AISC Seismic Provisions for OMF

[JAE]

OK all you seismic guru's out there....I need some help with a provision in the AISC Seismic Provisions for Structural Steel Buidlings - Section 11. Ordinary Moment Frames (OMF).

The section 11.2 calls out requirements for beam to column joints and connections where it says that FR moment connections need to be designed for a required flexural strength Mu = 1.1RyMp. This is simply the plastic moment capacity of the beam and makes sense that the largest moment that would ever get developed in the beam would be Mp.

Then, it says "or the maximum moment that can be delivered by the system". Usually, this is the ultimate capacity of your floor or roof diaphragm. Now this moment that would occur in the beam would be due to the gravity moments developed by the floor loads PLUS the moment developed in the beam by the maximum diaphragm shear capacity.

So far so good... I calculate Mp and also calculate the moment due to gravity plus diaphragm capcity shear and compare the two to choose the smaller.

In other words, the most moment that would ever get into the connection would be either the moment at the point of deck failure or the plastic moment of the beam. I follow all that.

Now in paragraph 11.2b, the required Vu for the connection is defined as the sum of two things:

1. Shear from load combination 1.2D + 0.5L + 0.2S
2. Shear resulting from Mu defined above.

I understand the first part as requiring the designer to include the gravity shear from the floor or roof. But I have a problem with the second. According to strict logical English, the shear resulting from Mu "above" could possibly include the gravity also as the "maximum moment that can be delivered by the system" has the deck diaphragm lateral forces in the analysis but also INCLUDES GRAVITY in the analysis. Thus, we'd be adding gravity shear twice, and that doesn't make any sense.

Can anyone clear this up for me?

Thanks,

 

[Kramer]

If you are using FEMA 350, the design procedure is based on using the full plastic moment at the beam ends, not the maximum load that can be delivered to the system. Therefore in the FEMA 350 procedure, the shear due to the plastic bending is twice the plastic moment divided by the distance between the plastic hinges. To this shear, the gravity loading must be added. Another approach you can take is to design the connection for the gravity loading plus omega times the seismic force. I often do this for moment frames using steel tubes and other shapes which do not conform to the FEMA 350 connection criteria. If you are using the 1.7 stress increase, be sure that you are using the strength basis forces (not divided by 1.4). In your analysis, where you are designing for the maximum diaphragm shear, if you are including gravity in your analysis of the frame, then you should not have to include it twice. I typically do not design using the "maximum force method". How do you determine the ultimate capacity of the floor or roof diaphragm?

 

[JAE]

First...thanks for the reply.

We are not using FEMA here....just IBC 2000 which references ASIC Seismic Provisions. Thus, the max. load delivered by the system can be included....and omega is not required for OMF. (this is east coast design not Calif.)

The main problem I have is with AISC's wording - it seems to create a non-logical jump by requiring gravity shear to be included twice in the sum - I know I won't do this...just wanted to understand their intent.

The capacity of a diaphragm.....tables provided by deck manufacturer's under SDI include a safety factor (such as 2.75) to get to an "ultimate" capacity. The moment developed by applying this "ultimate" diaphragm capacity shear on the system is then compared with Mp to find the appropriate moment to use.

 

[Taro]

JAE,
I think you are using the correct procedure to satisfy the intent of the code. Obviously, you do not need to include the gravity shear twice. Just remember that it is difficult to write code provisions that are completely foolproof.

 

[dougantholz]

JAE,
I think that Kramer has it right. The shear induced by Mu is what happens when you form plastic hinges at Lp + dc/2 (dc - is depth of the column). Take a look at FEMA 350 it explains what went into AISC Seismic provisions.
To explain it here, the max shear is the half the uniform gravity load plus - the reaction of the beam that is formed in the middle when you take two plastic hinges (one each end) and rotate the middle beam - you get a vertical reaction in the at a distance Lp from the column to counterbalance Mu.

We are in Missouri and we try to stay out of AISC Siesmic provisions, but often use FEMA 350, with some rational modifications, for design of SMF, OMF and Moment connections with an R of 3.