After getting the architectural plan of a building, one of the most important tasks for a structural engineer is to determine the position and orientation of columns. This decision impacts the layout of beams, slabs, stairs, and the selection of footings. In this article, we will explore how to decide the position of columns in a building plan during structural planning.
1. Column Placement at Corners and Intersections
One common practice is to place columns at or near the corners of a building, as well as at the intersections of beams or walls. The primary purpose of columns is to provide support for the beams, which are placed underneath the walls to help distribute the load.
This placement ensures that the columns effectively support the structure and maintain the building’s stability.
2. Placing Columns on the Property Line
In certain exceptional cases, it may not be feasible to place a column at the junction of walls, especially when the column is located along the property line. This can present challenges in providing the necessary footing for such columns due to space limitations.
To overcome this, columns can be shifted inward along a cross-wall to accommodate the footing within the property line. Alternatively, a strap footing or a combined footing can be used to resolve the issue.
3. Beam Span Considerations: Avoid Larger Spans for Cost Efficiency
The span of a beam is a critical factor in structural design, affecting both the size and cost of the beam. Larger spans should be avoided for economy reasons, as the required depth of the beam increases with span length. This increase in depth leads to higher self-weight and total load.
This is because the moment governing the beam design is directly proportional to the square of the span and the load as evident from the bending moment equation.
M = wl2/8
Therefore, longer spans lead to an increase in beam depth and cost. On the other hand, columns are generally more cost-effective than beams as their size does not significantly increase with height (as long as the column remains relatively short).
4. Optimizing Structural Layout for Cost-Effectiveness
Let’s explore two alternatives in structural layout:
- Alternative I: A single column placed at C to create a two-span continuous beam for A-B.
- Alternative II: Two columns placed at E and G to form a three-span continuous beam for A-B.
In Case I, spans AC and CB will be larger, and the beam will carry two point loads—one at E and the other at G—transferred from secondary beams. This will require a heavier section for the beam.
In Case II, with columns at E and G, the beam becomes a three-span continuous beam. The beam length is reduced, and it carries only one point load at C (located on the central span). This reduces the bending moments in the outer spans AE and GB, leading to a considerable reduction in beam cost.
Thus, Alternative II is more cost-effective in most cases.
5. Maximum Beam Span Limits
In general, for beams carrying live loads up to 4 KN/m², the maximum spans may be limited to these values:
6. Avoiding Excessive Column Spacing
Larger spacing between columns not only result in longer spans for beams but also increases the load on columns, leading to bulkier columns on lower floors. The use of larger column sections can also cause offsets from walls, which could obstruct available floor area.
7. Minimizing Bending Moments in Beams by Optimizing Column Position
When columns are located very close together, such as at the corner of a building or where walls intersect, it is advisable to provide only one column in these areas to minimize the bending moment in the beam.
Small offsets, like points P and Q, are sometimes included in building design for architectural reasons.
If a column is provided only at P, beam B1 will transfer a concentrated load at point Q, resulting in a longer span and higher bending moment in beam B3.
Conversely, if the column is placed at Q (with no column at P), the reaction from beam B2 at P generates a hogging moment in the cantilever PQ, reducing the sagging bending moment in beam B3. Therefore, placing the column at Q is a more cost-effective option.
Conclusion
Column placement is a crucial aspect of structural engineering, directly influencing the design and cost of beams, slabs, and foundations. By strategically positioning columns and optimizing beam spans, engineers can create a more economical and stable structure.
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