Column base connections for pre-engineered metal buildingsSeptember 18, 2014
One of the most important structural considerations that is often overlooked when designing or ordering a Pre-engineered metal building is the column base connection.
Column bases are the foundations for the columnar components of your steel structure. The column base transfers loads to the building foundation and act as a support for the structural loads the columns must meet. Bases come in many sizes and designs but there are two key methods of connecting the column base to the foundation. The foundation must be designed with the specific connection type in mind.
The base plate of the column transfers all column loads to a supporting member, the structural frame as a whole or to the foundation. This includes:
- Axial/gravity loads
- Moment loads
- Shear loads
- And axial/uplift loads
Base plates turn the column into a vertical cantilever eliminating the need for guy wires for additional support.
While base plate design is relatively open as far as size, shape and aesthetics, there are manufacturing issues that come into play. Some base plate designs are difficult to fabricate or call for materials that are expensive or rare. Others may not be available in the size requested. All these issues can raise the price of the base plate.
Some base plate designs are not stable enough to support the column without the extra support of guying.
A fixed base column is often compared to a flagpole; the entire moment, or bending, load is carried by the supporting foundation. Fixed-base columns are relatively inexpensive but require a much thicker and stronger foundation. This column base type must be specified before the foundation is designed; otherwise the savings from the base type will be eaten up by the expense of reworking the foundation or the columns after installation.
Pinned-base columns are fastened to the foundation with anchor bolts and transfer different loads to the foundation than fixed-base columns. Pinned-base columns transfer only transfer vertical and horizontal loads to the foundation. The steel structure carries the other loads.
There is no need for specific foundation design for a pinned-base column building. In fact, a pinned-base column is often assumed when foundations are built. Unfortunately, the column fastening is generally unknown until the bids for steel construction are complete. By this time the foundation is already in. You can imagine what happens if the resulting building type calls for fixed-base columns in this case. The foundation will not have been designed to support fixed column requiring extra support for the columns during erection and the hope that the foundation will not crack under the additional load.
To save money and time, you must specify, before the foundation is designed and installed, what column type will be used. With all in agreement from the beginning, time, money, and aggravation can be saved.
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