--- title: "Column slab base: sketch and design steps" subject: "Design of Steel and Timber Structure" chapter: 6 key: "semester_6/design_of_steel_and_timber_structure:slab-base-procedure-sketch" source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/slab-base-procedure-sketch site: BCT Engineering --- # Column slab base: sketch and design steps Design of Steel and Timber Structure · Chapter 6: Compression Members - Asked in: 2078 Chaitra, 6 marks; 2071 Magh, 8 marks; 2068 Bhadra (old course), 6 marks (asked 3 times) - Video: [Design Procedure of Slab Base | Design of Column Bases | Design of Steel Structures](https://www.youtube.com/watch?v=aQwjFPGuOmI) — Ganesh Shegar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/slab-base-procedure-sketch - Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-6 ## Question Draw neat sketch of column slab base and write down design procedure (design steps of column bases / process to find thickness of slab base foundation). ## Answer #### Slab base A slab base is a rectangular steel plate welded (or connected by cleats/angles) to the foot of the column. The column end is machined to bear on the plate, so most of the load passes by direct contact and the weld is nominal. The plate spreads the load over the concrete and is held down by anchor bolts. ```text elevation plan |<------- B ------->| | | column | | +-------------------+ - | | | | | o o | ^ | | | | weld | +---------+ | | +==+=+==========+=+==+ base plate (t_s) | a | column | a | L | concrete pedestal | | +---------+ | | | | | | | o b o | v | anchor bolts | +-------------------+ - o = anchor bolt; a, b = projections of the plate beyond the column ``` #### Design procedure (IS 800:2007 cl. 7.4) 1. **Factored load** $P$ and concrete grade; bearing strength of concrete $=0.45f_{ck}$ (cl. 7.4.1). 2. **Required area** $A_{req}=\dfrac{P}{0.45f_{ck}}$. 3. **Plate size:** choose $B\times L$ so that $B\cdot L\ge A_{req}$, with projections of about 50 mm or more beyond the column; keep the projections about equal so that the plate thickness is economical. 4. **Actual bearing pressure:** $w=\dfrac{P}{B\,L}\le0.45f_{ck}$. 5. **Projections:** $a$ = larger projection and $b$ = smaller projection of the plate beyond the column (beyond the flange tips and the flange faces). 6. **Thickness** (cl. 7.4.3.1): $$ t_s=\sqrt{\frac{2.5\,w\,(a^2-0.3b^2)\,\gamma_{m0}}{f_y}}\ \ge t_f $$ where $t_f$ is the column flange thickness and $\gamma_{m0}=1.10$. 7. **Connection:** connect the column to the plate by a nominal fillet weld all round (the load is carried by contact if the end is milled); provide at least 2 anchor bolts (usually 4) of 20 mm for fixing (design them for any uplift or moment, if present). 8. **Check:** the plate is thick enough ($t_s\ge$ calculated, rounded up to 12, 16, 20, 25 mm...), the concrete pressure is not exceeded, and the foundation (pedestal or footing) is checked separately for bearing and size. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a slab base, how is most of the column load transferred to the base plate? A. Through the fillet weld B. Through anchor bolts C. By direct contact of the machined column end D. Through gusset plates Answer: C. The column end is machined to bear on the plate; the weld is only nominal. (Key point: Slab base load transfer) 2. Which design bearing strength of concrete is used for the required area? A. 0.67 fck B. fck C. 0.45 fck D. 0.25 fck Answer: C. IS 800 cl. 7.4.1 gives 0.45 fck; A_req = P/(0.45 fck). (Key point: Bearing strength) 3. Plate thickness ts is computed from A. sqrt(w a / fy) B. P/(B fy) C. tf only D. sqrt(2.5 w (a^2 - 0.3 b^2) gamma_m0 / fy), and not less than flange thickness tf Answer: D. Cl. 7.4.3.1 formula with a larger and b smaller projection. (Key point: Thickness formula) 4. Why choose nearly equal projections beyond the column? A. To reduce anchor bolts B. To lower fy C. To avoid welding D. Economical plate thickness Answer: D. Equal projections keep thickness smaller and economical. (Key point: Plate size choice) ## Related in Design of Steel and Timber Structure - [Foundation base for ISHB 350 column](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Afoundation-base-ishb350-120kn) — Design of Steel and Timber Structure, chapter 6 - [Slab base for ISHB 350@710.2, 1000 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-ishb350-1000kn) — Design of Steel and Timber Structure, chapter 6, asked 2× - [Slab base for SC 300, 1200 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Acolumn-slab-base-sc300) — Design of Steel and Timber Structure, chapter 6 - [Slab base for SC 220, 1000 kN, M30](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-sc220-m30) — Design of Steel and Timber Structure, chapter 6 - [Slab base for ISMB 350, 850 kN service](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-ismb350-850kn) — Design of Steel and Timber Structure, chapter 6 - [Slab base for ISSC 200, 1200 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-isc200) — Design of Steel and Timber Structure, chapter 6 ## Related in other subjects - [Mat foundation design procedure](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Amat-foundation-design-procedure) — Design of RCC Structure, chapter 11, asked 3× - [Design steps for long column, unbraced frame](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-steps-for-long-column-unbraced) — Design of RCC Structure, chapter 10 - [Types of timber columns and column bases](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Atimber-column-types-and-bases) — Design of Timber and Masonry Structures, chapter 3 - [Design steps of axially loaded brick column](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aaxially-loaded-column-vs-wall) — Concrete Technology and Masonry Structure, chapter 8