Partical Moment Connection 5

[haynewp]

I have modeled a basic steel moment frame. The fabricator has shown a bolted angle at top and btm of the beam. I figure this to be about 60% of fixed. I cannot model partial moment connections in the software I have.

I am trying to determine the negative effects this would have on my frame, since it was modeled at 100% fixed. What comes to mind is increased deflection and p-delta effects.

What is the best way to figure the effects for partial fixity connections in frames?

 

[Ron]

Depending on configuration and stiffness ratio of beam-to-column, you can expect the fixed condition to yield a moment at the connnection of somewhere between wl^2/12 and wl^2/15 (possibly a bit lower, but not much) for reasonable span ratios. The variation between these values and a 60 percent value will have the effects you mentioned, but if you bracket those in hand calcs, you might find they are insignificant. The effect will be greater in the member with lower stiffness.

 

[flamby]

Normally we bother about end-moments in moment frames. With partially released moments at ends, the mid-span moments will go up and sections will need a check at mid-span too.

 

[haynewp]

What about for estimating the decrease in joint stiffness for the lateral design of the frame. I would imagine that some programs allow internal springs that would help account for partial fixity, but I do not have this available.

I am concerned about the increase in story drift I will now have.

 

[JAE]

If your software doesn't directly allow for partial restraint connections, the only other "trick" you can do is set a joint a small distance from the beam/column intersection joint (perhaps 6" away) and for the small 6" beam length, enter a smaller beam with lower Ix.

How much lower Ix is the tough part. You could do some hand calculations to get an estimate on the moment/rotation of the joint and somehow equate that to a flexural stiffness in the short section.

In any case, the less-stiff mini-beam would at least approach the exact solution and create a frame with less stiffness than your original fixed frame.

 

[haynewp]

JAE, I am using RISA 3D, are there any other tricks you know of for modeling PR connections in this program?

 

[Taro]

FEMA 356 Section 5.5.3 provides a simplified method for adjusting frame stiffness to account for partially restrained connections.

For bare steel PR connections:

Rotational stiffness, K = MCE/0.005

Where MCE is the expected moment strength of the connection.

Then, the adjusted stiffness of the beam is:

EIadjusted = 1/(6h/(L^2*K)+1/(EI))

See FEMA 356 for more info.

 

[Duchas]

When you consider that a beam is connected to a column, and that the column is not entirely rigid itself, then no matter how stiff the connection, the beam end will never be entirely 'fixed'.
If you accept that, you may consider that your beam end connection provides say 60% of full fixed end moment(FEM) for the beam; Is the applied moment (at the beam end) as determined by your analysis, greater or less than the 60% (say)of FEM?? If the applied moment is less than the 60%, then as far as that beam is concerned in that structure, its end is fixed.
Hope these are helpful thoughts.

 

[haynewp]

So I need to check that the moment capacity of the CONNECTION (at 60% of fixed) is actually greater than the moment that is required (if was completely rigid).

Then go to reanalyzing the members for the increased deflection and p-delta. Is this correct?

 

[haynewp]

In other words, 60% of the capacity of the connection has got to be greater than the 100% fixed moment required.
(Sorry I'm new to PR connections.)

 

[ChipB]

JAE's suggestion will work. Since you designed it as fully fixed, I would choose a beam for the short section that had a moment capacity of the 60%. I would also use a really short section, say 6", to imitate the connection. (oh, crap!!! Did I just say 6" was really short?? DAmmit, I've judged myself again!). The PIA of it is splitting each of those beams to meet your condition. To be accurate, you'd have to apply it to all of those beams, not just the worst one. It has to redistribute the stiffness matrix taking all those connections into account. I think the end restraints stay with the member too, so if your fixed on one end, and pinned on the other, once you divide your member, the original member number piece now has the same restraint and will need to be modified back to an accurate end condition.

You can always ask the fabricator to give you the connection calculations for that connection and verify it's adequecy.

However, as much as you hate to do it, if it is not your design intent.....REVISE AND RESUBMIT

 

[haynewp]

So in conclusion,

Theoretically I can place small beam segments at the ends of the actual frame beam. These segments will have about 60% of the actual beam's Inertia. Running the frame with lateral loads, I should get pretty accurate results regarding drift and p-delta etc. I will also have a redistribution of the gravity load moments due to the less stiff ends.

Next, I need to check the actual moment at the ends of the beams due to lateral and or gravity loads. This moment should be less than or equal to 60% of the actual beam capacity, and also less than 60% of the column capacity.

Finally, I will need to check that all AISC seismic provision requirements for detailing and design are also met (if I use an R>3 (IBC 2000)).

Does anyone disagree with this procedure.

 

[JAE]

Sounds good to me...but you also need to check the connection itself for the moment and shear that you get from your RISA analysis.

Like the saying goes...a building is not a bunch of beams and columns connected together at their joints. It is, rather, a bunch of connections that happen to be linked together with beams and columns.

 

[ronster]

Why not tell the fabricator to use a fully fixed end connection from the AIAC manual? You should not revise your design to accomondate a detail you feel is incorrect.

 

[haynewp]

This is his idea for reducing the cost of the building because of the owner complaints. There are many of these moment connections in the building.

Squeaky wheel gets you greasing.

 

[redhead]

Haynewp:
The questions everybody seems to avoid with PR connections are the additional story drift and the correct "k" factor. The standard nomograph only applies to fully fixed connections.
Don't give up stability for some cost savings.

 

[ajose]

haynewp,

The effect on the beams.
For rough stimating if you have a simple supported beam the moment in the mid of the span is (W*L^2)/8 and a bem fixed both end is (W*L^2)/10.
That's mean if you try to save on the end connection you have to increase for stresses or deflections (not shown).
The economy increase (small requeriments) if there is some continuity, the maximun economy is found arroud four or five continuos span, more span have no impact on the size of the beam.

The effect on columns.
Like redhead state, you are changing the slenderness factor of the column k, the column is more slender for that have less allowance stresses at the end.

The global behavior.
The columns fixed at ground and the beam simple supported at the columns. You have the more expensive structure, the stablity, deflection is just column responsability. The beam act like they are and have no economy.

The columns fixed at ground and the beams fixed to the columns. Is the most economical, taking advantage of the continuity for the beams. Unloading the columns ang giving some responsability for delection and stresses to beams. Your structure is located in this place.

Changing the restrains, and you go to an expensier structure more than the save for the changes. The posibility of reinforce the beam, or increase the size of the columns for the lateral movement control. The worse could be the change of member size (if the material if already bought).

Go for the safest way make an analisys with the changes, i normally use STAAD III 22.3WM this have an option for member end release partial moments.

 

[ronster]

Usually the most economical structure is with pinned connections and shear walls or x bracing to resist lateral loads. If your building is designed with fully fixed moment connections to resist lateral loads then you should stick with fully fixed connections. There is no easy way to justify using a partial moment connection to save money. If you do then you have to add something to increase the stiffness of the structure unless you just used fully fixed connections all over the place regardless if they were needed for lateral syupport.

 

[haynewp]

I talked to engineer at the fabricator today. His suggestion is based on that he feels the time and connections involved for full moment will be more costly than the extra steel for the partial connections.
I've never seen a cost analysis breakdown per connection, seems like a lot of variables could be involved, but I have asked to see one from him. I cannot have x braces in this case nor do I have shear walls to use.

 

[steve111]

If you've designed full momemt connections and the fabricator/supplier of the steel has bid on the work based on that then thats what shoud be supplied. period.... (bunch of them). changinging your analysis requires redesign all the way down to the bottom of the foundation. Connections are everything in structural modelling. Perhaps the sub-trade is willing to state the amount of money he would be willing to give back to your client for his/her brilliance (including redesign cost). Never sacrifice structural design for $$$$$. Their motive and intent is quite different than yours and besides at the end of the day they'll just say you reviewed it and approved it. Picking the right structural system at the front end of the design is an art. Battling with a low ball supplier who's now trying to save their companies bacon is a drag. Talk money and they usually go away.