Introduction (Anchor Bolts Outside & Inside Column Flange)

Base plates are critical steel connection elements used to safely transfer axial load, bending moment, and shear force from a steel column to the concrete foundation.
As per IS 800:2007 – General Construction in Steel, base plate design ensures adequate bearing, bending resistance, and anchorage safety.

This article explains the step-by-step design of a base plate with anchor bolts located outside the column flange, which is commonly adopted for heavily loaded columns and moment-resisting frames.

Base Plate Design – IS 800:2007

Base Plate Design Calculator (IS 800:2007)

Preliminary design of steel column base plates with anchor bolts inside or outside column flange.

Input Parameters

Factored Axial Load, Pu (kN)

Factored Moment, Mu (kNm)

Concrete Grade, fck (MPa)

Plate Yield Strength, fy (MPa)

Trial Plate Size L = B (mm)

Plate Projection, a (mm)

No. of Anchor Bolts

Bolt Lever Arm y (mm)

Bolt Position

Detailed Design Calculations (Auto-Generated)

Calculations will appear here automatically.

How This Sheet Works

Step 1: Bearing Pressure
q = Pu / (L × B)
Permissible q_allow = 0.45 fck
Ref: IS 800:2007 Clause 7.4.1–7.4.3

Step 2: Effect of Bending Moment
qmax = q + Mu / Z , qmin = q − Mu / Z
Z = L·B²/6
Ref: IS 800:2007 Clause 7.4.1

Step 3: Plate Thickness
M = qmax·a²/2 , t = √(6M/fy)
Ref: IS 800:2007 Clause 8.2.1.2

Step 4: Anchor Bolt Tension
Tb = Mu / (n·y)
Ref: IS 800:2007 Clause 7.4.4

Step 5: Auto Plate Size Suggestion
If qmax > qallow → Areq = Pu/qallow, suggest square plate.

Base Plate Plan & Section (Indicative)

Note: Preliminary design only. Check shear, welds, bolt capacity, edge distances per IS 800/IS 456.

⚠️ Engineering Disclaimer

This calculator is for preliminary design and educational use only. Final design must be verified by a qualified structural engineer as per IS 800:2007 and relevant codes.Design References (IS Codes)

IS 800:2007 – Steel design
IS 456:2000 – Concrete design
IS 5624 / IS 1367 – Anchor bolts
SP 6(1) – Steel tables

Typical Arrangement – Bolts Outside Flange

In this configuration:

Anchor bolts are placed outside the column section
Provides higher lever arm
Improves moment resistance
Easier tightening and inspection

Commonly used for:

Portal frames
Industrial buildings
High axial load + moment columns

Given Design Data (Typical)

Factored axial load, Pu
Factored moment, Mu
Column section (e.g., ISHB / ISMB)
Concrete grade (M20 / M25)
Plate material (Fe 250 / Fe 410)
Bolt grade (4.6 / 8.8)

Step 1: Base Plate Area from Bearing Pressure

Design Bearing Strength of Concrete

As per IS 800:2007, Cl. 7.4.3: $\sigma_{br} = 0.45 f_{ck} \sqrt{\frac{A_1}{A_2}} \le 0.9 f_{ck}$ σbr=0.45fckA2A1≤0.9fck

Where:

$A_1$ A1 = supporting concrete area
$A_2$ A2 = loaded area (base plate)

Required Base Plate Area

$A_p = \frac{P_u}{\sigma_{br}}$ Ap=σbrPu

Select plate length (L) and width (B) such that bearing pressure is within limit.

Step 2: Pressure Distribution Under Base Plate

For axial load + bending moment, pressure varies linearly.

Maximum Pressure:

$q_{max} = \frac{P_u}{A_p} + \frac{M_u}{Z_p}$ qmax=ApPu+ZpMu

Minimum Pressure:

$q_{min} = \frac{P_u}{A_p} – \frac{M_u}{Z_p}$ qmin=ApPu−ZpMu

Ensure qmin ≥ 0 (no uplift)
If uplift occurs → anchor bolts take tension

Step 3: Anchor Bolt Tension (Bolts Outside Flange)

When uplift is present: $T_b = \frac{M_u – P_u \times e}{\sum (n \times y)}$ Tb=∑(n×y)Mu−Pu×e

Where:

$T_b$ Tb = tension per bolt
$n$ n = number of bolts
$y$ y = distance of bolt from plate centroid

Bolt Capacity Check

$T_{db} = 0.9 f_{ub} A_n / \gamma_{mb}$ Tdb=0.9fubAn/γmb

Ensure: $T_b \le T_{db}$ Tb≤Tdb

Step 4: Base Plate Thickness Design

Plate behaves as a cantilever projection beyond column face.

Bending Moment per unit width:

$M = q_{max} \times a^2 / 2$ M=qmax×a2/2

Required Plate Thickness:

$t = \sqrt{\frac{6M}{f_y}}$ t=fy6M

Where:

$a$ a = projection beyond column
$f_y$ fy = yield strength of plate

Adopt next higher standard thickness (e.g., 20 mm, 25 mm).

Step 5: Weld Design (Column to Plate)

Continuous fillet weld
Weld force = column load transfer
As per IS 800:2007, Cl. 10

$P_w = 0.7 s l f_{wd}$ Pw=0.7slfwd

Where:

$s$ s = weld size
$l$ l = effective length

Step 6: Shear Resistance

Shear is resisted by:

Friction between plate & concrete
Anchor bolts (if required)

$V_d = \mu P_u$ Vd=μPu

If inadequate → design shear key or bolts in shear.

Advantages of Bolts Outside Column Flange

✅ Higher moment resistance
✅ Reduced bolt tension
✅ Better constructability
✅ Suitable for seismic & crane loads

Common Detailing Requirements

Minimum edge distance ≥ 1.5 × bolt dia
Washer plates under nuts
Grout thickness: 25–50 mm
Bolt projection ≥ 2 threads

Conclusion

Base plate design as per IS 800:2007 with anchor bolts outside the column flange is an efficient and safe solution for heavily loaded steel columns. Proper checks for bearing, bending, bolt tension, welds, and shear ensure long-term performance and code compliance.

This configuration is widely recommended for industrial buildings, portal frames, and moment-resisting structures.