Differences in Structural Design by Different Structural Engineers

In my experience, I have often encountered clients saying, “Your design requires more steel, while another structural engineer’s design is far more economical." To which my response has always been, "Yes, that may be true, but I would like to point out the technical inaccuracies in the other engineer's approach." Despite my explanation, many clients tend to opt for the more economical design, often without considering the structural implications.

Over the years, I have refused to provide structural drawings to clients who don't trust my expertise and insist on thinner sections or reduced reinforcement in structural elements. This pushed me to delve deeper and analyze why there is such a significant variation in structural designs provided by different engineers. Here’s what I discovered:

Key Reasons for Differences in Structural Design:

1. Simplified Beam Design: Some engineers opt for designing beams as continuous elements, rather than performing a comprehensive 3D frame analysis. This simplified approach assumes that the reactions from the continuous beams can be used to design columns, leading to neglected moments in the columns. This oversight results in an underestimation of the loads and forces acting on the structure, especially in critical areas like the columns.
2. Moment Release in 3D Frame Structures: In certain cases, engineers choose to release moments in the beams by 10 to 20%, which reduces the moments transferred to the columns. While this approach may give the appearance of economy, it actually leads to lower forces being considered for the columns. This can compromise the structural integrity, as the actual forces acting on the columns are greater than those accounted for in the design.
3. Underestimating Live Loads: Some engineers undercut the live load requirements. While IS 875 mandates a minimum live load of 2 KN/m², I've observed designs where engineers consider only 1 to 1.5 KN/m². This underestimation of live loads leads to compromised strength, especially when the structure is subjected to real-world conditions, increasing the risk of overstressed elements during use.
4. Lower Partial Safety Factor: The partial safety factor, as per standards is 1.5. Factored load shall be 1.5 times the working load. However, in some cases, engineers use partial safety factors ranging from 1 to 1.2, resulting in under-designed structures. This practice compromises safety and does not align with the required safety standards, especially under extreme load conditions.
5. Manual Column Design Using SP16 Charts: While software like STAAD offers column design solutions, many engineers prefer using manual methods such as the SP16 charts. These charts are based on assumptions such as 20 reinforcement bars for square or rectangular columns and 8 bars for circular columns. When a lesser number of bars is used, the resulting design is only an approximation, which may not reflect the true structural behavior, leading to under-reinforced columns.
6. Neglecting Moments in Footing Design: Often, footings are designed for axial loads only, ignoring the moments that may act on them. This can lead to under-reinforced footings, especially in cases where load combinations include significant moments. Such designs increase the risk of structural failure, as the footing is not fully prepared to handle the bending forces acting on it.
7. Incorrect Effective Length of Columns: The effective length of columns is a crucial factor in their design, as it influences their ability to resist buckling. Failing to use the correct effective length can lead to miscalculations, causing the column's capacity to be overestimated. This results in a weaker structure that may not perform as expected under actual load conditions.
8. Ignoring Eccentricity in Column Placement: When columns are placed at the intersection of beams, there is often an eccentricity that is overlooked. Engineers must use the offset command in design software to correctly account for the actual placement of columns relative to the beams. Failing to do so results in misalignment of load transfer, affecting the structural behavior and possibly leading to localized failures.
9. Considering Beams Only in One Direction: In some building structures, engineers only design beams in one direction, ignoring the contribution of beams in the other direction. This leads to an unbalanced load distribution and can result in higher stresses on some elements while underutilizing others, leading to inefficiency and potentially unsafe designs.