TSC Standards Series · Part 8 of 9
Foundation Design — TS 500 and ZTA 2020
Foundation design for buildings in Turkey: site investigation requirements and allowable bearing capacity per ZTA 2020, spread footing design per TS 500 including punching shear and bending reinforcement, pile foundation capacity, TSC 2018 §16 seismic foundation requirements, and a step-by-step worked example for a 5 × 5 m spread footing.
1. ZTA 2020 — Regulatory Framework
The ZTA 2020 (Geotechnical and Foundation Design Regulation) is issued by Turkey's Ministry of Environment, Urbanization and Climate Change. It became mandatory for all building permit applications after July 2020 and establishes requirements for:
- Geotechnical investigation scope and reporting
- Laboratory testing requirements
- Allowable bearing capacity determination methods
- Foundation type selection criteria
- Slope stability and retaining structure design
- Liquefaction hazard assessment
Legal requirement: ZTA 2020 requires a formal Geotechnical Investigation Report signed by a licensed geotechnical engineer for all new building projects regardless of size. The report must include borehole logs, laboratory results, and geotechnical design parameters. Without this report, a building permit cannot be issued.
2. Soil Investigation Requirements (ZTA 2020)
| Building Category | Min. Boreholes | Min. Depth | Required Tests |
|---|
| 1–3 storey residential (BKS 3) | 1 per 500 m² plan (min 2) | Bedrock or 10 m | SPT, grain size, Atterberg limits, natural moisture |
| 4–9 storey residential/office (BKS 2) | 1 per 400 m² (min 3) | 1.5 × foundation depth or 15 m | SPT/CPT, c-φ triaxial, density, consolidation |
| 10+ storey or BKS 1 | 1 per 300 m² (min 4) | 2× foundation depth or 20 m | Full suite: SPT/CPT/PMT, triaxial, consolidation, Vs measurement |
| Any building on soft soil (ZE/ZF) | Minimum 4 | Until stiff layer + 5 m | Liquefaction assessment, cyclic triaxial if required |
SPT (Standard Penetration Test) is the most commonly used in-situ test in Turkey. The blow count N (SPT-N value) at 300 mm penetration is the primary parameter for cohesionless soils. CPT (Cone Penetration Test) is increasingly used for soft cohesive soils. Pressuremeter tests (PMT) are required for bored pile design on difficult soils.
3. Allowable Bearing Capacity (ZTA 2020)
ZTA 2020 requires that allowable bearing capacity qa be determined from one of three approaches, in order of preference:
- Laboratory-based bearing capacity analysis using Meyerhof or Vesic bearing capacity factors applied to measured c and φ values from triaxial tests.
- In-situ test correlations using SPT-N or CPT-qc with empirical conversion factors specific to soil type and depth.
- Presumptive values from ZTA 2020 Table (for preliminary design only — not for final permit submission).
| Soil / Rock Type | Presumptive qa (kN/m²) | Notes |
|---|
| Hard, intact rock (ZA-type) | 3000 – 10000+ | Based on RQD and fracture spacing |
| Dense gravel (N > 50) | 500 – 1000 | Compactness confirmed by SPT |
| Medium dense gravel/sand (N = 30–50) | 300 – 500 | Settlement governs for large footings |
| Dense sand (N = 30–50) | 250 – 400 | Settlement check required |
| Medium dense sand (N = 10–30) | 150 – 250 | Liquefaction check per ZTA §12 |
| Stiff clay (cu = 100–200 kPa) | 200 – 350 | Long-term settlement governs |
| Firm clay (cu = 50–100 kPa) | 100 – 200 | Consolidation analysis required |
| Soft clay (cu < 50 kPa) | < 100 | Pile or deep foundation typically required |
Where Nc, Nq, Nγ are bearing capacity factors; s, d, i are shape, depth, and inclination correction factors respectively. The factor of safety FGS = 3.0 for permanent gravity loading; FGS = 2.0 is permitted under seismic loading (with qult from drained, undrained, or combined analysis as appropriate).
4. Spread Footing — Structural Design (TS 500 §13)
Once the bearing capacity is confirmed and footing plan dimensions L × B are established, TS 500 governs the structural design of the footing.
Punching Shear
Where Vp = factored punching shear force (total factored column load minus soil reaction within the critical perimeter); up = critical perimeter at distance d/2 from column face = 2·(a + d) + 2·(b + d) for rectangular column a × b; d = effective footing depth.
Punching shear critical perimeter: TS 500 takes the critical perimeter at d/2 from the column face (same as ACI 318 §22.6). In contrast, EN 1992-1-1 uses 2d from the column face. This difference means TS 500 is more conservative than Eurocode 2 for the same footing depth.
Flexural Design
Where qnet,d = factored net soil pressure (factored column load / footing area); a = column dimension in the direction considered; B = footing dimension perpendicular to the bending direction. The cantilever is measured from the column face, and critical moment is at the column face. Steel is sized for this moment using the standard flexural design procedure from TS 500 §8.
Minimum Reinforcement
TS 500 §13.4 requires ρmin = 0.0018 in both directions for footings (slightly higher than beam minimum to control shrinkage/temperature cracking in the full slab). Bar spacing must not exceed 3d or 300 mm.
5. Raft and Strip Foundations
Where soil bearing capacity is insufficient for spread footings, or building load is high and uniform, raft foundations are used. The raft distributes loads to a large area and also provides significant stiffness against differential settlement.
- Raft on elastic foundation: Modelled as a plate on Winkler springs (ks = subgrade modulus from plate load test or correlation with qa). TS 500 §13.5 covers design.
- Compensated raft: Where the weight of excavated soil approximately equals the building weight. Common in Istanbul for soft clay sites (ZD/ZE). Net contact pressure ≈ 0.
- Strip footing: Used under load-bearing walls or closely-spaced columns. Designed as an inverted beam between column loads, with soil reaction as the distributed load.
6. Pile Foundations
ZTA 2020 §11 and TS EN 1997-1 govern pile design in Turkey. Piles are used when soft soil extends to great depth, settlement must be minimized, or lateral loads are significant.
Where qs,k = unit shaft friction; As,i = shaft area in layer i; qb,k = unit base resistance; Ab = pile base area. Shaft friction values are derived from in-situ tests (SPT-N or CPT-qc) using ZTA correlation tables, or from laboratory cu tests for cohesive soils (α method: qs = α · cu, where α = 0.45–0.65 depending on cu).
Negative Skin Friction
In areas with recent fill, soft compressible layers, or regions subject to post-construction settlement (common in reclaimed land areas around İstanbul, İzmir Körfezi), ZTA 2020 §11.5 requires assessment of negative skin friction (downdrag). The pile must carry the additional load Fneg = qs,neg · As,settling as axial compression.
7. TSC 2018 §16 — Seismic Foundation Design
TSC 2018 Chapter 16 imposes capacity design principles at the foundation level for DTS 1 and DTS 2 buildings:
- Capacity design demands: The foundation must be designed for the forces corresponding to the full development of plastic mechanism in the superstructure (column base or wall base). The overstrength factor D (from TSC Table 4.1) is applied to amplify design forces.
- Seismic bearing capacity: Under seismic loading, the allowable bearing capacity may be increased by 33% (qa,seis = 1.33 × qa,static) provided the foundation does not rock excessively and the soil is not liquefiable.
- Liquefaction: For sites with potential liquefaction (ZF class, ZE class with N < 15 within 15 m depth), TSC §16.10 requires either deep foundations bypassing the liquefiable layer, ground improvement, or explicit liquefaction mitigation design.
- Foundation ties: TSC §16.4 requires tie beams connecting all foundations in DTS 1/2 buildings. Tie beams must carry ±0.1·Nd in tension or compression (where Nd = larger column axial load of the two connected columns).
Common omission: Foundation tie beams are frequently undersized. TSC §16.4 requires that the tie beam's axial capacity (both compression and tension) equals at least 10% of the larger column load. For a 3000 kN column: tie beam must carry ±300 kN. This requires a significant longitudinal reinforcement area, especially in tension.
8. Worked Example — 5 × 5 m Spread Footing
Example: Square Spread Footing, TS 500 + ZTA 2020
Given: Exterior column 500×500 mm. Factored loads: Nd = 2200 kN, Mx,d = 80 kN·m (both directions). Allowable bearing capacity qa = 250 kN/m² (from geotechnical report, ZD soil). Concrete C25/30, Steel B500C. Footing depth Df = 1.5 m below finished floor, d = 500 mm (assumed initial).
Step 1 — Footing size from bearing capacity:
Characteristic Nk ≈ Nd/1.3 = 2200/1.3 = 1692 kN (approximate unfactored)
Areq = Nk / qa = 1692 / 250 = 6.77 m² → use 2.6 × 2.6 m
Check with moment: e = Mx,k/Nk = (80/1.3)/1692 = 0.036 m → within L/6 = 0.43 m ✓ No uplift
Step 2 — Net factored soil pressure:
qnet,d = Nd / (B × L) = 2200 / (2.6 × 2.6) = 325 kN/m²
(Seismic increase not applied here — gravity governs)
Step 3 — Punching shear check:
Critical perimeter: up = 2(a + d) + 2(b + d) = 2(500 + 500) + 2(500 + 500) = 4000 mm
Vp = Nd − qnet,d × (a + d)(b + d) = 2200 − 325 × 1.0 × 1.0 = 1875 kN
Vpr = 0.65 × 1.20 × 4000 × 500 / 1000 = 1560 kN
Vp = 1875 kN > Vpr = 1560 kN ✗ → Increase d to 600 mm
Revised d = 600 mm:
up = 2(500+600) + 2(500+600) = 4400 mm
Vp = 2200 − 325 × 1.1 × 1.1 = 2200 − 393 = 1807 kN
Vpr = 0.65 × 1.20 × 4400 × 600 / 1000 = 2059 kN > 1807 kN ✓
Step 4 — Flexural design:
Md = 325 × (2.6 − 0.5)² / 8 × 2.6 = 325 × 4.41 / 8 × 2.6 = 466 kN·m
Solve for As: using C25/30 (fcd=16.7 MPa) and B500C (fyd=435 MPa), d=600 mm, b=2600 mm
Required As ≈ 2050 mm² per direction (from beam flexure equations)
As,min = 0.0018 × 2600 × 600 = 2808 mm² governs → use 10φ20 (As=3142 mm²) each direction ✓
Result: 2.6 × 2.6 × 0.7 m footing (h = d + cover = 600 + 70 = 670 mm, use 700 mm). 10φ20 @ 260 mm each way, bottom layer in both directions.
Preliminary design only. Foundation design requires site-specific geotechnical data from a licensed geotechnical engineer per ZTA 2020. Settlement analysis must be performed separately. Always verify with a licensed structural engineer.