×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Connection for UB sitting on top of UC

Connection for UB sitting on top of UC

Connection for UB sitting on top of UC

(OP)
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.

RE: Connection for UB sitting on top of UC

I assume the moment, shear, and axial load given are for the 410UB, not the 200UC.  The sizes are not exactly like the Australian sections I am used to, but that load case is no problem for the 410UB.

What is the purpose of the second plate between the column cap plate and the bottom beam flange?  I think it is superfluous.  Also, the bottom flange bolts are not shown correctly in Section 2-2.

RE: Connection for UB sitting on top of UC

I should have added...you should be asking for your mentor to review this connection.  While it is simple, we are not aware of your location, applicable codes, grade of steel, etc.

RE: Connection for UB sitting on top of UC

(OP)
Thank you. The load is for the 410UB. It is designed in accordance to BS5950. My mentor told me that for the two plates, one is for the bottom beam flange and the other for the column cap plate. I was just wondering about this connection type because I can't seem to find any similar detailing for it on the internet.

RE: Connection for UB sitting on top of UC

Don't depend on the internet for this type advice.  The beam connection is a typical moment connection.

There is no reason to use two plates.  The column cap plate can serve as your bottom flange splice plate.

RE: Connection for UB sitting on top of UC

(OP)
ok. Thank you very much.

RE: Connection for UB sitting on top of UC

You are welcome.

RE: Connection for UB sitting on top of UC

whken,

A few comments on this.

agree with the others that the column cap plate could be used as the splice plate thus negating the need for the additional plate. This also provides a platform that the beams can be supported and fixed to while the remaining parts of the connection are being assembled.

This also assumes that you have analysed the column as a fixed connection.

I have a few particular concerns regarding this connection detail.

Firstly, if your column is modelled as fixed then you will have positive and negative forces in line with the column flanges which may or may not require stiffeners. As you have not provided the column moment e.t.c then I cannot tell if these are likely to be necessary.

As the splice plates are bolted to the cross beam then you also need to satisfy yourself that this is not going to induce torsion into the cross beam.

 

RE: Connection for UB sitting on top of UC

(OP)
csd72,

This is a design for single column gas station canopy which is 8.54m x 8.54m. Hence I have to design everything as fixed including at the support. Therefore there is moment in the column.

The highest forces for the column at the support is Fz = 105kN, Fx = 102kN and My = 54kNm.

I have designed for a 400mm x 400mm x 25mm S275 base plate with 4M20 G8.8 bolts by using Prokon.

RE: Connection for UB sitting on top of UC

With that information, all my comments above are withdrawn.  Why didn't your define your problem in the first instance?  When you said the axial load in the beam was 1 kN, I assumed this was a gravity only column.  I think your whole structure has problems if it is balanced about one 200UB column, but you will have to get someone else to help!  

RE: Connection for UB sitting on top of UC

Just one more blast...how can you have 105kN force at the base of a cantilevered column, and only a moment of 54kNm?  Is your canopy only 0.5 metre high?  Get help!!  You have no idea what you are doing.

RE: Connection for UB sitting on top of UC

(OP)
Sorry. I didn't address my problem properly. My issue was the balance of roofing structure and if this connection is capable of supporting it. I checked the capacity and other details and its all good except for deflections due to wind load.

My worry lies on whether the connection is good enough or not. I'm hoping that the balance will be supported by the 15mm plate since the shear capacity of the plate is bigger than the force from the beam's moment.

RE: Connection for UB sitting on top of UC

(OP)
The height is for the column is 5m and it is only supporting the canopy. The moment force mainly comes from the wind load affecting the uplift of the roof. The uplift is critical because it is upwards on one side and downwards on the other side.

RE: Connection for UB sitting on top of UC

You said Fz and Fx are both greater than 100kN.  I am not sure of your x, y, z coordinates, but assume at least one of those is the horizontal reaction at the base.  If so, your column moment is at least 100 x 5 = 500 kNm, and there is no way you can resist that with a 200UB, even bent about the strong axis, and you will have roughly the same moment about the weak axis.  Your basic understanding of how structures work is lacking, so you need a good mentor if you intend to do structural engineering work.
 

RE: Connection for UB sitting on top of UC

whken,

I agree with others above in that there is something that does not make sense here. Can you try and explain yourself more clearly.

I think maybe you are trying to combine several load cases and take the worst reaction from each but I am not sure. If this is the case then you really need to consider all load combinations and not just one composite one as it is not always possible to see what is going to be critical..

I am not confident you understand my questions regarding web buckling and induced torsion.

Is there also a secondary moment in the minor axis of the column? the fact that you have shear in two directions would indicate this. Anyhow you should be allowing for a minimum eccentricity.

Can you please provide more information on:

The structural system that this is part of
Where the loads are coming from (wind e.t.c.)
any other pertinent information.

 

RE: Connection for UB sitting on top of UC

(OP)
hokie66,

Sorry. This is my first project. Therefore there is still a lot of things that I am not sure.

I got the reactions from running S-Frame.


csd72,

Yes. You are right. I did take the values from several load cases because I used 26 load combinations and I don't want to confuse ppl by putting all of it here. But it seems that I got the opposite effect. Sorry.

There is only 1 force for for the axial reaction at the support which is either on the x-axis or y-axis depending on my wind load combinations. Fz is the downward force. Therefore there is no secondary moment in the column.

For the web buckling, I did not check because according to BS5950 Cl4.4.4.1, my d/t ratio = 14.25 which is smaller than 62E therefore it is assumed to be not susceptible to shear buckling. For the moment capacity, I take Mc as 275N/mm2 x 456000mm3 (Weak axis) = 121kNm which is more than max moment of 100kNm. Therefore i assumed that it is adequate.

As for induced torsion, do you mean lateral torsional buckling? This is because the code check from S-Steel tells me that it is adequate.

Sorry if I misunderstand your questions again.

Thx for all the help from you guys. I really appreciate it. My senior engineer told me that it is adequate but I just want to be very sure.

RE: Connection for UB sitting on top of UC

Don't rely on computer analysis for your first project without checking the calculations by hand. Also Moment capacity is PySyy for weak axis. Which i would estimate as 102KNm for 203x203UC71 when you check overall buckling the section will fail. Not to mention deflections or stability in the weak axis.
typically these canopies use square hollow section columns for stability in both directions.

Get help from your mentor.

Kieran
 

RE: Connection for UB sitting on top of UC

(OP)
kieran1,

Thanks for your advice. Initially I proposed 300x300x8 SHS for my boss however it was rejected due to material availability issue. Hence I followed the standard size given by the gas station retailer which is 203x203x86UC which gives a moment capacity of 121kNm.

My main concern lies on the connection part regarding the balance of the beams sitting on top of the UC. I have checked for the capacity of the plate and everything seems ok.

I have also looked for help from contractors doing similar jobs and they told me that the beam usually sits on top of the column. This is because normal design is tollgate design where the beam and column orientation is the same. However in some cases, the architect will specify for the column to be rotated 30 degrees while maintaining the same beam orientation. Therefore unless if the column is a CHS, the beam must sit on top of the column.

Thanks for all your help.

RE: Connection for UB sitting on top of UC

whken,

There are a LOT of unanswered questions here and it is great cause for concern.

firstly Reagarding your response to my email:

I agree that summarising load cases can be difficult but giving the worst case for each load can actually be less critical than having a smaller load on one side and a larger on the other. Usually you can simplify it by thinking of: Maximum dead plus live load/snow, Maximum dL plus out of balance live/snow, worst wind overturning and worst uplift.

What rings alarm bells is the fact that the beams are so much deeper than the column despite the fact that they both should be taking similar moments.

I am not referring to shear buckling but to the buckling at the support bearing which is a different case entirely.

By torsion I mean torsion not lateral torsional buckling, but if you only have 1 column this may not be an issue and by the way is the least of your problems.

A 300x300x8SHS may not be a standard item but I think a 250x250 is, I suggest you check.

By the way, a decent boss will not only be happy to answer any question once but will expect them. We were all graduates once.

 

RE: Connection for UB sitting on top of UC

(OP)
Thx csd72, I will recheck my calculation again. The capacity of a smaller sized beam would be sufficient. However, due to deflection, I increased the size so that I can minimize the deflection even though they still exceed the serviceability limit states.

Currently I am faced with another problem regarding the fascia truss. The fascia truss will be welded to the beams. What I want to ask is regarding the length of the truss that I should take to calculate member slenderness.

Lets say the lattice truss span is full span is 5m and is weld connected to UB at each end. The span between each diagonal & vertical member is 1m. Assume the member have a compression force of 90kn, shear force of 4kn, moment-x of 1kNm and moment-y of 1kNm.

Top & bottom chord = 80 x 40 x 5 RHS
Diagonal and vertical chord = 40 x 40 x 5 SHS

I take the slenderness for the x-axis (downwards force - strong axis) of the beam as Lex/rx where Lex = 1m.

1) For the y-axis (sidewards), should I take the Ley as 5m or 1m? When using 5m, the member will fail in slenderness, compression capacity and overall buckling.

2) The beam will be welded to the bottom and middle chord. Can I assume that the middle chord would be sufficient to brace the top chord? This is because I am not allowed to brace the top chord since the level of the purlin is lower than the top chord and I'm not permitted to have anything exceeding the purlin top level with the exception of flashing.

3) If needed, can I use a 40 x 40 x 5 angle welded to the bottom flange of the rafter beam and weld it to the bottom chord at middle span of the truss to reduce the Ley (for both top and bottom chord, assuming the middlle chord acts as a brace) to 2.5m?

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

(OP)
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.

RE: Connection for UB sitting on top of UC

(OP)
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.

RE: Connection for UB sitting on top of UC

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.  

RE: Connection for UB sitting on top of UC

(OP)
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.

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

(OP)
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.

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

(OP)
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

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

(OP)
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.
 

RE: Connection for UB sitting on top of UC

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.

RE: Connection for UB sitting on top of UC

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources