Base Plate and Moment Frames for buildings

[Calif]

Ok, I have always been told that for base plates supporting a moment frame, you design the base plate for just axial load and not for moment even if it is part of a moment frame. The idea is that it frame will develop a hinge near the base plate but not at the base plate. If that is the case, do you typically bolt within in the interior depth of the column or do you bolt on the outside the column depth? This is a question in reference to building design and I just asking for your opinion.

Thanks


The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[mmillerpe]

It would depend on how you want the moment frame to act and how you modeled the frame. If you are relying on a fixety or partial fixity at the base, then you would need to ensure that the base plates, anchor rods, piers, footings, etc have been designed for the proper moment. In this case, you would generally see the anchor rods outside the column depth.

If you are treating the base of the column as a pin connection than you can design for only the axial load, and the location of the anchor rods is less important. Keep in mind that you may still need to check OSHA requirements.

More info on the design of base plates for moment can be found in AISC Design Guide 1.

 

[Calif]

Hi mmillerpe

I have the design guide in PDF and hardcopy but this has always confounded me. I use Etabs at work yet, I have always been told that we design moment frames hinged at the base. I know from statics that assigning a hinge condition at the based applies more moment to the frame versus assigning a fixed condition at the base which reduces moment throughout the frame. What gets me is that although we assign a hinged condition at the base, we know that the base plate has moment yet the prevailing idea in building engineering is that it should be assigned a hinge support. If it is hinged and part of a moment frame, I would think the bolting should be closer to the COG of the base plate and moment resisting base plate, you want the bolting closer to outside of the base plate. I do not use OSHA for design since I work for a private firm.

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[mmillerpe]

I am not sure that the prevailing idea in building design is to provide a hinge at the base. I have designed a number of moment frames, some as a hinged base, others utlizing a "fixed" base. The type is dependends on the situation.

Keep in mind that you never get a truly pinned or completly fixed condition, the actual condition will be somewhere in between.

Providing a hinged base is simpler detailing wise, and has less of an effect on foundation sizes, however will show much higher deflections. When hinging the base, it would be best to provide the anchor rods at the column centerline, which when you look at many older projects, where only two rods were required, is what they did. These days where for columns typically (4) anchor rods are required it is difficult to do, and therefore will always have some moment capacity. With that, you have to think about where and how is the hinge going to form. Is is the column section itself, the weld to the base plate, the base plate, the anchor rod?

Likewise, when providing some fixity at the base, it is better to place the anchor rods outside the column profile. Depending on the amount of moment, you may need to end up oversizing the plate to get a larger moment arm in order to keep the tension in the anchor rods below their allowable stresses.

Although you work for a private firm, the projects you design likely fall under OSHA regulations. Typically, the GC is required to follow them, and most do not affect the design of the structure. On that note, a number of years ago they introduced a new OSHA reg that all columns are to be anchored with a minimum of (4) anchor rods. I believe the theory was to provide a minimum amount of moment capacity at the base so that the column would be stable if an iron worker was hanging off the side of it.

 

[vandede427]

if you modeled your column with a pinned base and fixed top, then yeah you can do that. but if you modeled it fixed-fixed, then you've got to develop that moment.

 

[asixth]

If you are prepared to provide larger footings and a thicker baseplate than the frame can be designed with a fixed base. This will increase the economy of the steelwork.

Building practice in my part of the world has dictated using the anchors within the column flanges like you suggested. The baseplate thickness is reduced, the hold-down bolts are reduced and the footing design is simple while still maintaining and economical frame.

Agree with mmillerpe that despite that baseplate being detailed as a hinge, it will still have reserve capacity to resist a small moment at the baseplate (say 5% of the moment at the knee joint). I do not and would not recommend relying on this fixity for ultimate design checks. However at serviceability loads (i.e. checking drifts), I do rely on the baseplate and footing to resist this small moment to reduce the calculated deflections to within acceptable levels.

I would provide a minimum of 4 bolts to give the structure some robustness during erection.