The Design of Structures B notes focus on foundations, column bases, and complete structural system design for students in the Bachelor of Quantity Surveying and Construction Management program. This resource covers essential topics such as the types and behavior of column bases, slab base design, and the integration of column bases within a structural frame. Key learning outcomes include designing base plates for concentrically loaded columns and analyzing base plate behavior under eccentric loading. The notes reference Duggal, McCormac, and Salmon-Johnson, providing a comprehensive guide for third-year students in their second semester.

Key Points

  • Covers week 10 topics on foundations and column bases in structural design.
  • Explains the design of slab base plates for concentrically loaded columns.
  • Details the classification of column bases and their applications in construction.
  • Includes analysis of base plate behavior under eccentric loading conditions.
openda.aa
23 pages
Language:English
Type:Lecture Notes
Structural Engineering
openda.aa
23 pages
Language:English
Type:Lecture Notes
Structural Engineering
366
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EACQ 3278 / EACR 3287 — Design of Structures B | Week 10: Foundations, Column Bases & Complete System Design
Bachelor of QS & Construction Management | Prepared from Duggal, McCormac & Salmon-Johnson | Page 1 of 23
EACQ 3278 / EACR 3287 — DESIGN OF STRUCTURES B
WEEK 10 LECTURE NOTES
Foundations, Column Bases & Complete Structural System Design
3-Hour Class | Year 3 Semester 2 | Bachelor of QS & Construction Management
References
Duggal Ch. 5 (pp. 189–223) | McCormac §7.7 & Appendix D (pp. 218–231, 688–
696) | Salmon & Johnson §13.9, §15.3–15.4 (pp. 746–752, 803–819)
LEARNING OUTCOMES — By the end of this session students will be able to:
1. Identify the structural purpose of column bases and select the appropriate type for a given
loading condition.
2. Design slab base plates for concentrically loaded columns using both the working stress
method (IS code, Duggal) and the LRFD/ASD method (AISC, McCormac).
3. Design gusset base plates for heavily loaded columns, including connection details.
4. Analyse base plate behaviour under eccentric loading and determine stress distributions and
anchor bolt forces.
5. Size anchor bolts and hold-down angles for uplift conditions.
6. Explain the role of column bases within a complete structural frame and demonstrate
system-level design integration.
EACQ 3278 / EACR 3287 — Design of Structures B | Week 10: Foundations, Column Bases & Complete System Design
Bachelor of QS & Construction Management | Prepared from Duggal, McCormac & Salmon-Johnson | Page 2 of 23
PART 1 | COLUMN BASES — TYPES, BEHAVIOUR & SLAB BASE DESIGN
1.1 What Is a Column Base and Why Is It Needed?
A steel column transmits its compressive load — often hundreds of kilonewtons — down to a concrete footing or
pedestal. Without a proper base, the column bearing area would be far too small, producing bearing stresses that
crush the concrete. The column base plate serves three purposes:
• Load distribution: spreads the concentrated column load over a sufficient bearing area so that contact
stresses in the concrete remain within permissible limits.
• Alignment: maintains the column plumb and at its correct plan position during and after erection.
• Connection to foundation: transfers any bending moments or horizontal shear forces into the concrete
pedestal, particularly important under wind and seismic loading.
Reference: Duggal §5.1, p. 189
1.2 Types of Column Bases
Duggal (§5.2) classifies column bases into two broad categories:
Type
Description
When Used
Slab Base
A single flat plate — no stiffeners.
Simple, economical.
Concentrically loaded columns; light to
moderate loads. IS code calls this a
'pinned base'.
Gusset Base
Base plate + two gusset plates +
gusset angles. Greater bearing area
and rigidity.
Heavily loaded columns; or where
bending moment is present. Can be
treated as a rigid base.
Welded Base
(Moment-
Resisting)
Thick plates with welded stiffeners,
connected to moment-resisting
frame. Covered in Appendix D of
McCormac.
Portal frames, moment frames,
cantilever columns — wherever full
fixity at base is required.
Grillage Footing
Two layers of steel I-beams
embedded in concrete. Very large
bearing areas.
Soft ground with very low soil bearing
capacity making large concrete blocks
uneconomical.
1.3 Slab Base — Structural Theory
The slab base design follows the cantilever plate bending model. The concrete pedestal applies a uniform upward
bearing pressure w (N/mm²) to the underside of the plate. The plate overhangs the column edge in two directions
— overhang a (greater) and overhang b (smaller).
1.3.1 Pressure Under the Plate
EACQ 3278 / EACR 3287 — Design of Structures B | Week 10: Foundations, Column Bases & Complete System Design
Bachelor of QS & Construction Management | Prepared from Duggal, McCormac & Salmon-Johnson | Page 3 of 23
w = P / A₁ [N/mm²]
where P = total axial load on column (N), and A₁ = area of base plate provided (mm²). The pressure is assumed to
be uniform for a concentrically loaded column (Duggal §5.3).
1.3.2 Bending Moments in the Plate
Taking a 1 mm wide strip cantilevered from the column edge, the maximum bending moment per unit width for
overhang a is:
Mₐ = w·a² / 2 [N·mm/mm]
And for overhang b:
M_b = w·b² / 2 [N·mm/mm]
The plate bends simultaneously about both principal axes. The stress from bending about one axis restrains the
bending about the other axis due to Poisson's ratio effects (μ = 0.25 for steel). The net bending stress relationship
(Duggal Eq. 5.6) is:
DERIVED THICKNESS FORMULA (Duggal Eq. 5.6):
t = √( 3w(a² − μb²) / σ_bs ) for a ≥ b
Where: t = minimum thickness of slab base (mm)
w = intensity of bearing pressure (N/mm²)
a = greater overhang of base plate (mm)
b = smaller overhang of base plate (mm)
μ = Poisson's ratio = 0.25 for steel
σ_bs = permissible bending stress in base = 185 MPa (IS code)
📝
When a ≈ b (near-square overhang), the thickness formula simplifies approximately to: t ≈
√(3w·a²·(1−μ)/σ_bs). In practice, designers often make the overhangs equal for simplicity.
1.4 Slab Base — Design Procedure (IS Code / Duggal §5.3)
The following step-by-step procedure is directly from Duggal Chapter 5:
1. Assume a suitable allowable bearing pressure on the concrete pedestal based on its grade. For M-15
concrete (fck = 15 N/mm²), the allowable bearing pressure = 0.25 fck = 3.75 N/mm².
2. Compute the required area of the base plate:
A_required = P / (allowable bearing pressure) [mm²]
3. Select the plate dimensions. For a square plate:
L = B = √A_required [mm] (round up to nearest 5 mm or 10 mm)
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FAQs

What are the main purposes of a column base in structural design?
A column base serves three primary purposes: it distributes the concentrated load from the steel column over a sufficient bearing area to prevent excessive contact stresses in the concrete, maintains the column's alignment during and after erection, and transfers bending moments or horizontal shear forces into the concrete pedestal, which is particularly crucial under wind and seismic loading.
What are the different types of column bases and their applications?
Column bases are classified into several types: slab bases are used for concentrically loaded columns with light to moderate loads; gusset bases are for heavily loaded columns or where bending moments are present; welded bases are for moment-resisting frames requiring full fixity; and grillage footings are used in soft ground conditions where large concrete blocks are uneconomical.
How is the slab base design procedure structured according to Duggal?
The slab base design procedure involves several steps: first, assume a suitable allowable bearing pressure based on the concrete grade; second, compute the required area of the base plate; third, select the plate dimensions; fourth, calculate the actual bearing pressure; and finally, determine the overhangs and compute the minimum plate thickness using the derived thickness formula.
What is the significance of gusset plates in gusset base design?
Gusset plates are significant in gusset base design as they increase the active bearing area without necessitating an excessively thick base plate. This design allows for a more efficient load transmission, where the column load is partly supported by direct bearing at the column end and partly through the gusset plates and angles to the base plate, resulting in a thinner base plate compared to a slab base.
How does eccentric loading affect base plate design?
Eccentric loading introduces complexity in base plate design because the bearing pressure under the plate is no longer uniform. The combined stress at any point can be calculated using the formula that accounts for both the axial load and the bending moment, leading to different design cases based on the level of eccentricity. Each case dictates specific approaches for determining the bearing stress distribution and thickness.
What is the derived thickness formula for slab bases according to Duggal?
The derived thickness formula for slab bases is given by t = √(3w(a² − μb²) / σ_bs), where 't' is the minimum thickness of the slab base, 'w' is the intensity of bearing pressure, 'a' and 'b' are the overhang dimensions, 'μ' is Poisson's ratio, and 'σ_bs' is the permissible bending stress in the base. This formula helps ensure the slab base can adequately support the loads.
What is the role of anchor bolts in the design of column bases?
Anchor bolts play a crucial role in the design of column bases by securing the column to the foundation and resisting uplift forces. When a column is subjected to eccentric loading, the anchor bolts must be designed to withstand significant tensile forces, especially when the eccentricity exceeds certain limits, requiring careful analysis of the forces acting on them to ensure structural stability.

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