Portal frame design

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Nakajo

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I'm building a 6.5m x 4.5m x 3.2m high summerhouse/garden shed. It has a steel structure comprised of two portal frames and some cross members. All the steels are mitred 152 x 89 UB 16s, off of 200 x 200 x 12mm thk base plates.

I want to avoid the use of apex/knee braces and instead provide apex/haunch brackets. Is there a rule-of-thumb for sizing/designing these elements?
 
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Couple of rules of thumb for haunch brackets are:
Length of haunch = 10 % of span
Depth of haunch = same depth as rafter

However, for a 3.2m high building with frames at 4.5m centres, I would have expected larger sections than 152 x 89 UBs.
 
Thanks for your input; I'm happy with the beam section sizes - but unless I miss my terminology, you're talking about 'braces' - the precise thing I want to avoid (if at all possible)?!?
 

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Najako,

He is talking about Haunch Brackets, being the solid sections fully connected along their length to the main structure. Haunch Braces are, like braces in general, only connected to the main structure at their ends.

His rules of thumb are correct, although obviously you need the connections fully designed later.
 
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The terminology seems to differ from site to site. My intention is to provide the structure as per the sketch previously attached.
 
Nakajo said:
The terminology seems to differ from site to site. My intention is to provide the structure as per the sketch previously attached.
Click to expand...
But with the addition of haunches? It's possible (although not likely) that you won't need any additional haunches. Usually haunches are cut directly from stub sections of the main sections being used for the portal frame, so your haunches would be cut from a 152x89 UB.
 
I only want the elements shown in the sketch, so apologies if my terminology is wrong. The intention is to fabricate the members offsite but weld them together on a jig on site
 
Nakajo said:
I only want the elements shown in the sketch, so apologies if my terminology is wrong. The intention is to fabricate the members offsite but weld them together on a jig on site
Click to expand...
Right, I'm with you, I think...

Is there a reason you would prefer to site weld? I would fabricate off site and put splice connections somewhere near the points of contraflexure - which would be somewhere around half way along the cranked members.
Then you can fully butt weld the cranks to the correct angles and dimensions in the workshop so it should bolt together on site pretty easily. Personally I think site welded structural connections are best avoided unless there is no alternative.

Just so that I fully understand your proposal, you propose to weld stubs (shaded) at the correct angles and weld them to the beams on site?
 
It's just the difficulty of bringing large elements on to site - but the splicing idea is a good one. And yes, I propose to weld stubs (shaded) at the correct angles and weld them to the beams on site.
 
Nakajo said:
It's just the difficulty of bringing large elements on to site - but the splicing idea is a good one. And yes, I propose to weld stubs (shaded) at the correct angles and weld them to the beams on site.
Click to expand...
If you do choose to do as you propose, it's really a case of designing the welds to deal with the forces involved. If you do a full penetration butt weld then they are as strong as the parent material so position of the weld is irrelevant, but the quality of the weld is a big issue.
I get the feeling you will have trouble welding this on site. You will either have to have positional (e.g. vertical or overhead) welds, or you will spend a lot of time flipping large sections so that they can be welded downhand.

You could, of course, use bolted endplate connections to the stubs if you can get them designed to work.
 
Engineer-in-Chief Raygun is right - you try and arrange site connections to be via bolts, with welding done in the shop.
Site welding is prone to quality problems.

You say you're happy with the 152x89 - has this been checked? If there are to be any bolted connections via welded end plates,
a deeper beam might be more practical.
 
Yes, the engineer has confirmed the sizes - but he's doing it as a freebie, so I'm reluctant to constantly pester him to plug the gaps in my knowledge - that's what you're here for ;)
 
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