structural steel connection design 1

[rfd23]

I am working in North America. client/upper management hires a korean detailing company for structural steel connection. Their typical shear connection detail is close to shear tab/ beam splice connection. We can use this type of connection as per AISC-ASD manual if we incorporate eccentricity moment in connections(refer.pg3-123- vol.2).
I will explain the detailas follows:
beam stops at the edge of girder flange,shear plate is welded to full depth of girder like a stiffener(it is on both side of web). Two plates similar to splice connection(both side of webs) make a connection between girder and beam using bolts to fulldepth of beam. (this is 100% similar to splice connection).45 mm is distance between beam bolts and face of girder flange.
koreans are using e=45mm eccentricity (for all beams) and Mcon= R*45 KN-mm. My opinion is that eccentricity should be equal to the 1/2 of flange width of girder (Mconn=.5*gider flange width*R) as beam stops outside of girder flange face.
Now this moment should be resolved into components and victorically added to shear to get max bolt forces.

Please give me your feed back. I would be greatfull to all of you.

 

[hokie66]

BA,

The Kiwis are onto it.

http://news.ninemsn.com.au/world/796175/kiwis-cook-up-plans-to-eat-wallaby

 

[asixth]

And we qualified for the world cup

 

[rfd23]

BA
sorry for late response, actually i was in office and did not have internet access to some web sites.
vertical spacing 60mm in calculation is right. detail is for a typical connection (can be differ beam to beam).
Both girder and beam bolts are in double shear. Distance between girder web and beam bolts is 150+5+40.
hookie66
project is in south Africa.
I would take Mconn=R*195
and then resolve this moment by T/C to bolts
T/c=Mcon/lever arm, lever arm is 90 in this case.
I would design all plates to this moment.please advise.
WE are sending our engineer to Korea to resolve these kind of issues.
Thanks everybody for this usefull converstaion.

 

[hokie66]

rfd23,

I agree. I think we may have misinterpreted your last post. No matter if the Japanese and Koreans use this detail, I still would not allow it on my projects. Not compact enough to suit my sense of structural logic. Good luck with the project. The joys of worldwide commerce. Design in North America, project in South Africa, detailing in Korea, fabrication somewhere. Wow! You would think whatever savings are achieved would soon be consumed in travel, transport, and miscommunication.

 

[BAretired]

I would take Mconn=R*195
and then resolve this moment by T/C to bolts
T/c=Mcon/lever arm, lever arm is 90 in this case.
I would design all plates to this moment.please advise.

You lost me on the third line. Where does the lever arm of 90 mm come into play?

Let us say the factored load R is 100 kN. Then the moment at the three bolt connection to the beam is 19.5 kN-m. The old fashioned way of doing this would be to say that each bolt takes 100/3 = 33.3 kN vertically. The top and bottom bolt take an equal and opposite horizontal reaction of 19500/120 = 162.5 kN. The resultant reaction for each of the top and bottom bolts is 165.9 kN. One bolt in Double shear is deemed okay.

The modern way of doing it is to consider an instantaneous center. It is a total pain in the neck to figure out where the I.C. is, so CSA S-16 list a variety of connection configurations along with variable eccentricities and variable pitch. Unfortunately, the lowest pitch listed is 80 mm so I can't tell you what the shear is with pitch of 60.

For a 3 bolt connection, pitch of 80 and eccentricity of 200, the coefficient C = 0.79 which means that the maximum resultant bolt shear in the group would be R/C = 126.6 kN. The old fashioned way would yield 129.4 kN, so it appears to be conservative.



Best regards,

BA

 

[hokie66]

BA,

The 90 mm is the horizontal distance between bolts. His vertical pitch is actually 70, which by the way is what we use in Australia.

 

[BAretired]

hokie,

In his last post, rfd23 confirmed the vertical spacing of the bolts is 60. I assumed he meant pitch.

If the two lines of bolts are 90 mm apart, then one line is 105 mm from the girder centerline and the other is 195 mm away. For a reaction of 100 kN, the moments are 10.5 and 19.5 kN-m for each bolt group respectively. There is no need to check the closer group if it is identical to the more critical one.

The plates connecting the two lines of bolts must be designed for the higher moment, i.e. 19.5 kN-m.

Best regards,

BA

 

[hokie66]

My mistake on the pitch. I was looking at his original drawing which showed 70. I agree with your calculation.

 

[rfd23]

BA
I am not sure I am doing it right.
As per your calculation it appears I am going in wrong direction. I Still want to present it so you can advise me why I am thinking this way.
I would resolve Mcon 19.5 as a couple between two bolt lines(girder and beam)which would be 19.5/90=216KN. 90mm is horizontal distance between girder and beam bolt lines.
This(216) would be my total additional shear on each side of bolts.
for each bolt it would be 216/3+100/3= 72KN<82 KN
82 KN is double shear capacity of single bolt.
As per your calculation (old fashioned way)165KN bolts can not take this (allowable capacity is 82KN-3/4 inch bolt double shear)
please comment on my way of calculation.
Thanks for both of you for your time and usefull thoughts.

 

[burtonSTR]

I don't think that using eccentricity=45+flang width/2 is a good practice - if the girder is big, let say 3m and the neam is small (W21 or W24).

How to calculation the eccentricity of beam to column connetion?

 

[hokie66]

rfd23,

BA's method is correct. Think of the top and bottom bolts as the flanges which take the bending.

For the bolt strengths, I think you are talking allowable design, while BA's assumed 100 kN is factored. Assuming you are using Grade 8.8 bolts (similar to A325), an M20 bolt would take 185 kN in double shear (Australian standard). Permissible load strength of same bolt was 94 kN.

 

[BAretired]

rfd23,

The shear in the bolts is a matter of statics. Please look at the attached sketch.

In Case 1, the two members overlap. The reactions in the two lines of bolts are R(a + b)/b and Ra/b as shown.

In Case 2, the members do not overlap. Instead, they are connected by a plate which is free to rotate. The three bolts on the left must carry the moment, R*a and the shear, R. The three bolts on the right must carry the moment R(a + b) and the shear, R.

The bolt shears in Case 2 are much higher than in Case 1.

Hokie was correct about the factored load. In Canada, we have been using LSD (Limit States Design) for several years now, so my 100 kN was a factored load. A 3/4" diameter A325 Bolt has a factored shear resistance of 113 kN if threads are excluded from the shear plane and 79 kN if threads are intercepted. In double shear, the values are doubled.

I do not like this connection and would try to persuade the steel detailers to use a different connection which does not depend on a group of three bolts carrying the entire moment at that section. In my opinion, it is not good practice even if it can be made to work.

Best regards,

BA

 

[kslee1000]

cfd23:

Treat the connection as two systems with free body diagram as shown on the attached sketch. Hope it helps.

 

[kslee1000]

Add "sum M = 0".

 

[hokie66]

kslee,

He wants to avoid torsion on the girder, so the connection should be designed for the moment based on the distance from the beam bolts to the girder centreline, as advised above several times. Your method is not appropriate to this connection. It is a poor connection, none of us like it, but the statics work as BA describes.

 

[rfd23]

Thank you everybody for this usefull inputs.
I myself do not like this connection. I prefered double angle connection direct to girder web as we usued in northamerica. But unfortunately during initial stages of this project, this type of connection was oversees and approved by Leed engineer under pressure(by higher mangement) to meet schedule.
WE can not reject this connection at this stage, however I want to make sure that total number of bolts are enough to take additional moment.

 

[kslee1000]

Hokie:

It's not the question of he (rfd) likes it or not, it's his responsibility to accept or reject the connection details. He has some thoughts and doubts on how to look this issue, I was providing my view on how the detail should be analyzed. He needs strong, solid reasons to "REJECT" based on the design code stipulated in the contract document. He has no position to simply say: "I don't like it, do it my way or no way".

 

[kslee1000]

Provides rfd is quite prudent, he, as you, did not like it but with doubts in mind. Please help him to clear his mind and this matter in the simplest form you can.

 

[hokie66]

"Clearing his mind" has been done by BA, thank you. Your supplementary suggestion was unhelpful, as it was incorrect. In your nomenclature, M= R x(d1 + d2).

You don't have to lecture me on his responsibility. I know, and he agrees with me and BA that the detail is less than desirable, but it is out of his hands.