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.

 

[hokie66]

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.

 

[hokie66]

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.

 

[whken]

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.

 

[hokie66]

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.

 

[whken]

ok. Thank you very much.

 

[hokie66]

You are welcome.

 

[csd72]

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.

 

[whken]

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.

 

[hokie66]

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!

 

[hokie66]

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.

 

[whken]

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.

 

[whken]

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.

 

[hokie66]

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.

 

[csd72]

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.

 

[whken]

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.

 

[kieran1]

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

 

[whken]

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.

 

[csd72]

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.

 

[whken]

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?