Max Span of Portal Frame Rafter?
Max Span of Portal Frame Rafter?
(OP)
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!
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!






RE: Max Span of Portal Frame Rafter?
Mike McCann
MMC Engineering
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
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.
RE: Max Span of Portal Frame Rafter?
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
:)
RE: Max Span of Portal Frame Rafter?
Ta!
RE: Max Span of Portal Frame Rafter?
Kieran
RE: Max Span of Portal Frame Rafter?
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.
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Max Span of Portal Frame Rafter?
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?
RE: Max Span of Portal Frame Rafter?
check maths in your last post.
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
Everyone has been very helpful. Thanks for correcting my maths tony1851 :)
RE: Max Span of Portal Frame Rafter?
Ta
RE: Max Span of Portal Frame Rafter?
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.
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
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?
RE: Max Span of Portal Frame Rafter?
RE: Max Span of Portal Frame Rafter?
As for frame spacing, this needs to be with consideration of the purlins as stated. A lot of the cost of a portal frame structure is in the purlins. At least here in Australia.
Depending on your loadings (wind/snow), I don't see many frames spaced past 10m. Most around 8m or so. We like to keep the purlins to 250mm deep or below, above this a few other aspects bump up, such as cleats and bolt sizes/grades.
12m is about the accepted maximum span for transport here too. We can certainly do greater, but 12m is a standard truck and helps a lot keeping costs down. A splice just after the haunch is common (either haunch, or a thicker/bigger member).
Good luck!