Question

Your client has asked you to assist with a four storey steel frame building. Take ground floor as first floor. The frame is to be made of four columns line and three beam lines in the front elevation. Column height between floor to be 8m and the span of beam between columns is 9m. There are four bays of frames in the side elevation. Roof is made out of trusses. As part of your planning you are requested to prepare the following. a. Roughly Sketch the frame b. Indicate different loads you will consider and the combination of loads. c. Show the load path. Of the vertical and Horizontal load being carried from the roof to the soil d. Give examples of sub frames you will use to analyse elements to get values of B.M, S.F and Axial force. e. Discuss the method of analysis you will adapt and why? f. Kind of connections and bracing you will use and why? g. Discuss the steps you will take to meet the requirements as regard to Fire resistance, fatigue failure and corrosion protection. I need a better solution than the one in your site. Also please do not mention lacing and battern system. Can I please have clear answer with explanation ?

Answer 1

It is not a necessity to always keep the size of columns the same in all floors, but practically it is better to reduce the amount of reinforcement rather than changing the size of column as we go to higher floors.

The reduction of amount of reinforcement in higher floors is always true if we only consider the gravity forces, but this condition sometimes changes when lateral forces come to action. In this case, the column design may be controlled either by gravity forces or biaxial moments. Since the gravity forces are low in higher floors, usually their behavior is controlled by the moments and, thus, you will see more amount of reinforcement in these columns than the columns in the lower floors in which the gravity forces are higher and thus design is controlled by axial forces. As a result, not only the gravity forces but also the lateral forces have great effect on column designs.

That is right that the load of columns in multi-storey buildings change at the height (for sure longitudinal force decreases with increasing the height; and not necessarily the bending moments) - hence, it may seem natural to change the cross-section of the columns.

Often the reason for leaving the constant cross-section of the column are architectural considerations, rather than structural.

Quite frequently multi-storey buildings are office buildings with glass facades and architects would like to get line of visible, external columns of constant thickness.

Often, the real problem is construction of the lower storey columns with a small cross-section, and not a problem for a large cross-section of the upper columns.

Then you can perform concrete columns with high strength concrete or steel-concrete composite columns.