Connection for UB sitting on top of UC

[whken]

I have designed for the 406 178 54 UB to sit on top of the 203 203 68 UC. But the problem is that I searched the for similar type of connections but was unable to find any. Therefore I am not sure if the design is adequate.
I have checked for the capacity of the bolt and plate and everything seems ok.
I want the connection type to be moment and shear connection.
For the load case, the highest moment = 100kN.m, shear = 54kN and axial load = 1kn.

I am looking for some opinion regarding the connection if it is workable or not.

 

[csd72]

1) 5m unless there are perpendicular members that provide restraint to the compression chord.
2)Use a vertical member to act as a cantilever brace for the top chord, ensure that it has sufficient strength and stiffness to take the restraint force in addition to any truss forces.
3) I would need to see a sketch for this one.

Also, it is common for this type of thing to avoid the complications of a middle chord by simply designing a truss between the two top chords to take the full load and then hanging the bottom chord from this.

 

[hokie66]

I will reluctantly pitch in here again, only because of concern with safety of the public.

How do you have a 5 metre span fascia truss with a square 8.54 metre roof which is centrally supported? I would assume the beams cantilever to pick up the fascia truss, not the other way around. If you can provide a roof framing plan, maybe this whole thing will make a bit more sense. An idea of the roof drainage scheme would also help.

 

[whken]

Thx csd72, I will bear in mind your advice the next time when I design for the trusses.

I have attached the layout of my trusses and the detailings. As for the vertical member, I used a bigger size truss (same as top and bottom chord) + a 12mm thick plate.

 

[whken]

Sorry for double post. I cant find the way to upload 2 files in the same post.

hokie66,

thx for your concern. I really appreciate it as well. I keep revising my design due to safety concerns as well.

My design includes 1 main canopy that is attached to the building where the canopy size is 18.54m x 26.27m and the single column canopy. Initially, my main issue was regarding the single column canopy's detailing.

 

[hokie66]

whken,
You have moved on from the single column canopy to another one which I don't understand either, but I would like to return to the single column canopy.

My simple question to you is: how are the corners of the canopy supported? If the corners rely on the secondary beams being balanced and cantilevering from the primary beam, then you have big problems with torsion in the primary beam. The 400UB section is of little use in torsion.

 

[whken]

The corners are cantilever fascia trusses supported by the primary beams. I will try to read more into torsion design.

Currently I'm thinking of using 20mm diameter galv. bracings to brace the corners and rearranging the purlins so that they are in line with the diagonal trusses. Then they will be bolted to a MS plate that is welded to the middle of the diagonal trusses. As for the remaining sides of the trusses that is not braced, I'm thinking of using tie rods from the purlins to brace the diagonal chords as well.

 

[hokie66]

Your terminology is confusing me. What do you mean by "diagonal trusses" and "diagonal chords"? As before, a framing plan would help me to understand your situation.

If the roof is a series of balanced cantilevers about the primary beam, then any unbalanced loading will cause torsion in the primary beam, and a UB section does not cope with torsion.

 

[whken]

hokie66,

Sorry. What I really meant was the vertical members of the truss. My mentor asked me to first try rotating the top and bottom chords so that the members strong axis (rx) is resisting the sidewards force. This way, the slenderness of the compression members will be reduced and only critical members that failed will have to be provided with additional bracing.

As for torsion,
maximum torsion for primary beam = 0.01kNm
maximum torsion for the fascia trusses = 0.253 kNm.

One of my senior's engineer advice is that since the torsion value is small and does not exceed the maximum moment value, therefore it is ok. I'm still trying to find a guide book for checking torsion according to the British Standard. Any advice will be greatly appreciated. Thx.

 

[hokie66]

With such a small amount of torsion, I must be misunderstanding your framing scheme. The only way I can see that torsion would not be an issue is if the primary beams and column are situated diagonally, so that the fascias are supported at the corners.

 

[whken]

hokie66, I have uploaded the details for the roof. However, this is the old details and I have not yet rotated the top and bottom chord of the truss.

Currently I'm still confused regarding the effective length that I should take. I understand that for Lex, I can take the effective length as restraint to restraint which is between the vertical truss members ~ 1m. As for Ley (sidewards), I take the effective length as 4.27m due to the point of restraint being my primary beam. Is this correct?

However, as you can see in my attachments, the primary beams only restraints the bottom chords. The issue is that the top chord is in compression and I'm not sure if the vertical member connecting the top and bottom chord counts as a brace or not.

In addition to that, I assume one end of the compression member as restrained torsionally and laterally. For the other end, I assume that it as free and unrestrained even though it is connected perpendicularly to the other cantilever members of the truss. Therefore I take the effective length as 1.0L. Is this way correct?

I read about some information regarding the trusses. Is my case similar to pony truss bridge design? It seems that the more I read, the more questions I have unanswered. =p

 

[hokie66]

I won't try to answer your questions about the fascia trusses, because you have a more fundamental problem with your design. As I thought, the fascias are orthogonal with the main members. Therefore, unbalanced loading must twist the main members, and as I said before, UB members are no good in resisting torsion. You seemed to realize this in your earlier comments, as you stated that the wind can be upwards on one side and downwards on the other.

 

[hokie66]

I will try another approach. Your main problem with the single column supported roof is instability, not strength. It would be stable with balanced loading, but unbalanced loading will tend to tip the outer edges. I know this is your first job, and you are being diligent if a bit hard-headed, but you have to get help from your mentor or someone in your office who has experience in concept design.

 

[whken]

hokie66, thanks for your concern. I did look for help from my mentor as well as all other senior engineers as well. There are many factors affecting my design such as material availability and previous similar designs. This is because the boss said that previous design can adopt smaller members, then there is no reason for me to increase the size since it will be overdesigned. The client will choose the design from a few consultants hence overdesign needs to be avoided.

I checked the wind speed and other loading factors. My loadings is smaller than the previous designs.

The capacity of the members are adequate. The only failures are deflections of the structure which reaches 125mm in serviceability limits check. My mentor said it is not preferable but acceptable when there is no other choice.

Perhaps the design in my country is a bit different because we don't have snow. Hence we only check for wind loads.

The concern about instability comes only from myself because I can't find similar design from books. Hence I wanted to know more regarding how to check even after my mentor said the design was adequate. I thought if I can find out more and prove my concern about instability, then I can discuss with my boss and change the previous design. However, I checked with my contractor friends and it seems that they all use the same standard size and connection. Hence I can't really say to my boss that my single column canopy is unstable and I need to further increase my member sizes compared to standard designs.

 

[hokie66]

I am not saying the column or the main beams are unstable. There may be strength issues, but not stability. It is the fascia trusses which support the purlins that are unstable under unbalanced wind loading. You will have a see-saw condition.

Amazing that you are going this far with a design which is in competition with others, but that probably explains the cavalier attitude of your mentor in assigning you this task as your first job.

 

[whken]

I see. Then I should provide bracing to the end corners.

My issue with the truss is due to the comparison with previous design. That the designer in charge of the previous design already left. And he designed the top and bottom chord as 80 x 40 x 5 RHS. Hence my boss jumped up when I showed him my design that requires 100 x 100 x 5 SHS for top and bottom chord in order for the slenderness ratio to pass. However, I made a mistake by taking the length as support to support for my restraint. When I changed the length of the truss to be joint to joint, 80 x 40 x 5 RHS member is sufficient.

After discussing with my other senior engineer, he told me that it is correct but only for the vertical axis. As for horizontal axis, he thinks that the length taken should be between support and support. Then my problem comes back again. The member now faces slenderness issue again. In addition to that, he said only my bottom chord is braced at support while my top chord (compression) is unbraced. He said it is similar to U-bridge and asked me to try find out more.