Recommended Stress in Moment Connection
Recommended Stress in Moment Connection
(OP)
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
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






RE: Recommended Stress in Moment Connection
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.
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
Tata
RE: Recommended Stress in Moment Connection
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
RE: Recommended Stress in Moment Connection
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.
RE: Recommended Stress in Moment Connection
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?
RE: Recommended Stress in Moment Connection
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
RE: Recommended Stress in Moment Connection
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?
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
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
RE: Recommended Stress in Moment Connection
now you have me worried that my analysis software is not checking the column shear properly!!!
RE: Recommended Stress in Moment Connection
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 ...
RE: Recommended Stress in Moment Connection
RE: Recommended Stress in Moment Connection
running a small, simple model with your prescribed method of determining the shear force and I am getting some alarming numbers.
RE: Recommended Stress in Moment Connection
Can you point me to the section of ASD 9 that you are referring to?
RE: Recommended Stress in Moment Connection
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
RE: Recommended Stress in Moment Connection
The picture and information is also in the 2005 Spec Commentary. p16.1-359-360
http://www.FerrellEngineering.com