Over the next paragraph, I have summarized the 3 most important criteria in design.

#1. Shear Failure

When it comes to code based design criteria, either on Special Moment Frame (SMRF) or on Dual System (SMRF + Shear wall), it is best to start with the most critical scenario. Take shear action to be in top hierarchy, being an abrupt and brittle type of failure. While flexure action comes next, being a ductile and a more gradual type of action.

Generally, shear is labeled as force controlled action while flexure as a deformation  controlled action. This explains why Ø (phi) factor value for shear capacity is more conservative.

i. Girder

For the special moment frame, girders framing into columns are prone to hinging during a strong earthquake event. Amount of actual longitudinal bars on girder ends resist the inelastic response, however these longitudinal bars exhibits probable moments. It is the calculated moment strength of the cross section while assuming the rebar yield strength is at least 1.25fy. This will be used as a parameter for the beam probable shear. In short, the higher is the probable moment, the higher is the required beam shear capacity. So, it is important to avoid excessive flexural rebars. At times, it is suggested that shear strength at the hinge zone (i.e. at distance 2*h from the support, where: h=depth) be resisted by the stirrups alone (i.e. Vc=0). This is to ensure that as shear strength degrades, due to the sway reversals, and as concrete cracking, stirrups alone will resist the probable shear demand.

ii. Column

Apart from the vertical reinforcements, columns highly rely on confinement for its full capacity and confinement requirements. Certain number of confinement legs (i.e. hoops) needs to be satisfied depending on hoop size, column size and concrete compressive strength. Most of the time, the number of hoop legs required from the confinement criteria, governs over the shear demand from analysis.

Finally, hoops shear capacity can be checked from the column probable moment produced from vertical reinforcements. That way, the approach resembles like a girder.

iii.  Column Joint

In a moment frame, girders framing into the column are assumed to yield. The rotation on girder ends produce joint shear on column-girder joint. The amount of shear is mostly governed by the probable moment of these girders. The joint resistance depends on the column concrete strength (fc’), as well as the fictitious concrete area within the joint. The effective width of the framing girders affects the area of the joint. Strength coefficient and joint configurations (i.e. how many girders are framing into the column) completes the parameter of the joint shear capacity.

#2. Strong Column – Weak Beam

 Failure of a column is more critical than failure of a girder. A column supports the load of the entire story above it, while a girder supports only floor load from a single plane.

To address this reality, the code requires that columns be stronger than the girders framing to it. This is where the strong column-weak beam principle comes in. the criteria is to make sure that the summation of column strength should be 20% higher than a pair of girders connected to a column.

#3. Back-Up Frame

 For a Dual System to be valid, the code requires that the moment frame resist at least 25% of the total design base shear. This ensures enough redundancy corresponding to the R value specified for the dual system type.

Two models are generated to apply this criteria. First, is the dual system (i.e. SMRF + Shear wall). Second, is the frame only model (i.e. without the walls). The response spectrum is scaled such that 25% of the total base shear be applied to the frame only model. While the two systems should resist the total base shear scaled from equivalent static base shear.

And One More Thing, Detailing

 A safe design cannot be achieved only by accurate design, but up to satisfying seismic detailing requirements.

 Proper detailing enhances the member capacity to withstand multiple deformation (i.e. column hinging, girder hinging). The goal is to achieve a ductile behavior for these members (i.e. columns, girders).

The maximum and minimum reinforcement limits are to improve constructability and ensures that a ductile flexural capacity is achieved.

The requirements on minimum amount of continuous reinforcements ensures enough capacity during moment reversals, all throughout the cross section.

Conclusion

Generally, the secret on safety is capacity design, that is why is it required on forced controlled actions (i.e. shear). Remember that, earthquake forces tend to be large on areas with high stiffness. For example, rightful amount of longitudinal bars on girders reduces the shear demand on column joint and on girder ends itself.

Similarly, take note that modeling and analysis should be taken cared as well. A realistic structure model usually captures the dominant behavior of the actual building.

Usually, you may not be able to achieve a perfect model, but don’t worry, it should never be. The goal is to capture important behavioral aspects of the building, in order to guide you in your design judgment.

A sound assumption on loadings, boundary conditions and member fixity leads to a more ideal analysis. There is no such thing as an exact analysis, at best your experience and gut fell will guide your instincts.

Finally, addressing the 3 most important criteria in code based design of buildings, ensures that the moment frame inelastic deformation, behaves in a ductile manner.  Which is a good place to be.

So, what do you think are the other important criteria to look into?