Recommended Stress in Moment Connection

[sybie99]

Hi Guys

When designing a steel beam with end moments, ie fixed rigidly to a column for instance, is there a rule of thumb as to the capacity usage of the beam to make the connection possible?

I know that if your member is stressed to say 0.98 or 98% of capacity, making the connection work is not easy. When I say connection I am referring to bolted end plate, or extended endplate connection, not welded.

Is it better to only stress members to say 80% of capacity?

Was just wondering if anyone can inform me on what their rule of thumb is and what problems occur in the bolted connection design if it has to transfer 100% of the member capacity stresses.

Thanks

 

[asixth]

Is the column-beam forming part of the lateral force resisting system? If so, the drift limits will be governed by the size of your column and to a lesser degree the stiffness of the beam.

If I had the option between increasing steel member sizes or increasing end-plate thickness I would increase the end-plate thickness and design the connection for 100% member capacity.

Fabrication costs will be similar, the detail still requires a welded end-plate and tensioned bolts. In my part of the world hot rolled steel goes for AU$2000/tonne (US$1800/t). Economically it would be better to increase the size of the connection.

 

[rb1957]

you design the joint so that capacity < load ... sometimes the fasteners are critical, sometimes the elements being joined together

 

[rb1957]

dammit ... spot the obvoius mistake !!

 

[slickdeals]

Sometimes I wish eng-tips would have the option to edit your own post much like many other forums do. I am not sure why it is not so.

 

[corus]

I would have thought you'd use a structural design code for assessing your beam rather than a rule of thumb, fingers crossed kinda method. There are specific codes for the UK, USA, and many other countries.

Tata

 

[connectegr]

sybie99
Bravo for considering the connection design.

The answer will be relative to the actual beam size, ie the flange forces to be considered. For small beams, with flanges less than 3/8" thick, full-moment is not difficult in terms of the endplate design. And may even be achieved with fillet welded flanges. But the column size may be an issue, thin column flanges and webs may require reinforcement. Doubler plates and stiffener plates can get expensive.

For heavier sections, relative to their depth, the flange forces can get very large. The tension per bolt will be very large. Therefore the plate thickness increases and the yield line of the column may require multiple levels of stiffeners. Stiffeners between bolt lines add the additional cost of difficult installation with limited access for welding.

Therefore, decreasing the moment percentage by increasing member weight will do little to help the connection design. But, if increasing the member's depth is an option the decreased percent moment will be beneficial. I agree with your comment or "rule of thumb" 80-85% of the moment capacity.

If this is a moment frame design. Provide the connection engineer with the actual lateral and gravity moments. This will provide a more accurate design of the column shear forces and reinforcement if necessary.

In the US the fabrication labor costs will nearly always out weigh the additional material cost.

http://www.FerrellEngineering.com

 

[CTSeng]

I agree, beam end moments should never be that high when drift commonly controls.

Also, providing good end reactions is the only economical option. I've seen too many projects recently where the EOR requirements are simply CJP flanges. Had one recently where they specified 'full beam moment' capacity while their detail showed CJP flanges with a clip connection in the web. Assuming full flange capacity generally only provides 75% to 80% of full beam moment but for most typical building moment frames I've been invloved in the moments are still below this range.

 

[ToadJones]

I am little bit lost here....
Why not design the connection(s) on the basis of the moments generated during an analysis; whether they be from gravity or lateral loads or from drift/deflection limits?

 

[connectegr]

ToadJones
As far as the beam connection is concerned the lateral vs gravity is irrelevant. But for column shear the additive lateral moments are necessary to check the column. All moments are not equal and opposite at the column. That is one of the biggest mistakes I see in connection design software.

see attached

http://www.FerrellEngineering.com

 

[ToadJones]

Connect-
I agree with what you are saying as I too have had similar concerns with Transfer Forces being left out of connection design.
I guess my question was really regarding the original poster's suggestion of using percentages of beam capacity for the connection design.
This approach could be very conservative/expensive, no?

 

[ToadJones]

Care to explain what the moments in your sketch are from?

 

[ToadJones]

ok, never mind, I am assuming that MG = moment due to gravity and ML = moment due to lateral.

 

[connectegr]

Lateral moments due to wind forces applied to the frame. But as fixed connections the beams also have gravity moments. The gravity moments are opposing forces at the columns. Opposing tension forces at the top flange and opposing compression forces at the bottom flange. But lateral forces may be additive across the column. A similar detail is shown in the 9th Edition AISC.

But, this break down is rarely given to the fabricator or connection engineer. General instructions "design for full moment capacity of beam", is ridiculously conservative since the beam size is rarely determined by the moment. If the column is design for full moment 100% additive across the column web, a doubler is nearly always required. We have developed programs that estimate the percentage gravity moment based on the given shear forces. But, this is still conservative and costly.

http://www.FerrellEngineering.com

 

[ToadJones]

connect-
now you have me worried that my analysis software is not checking the column shear properly!!!

 

[rb1957]

why would wind (nice aliteration, eh) loads act in the same direction as gravity loads ?

the moment at the support due to a lateral (horizontal) load on the span (ie wind) would act in a different direction compared to the moment due to vertical loads ...

 

[ToadJones]

moments at the end of the beam due to lateral can either be in the same direction or opposite the moments due to gravity.

 

[ToadJones]

Connect-
running a small, simple model with your prescribed method of determining the shear force and I am getting some alarming numbers.

 

[ToadJones]

connect-
Can you point me to the section of ASD 9 that you are referring to?

 

[connectegr]

9th Edition reference attached

My software comment above, was not specifically related to structural analysis. If the the user of a connection design program does not recognize the difference in the moment connections. (Moment (wind) frames vs. (gravity) cantilevers, or the combinations) Then the force inputs will be incomplete or incorrect. If a moment frame, connection design, is input as only full moments (with same beam size and elevation) all of the column shears will be zero, incorrectly. All of the limit states will check and the connection will "pass". Even the column shear will pass in the software analysis. But the force is not zero!

While the EOR's instruction of develop the full moment capacity is very conservative, the connection detail could be incorrect and inadequate.

http://www.FerrellEngineering.com