so the purlins will have to step up.

 

[civengzak]

kieran1 and hokie66...thanking you two very much for increasing my knowledge

 

[paddingtongreen]

The only thing I would add to this excellent advice is, because these will be fairly big members, to call for the splices to be shop fitted/shop matched before transport.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[civengzak]

Thanks paddingtongreen.

Is there any restriction for frame spacing?
For 115m long factory I'm providing 15m spacing between frames so that there will be 10 frames along 115m. In practice, spacing taken by Length/10?

 

[tony1851]

@civengzak;
check maths in your last post.

 

[hokie66]

The frame spacing will depend on the purlins and girts used, and also on the relative size of the frame members for the different spacings. You want to use available girt sizes, and also available rolled steel sections for the frames. This will require some preliminary design, with several spacings required. I don't know where you are located, but 15 m spacings sounds a lot, requiring very deep purlins, perhaps not readily available.

 

[civengzak]

hokie66, you're just fantastic. Thanks a lot for sharing your knowledge with us

Everyone has been very helpful. Thanks for correcting my maths tony1851

 

[civengzak]

I've attached the STAAD analysis output image.

Ta

 

[hokie66]

That looks like the gravity loading moment diagram, at maybe your 15 metre spacing. So as you can see, already you have very big member sizes. Reducing the spacing will bring the sizes down to something more manageable. Consider the craneage required to erect the frames, and remember that you will need to erect a pair of frames along with all their bracing before you have a stable structure to go on with. Perhaps halving your spacing to something like 7.5 or 8.0 metres?

Now you have all your other load cases to consider, wind, seismic, in both orthogonal directions. But as you said you already have experience with smaller frames, I assume you know that.

I would be looking to attract more moment to the knee joints by haunching the part of the rafter closest to the column, and probably increasing the column size rather than the rafter.

 

[civengzak]

hokie66, I can't thank you enough for your kind and helpful replies. You're a star

 

[hokie66]

You are welcome. Good luck with your project.

 

[civengzak]

Cheers!

As it's not gonna be an economical design, I liaised with the architect and managed to give a column in the middle, at 20m.
Just wondering if you have any experience working on RAM Connection SS3 for moment connections?

 

[hokie66]

I don't. But others here do. Perhaps another thread with your RAM questions might be better.