Foundation and Retaining Wall Design Presentation 2.pptx

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1. PCSP 411 Professional Course SP1: Foundation and Retaining Wall Design Engr. Raymond Jay G. Severo Civil Engineer

2. 01 Foundation/ Footing Footing Foundation Different types of Footing foundation Footing Design Concepts Provisions and formula for design and factors to consider in designing foundation Introduction NSCP 2015 Provision (Earthwork, Foundation and Footings) 02 03 Topic Overview

3. Introduction: (NSCP 2015 Provisions) Foundation Investigation (NSCP 2015) Soil Exploration shall be required for buildings, towers and other vertical structures falling under categories I, II, III and IV in accordance with Table 103-2 of NSCP 2015 or as required by the Building Official or if the site-specific conditions make the foundation investigation necessary.

4. Introduction: (NSCP 2015 Provisions) Excavations – Footings: Existing footing or foundation which may be undermined by any excavation shall be underpinned adequately or otherwise protected against settlement and shall be protected against lateral movement.   Chapter 3: Earthworks and Foundation Section 303: Foundation Investigation: Foundation investigation shall be conducted and a Professional Report shall be submitted at each building site. For Structures two storeys or higher, an exhaustive geotechnical study shall be performed to evaluate in-situ soil parameters for foundation design and analysis. The minimum required number of boreholes per structure based on footprint area is summarized in the table 2 below.

5. Introduction: (NSCP 2015 Provisions) All these boreholes should fall within the footprint of the structure and should generally be uniformly distributed throughout the building footprint. Unless specified by the consulting Geotechnical Engineer, all boreholes should be drilled to a depth of at least five meters into hard strata or until a suitable bearing layer is reached. For buildings with basements, the depth of boring should extend to twice the least dimension of the structures footprint (2B) added to the depth of the basement.

6. Introduction: (NSCP 2015 Provisions) Table 2: Minimum required number of boreholes per structure Foot print area of Structure (m 2 ) Minimum Required Number of Boreholes A ≤ 50 1 50 < A ≤ 500 2 A ≥ 500 2 + (A/100) *The minimum required number of boreholes should in no way be constructed as an upper limit value. ** “A” corresponds to the footprint area of the structure in m 2

7. Introduction: (NSCP 2015 Provisions) An exhaustive geotechnical investigation should also be conducted in case of: Questionable soil, expansive soils, or problematic soil (e.g., liquefiable, organic, compressible, sensitive, etc.); To determine whether the existing groundwater table is above or within 1.5 meter below the elevation of the lowest floor level; Where such floor is located below the finished ground level adjacent to the foundation; In case where the use of pile foundation and/or ground improvement are anticipated; In areas underlain by rock strata where the rock is suspected to be a questionable characteristic or indicate variations in the structure of the rock or where solution cavities or voids are expected to be present in the rock; and Other cases deemed necessary by the Geotechnical Engineer.   The Building Official may require that the interpretation and evaluation of the results of the foundation investigation be made by a geotechnical engineer.

8. Introduction: (NSCP 2015 Provisions) Section 304 Allowable Foundation and Lateral Pressure 304-1: From Geotechnical Site Investigation and Assessment The recommended allowable foundation and lateral pressure shall be estimated from a reasonably exhaustive geotechnical site investigation and assessment, which shall include at least the following: Description of Regional geologic characteristics; Characterization of in-situ geotechnical conditions; Factual report on the in-situ and laboratory tests performed to characterize the site for a list of in-situ and laboratory test commonly carried out for geotechnical site characterization);

9. Introduction: (NSCP 2015 Provisions) Disclosure of the assumptions and the applicable analytical or empirical models used in estimating the allowable foundation and lateral pressures; Calculations carried out and factor of safety (FS) assumed in arriving at the recommended allowable foundation and lateral pressure; and Evaluation of existing potential geologic hazards and those that may be induced or triggered by the construction? Installation of the structure.   The geological site investigation and assessment shall be performed by a geotechnical engineer.

10. Introduction: (NSCP 2015 Provisions) A geotechnical investigation and assessment shall be presented in a report. The report, together with a brief resume and a sworn statement of accountability of the geotechnical engineering consultant who prepared it, shall be included in the submittals to be reviewed and examined by the building official or government authority in charge of issuing the relevant permits such as environmental compliance certificate and/or building permit.  

11. Introduction: (NSCP 2015 Provisions) 305: Footings: General: Footing and foundation shall be constructed of masonry concrete or treated wood in conformance with chapter 4, 6 and 7. Footing of concrete and masonry shall be of solid material. Foundation supporting wood shall extend at least 150 mm above the adjacent finish grade.   Minimum Requirements for foundation (Section 305)   The minimum depth of footing is given in Table unless another depth is warranted, as established by a foundation investigation.

12. Introduction: (NSCP 2015 Provisions)

13. Introduction: (NSCP 2015 Provisions)

14. Foundation and Footing W hat is Footing/ Foundation? Footing are structural members used to support columns or walls and transmit their loads to the underlying soils. Reinforced concrete is the most suited materials for footing for reinforced concrete and structural steel buildings, walls, towers, bridges and other structures.

15. Different Types of Foundation: 01 S hallow Foundation 02 Deep Foundation Shallow foundation transfer the load from the superstructure to a soil stratum that is relatively close to the ground surface Deep foundation is generally any type of foundation that extends below strata of poor soil to a level where the sil is adequate to support the loads.

16. T ypes of shallow foundation Footing and mats are two types of shallow foundation Isolated spread footing – Carries a single column. (Square or rectangular footing) Wall footing – is similar to a spread footing and is usually continuous under the length of the wall Combined Footing – is a special type of spread footing that supports multiple columns or wall on the same footing. Strap Footing or cantilever footing – consist of an eccentrically loaded footing connected to an adjacent footing by a strap beam. Spread Footing - Supports one or more vertical elements (Column). Mat Foundation – is a large concrete slab that supports some or all of the columns and/ or walls in the building.

17. T ypes of deep foundation T he most common types of deep foundations utilized under building are piles abd drilled piers Piles – are foundation members that have relatively small-cross sectional dimensions compared with their length. They are available in various materials and shapes, and most concrete piles are circular, square, or octagonal in cross-section. Piles are typically placed vertically into the soil but can be installed at a slight inclination to help resist lateral loads. Drilled Pier – is similar to a cast-in-place pile in that it is a shaft that is drilled into the soil. The shaft may be lined with a casing that may or may not be extracted as the shaft is filled with concrete. A permanent casing is generally required where unstable soil conditions are encountered, which could lead to the soil caving into the drilled shaft. A drilled pier is also referred to as a pier or caisson.

18. Designing a Footing: Loads and Reaction – design of reinforced concrete member, footing must be proportioned to resist the effect from the governing factored loads determined in accordance with the ACI. T he thickness of a footing and the required area of flexural reinforcement are determined using the strength design method, which utilizes factored load effects. Shear requirements must be satisfied using factored shear forces and design shear strength . Design Consideration:

19. Designing a Footing: Design Consideration: Sizing the base area – the base dimension of a footing are determined using the unfactored loads and allowable soil bearing capacities Allowable Bearing Capacity – bearing capacity of soil or rock can be obtain from soil borings and tests performed by geotechnical engineer. Soil Pressure Distribution - after determining the bearing capacity of the soil, the next step in designing is to determine the soil pressure distribution at the base of the footing.

20. Designing a Footing:

21. Designing a Footing:

22. Designing a Footing: Design Consideration: Sizing the Thickness – Once the required area of the footing has been established on the basis of the service loads and the allowable bearing capacity of the soil, the thickness h of a footing must be determined considering both flexural and shear.

23. Designing a Footing: Design Consideration: Design for Shear – provision for shear strength in footing are the same as those required for slab and are given by ACI 11.11. Requirements for both one- and two - way shear must be satisfied. One-way shear – the factored shear force at the critical section, based on the factored pressure at the base of the footing within the tributary area, must be equal or less than the design shear strength determined in accordance with ACI 11.11.2.1. Critical section is located a distance “d” from the face of the column. Two-way shear – the factored shear force at the critical section, based on the factored pressure at the base of the footing within the tributary area, must be equal to or less than the design shear strength determined in accordance with ACI 11.11.2.1. Critical section is located a distance “d/2” from the face of the column perimeter.

24. Designing a Footing:

25. Designing a Footing: Design Consideration: Design for Flexure – after obtaining the dimension and thickness of the footing, it must be test and check for it shear capacity if the thickness of the footing can resist the shear and flexure force on the footing. Critical Section – a spread footing must be designed for the bending moments that are induced because of the pressure developed at the base of the footing from the factored load. Determining the required reinforcement - once maximum factored moment at the critical section has been determined, the required area of reinforcing steel can be calculated using the strength design requirements.

26. Designing a Footing:

27. Designing a Footing:

28. Designing a Footing: Design Consideration: Detailing the Reinforcement – requirements for the distribution of flexural reinforcement in footing are given in ACI 15.4. For one-way (wall footing) and two way square footing, reinforcement is to be distributed uniformly across the entire width of the footing. Flexural reinforcement for two-way rectangular footings must be distributed in accordance with ACI 15.4.4. Reinforcement in the long direction is uniformly distributed across the entire width of the footing. In the short direction, a portion of the total reinforcement must be uniformly distributed over a band width centered on the column or pedestal that is equal to the length of the short side of the footing. Development of Reinforcement - Flexural reinforcement in footing must be fully developed in accordance with the applicable provision of ACI chap. 12.The bars must extend at least a tension development length beyond the critical section defined by ACI 15.4.2.

29. Designing a Footing:

30. Designing a Footing: Design Consideration: Bar Size (No.) Diameter (mm) Development Length L d Inches mm 4 12 mm 19 482.6 5 16 mm 24 609.6 6 20 mm 29 736.6 8 25 mm 48 1219.2 9 28 mm 54 1371.6 10 32 mm 60 1524.0 Minimum Tension Development Length for Flexural Reinforcement in Footings

31. Designing a Footing: Design Procedure: The following design procedure can be used in the design of footings. Included is the information presented in the previous section on analysis, design and detailing. Step 1: Determine the area of the footing. Step 2: Determine the thickness of the footing by shearing stress; one-way and two-way shear Step 3: Flexure and steel reinforcement Step 4: Developmental length of the reinforcement

32. Designing a Footing: Step 1: Determine the area of the footing.     Assume thickness of footing to be used in getting the effective soil pressure     for square footing for rectangular footing  

33. Designing a Footing: Step 2: Determine the thickness of the footing   Determining thickness by Shearing; One-way shear and Two-way (punching) shear which ever is greater   One-way shear Two-way shear  

34. Designing a Footing: Step 3: Flexure and steel reinforcement   Steel Requirements:   Check:   No. of bars = Spacing of bars =  

35. Sample Problems

36. Spread Footing ( Square Footing) A squared column footing is to support a 400mm x 400 mm squared tied column with 12 pcs of 25 mmØ bar, the top of the footing is located 1.5 m below the natural grade line. The column is to carry a service dead load of 1500 kN and a live load of 1000 kN. Allowable soil pressure is 300 kPa as per boring test conducted by geotechnical engineer, also, soil unit weight is determined to be 17.95 kN /m 3 . The site location is determined to be flood prone area and there is always an average surcharge pressure of 25 kN /m 2 . Assume that the concrete unit weight for both column and footing is 23.5 kN /m 3 , also, concrete cover is located 75 mm from the nearest face of the reinforcement bar . Design the square footing having 25mm bars. Assume f’c = 27.5 MPa and fy = 275 MPa.

37. Spread Footing ( Square Footing) A squared column footing is to support a 450mm x 450 mm squared tied column with 16 pcs of 25 mmØ bar, the bottom of the footing is located 2 m below the natural grade line. Allowable soil pressure is 300 kPa as per boring test conducted by geotechnical engineer, also, soil unit weight is determined to be 18 kN /m 3 . Assume that the concrete unit weight for both column and footing is 23.5 kN /m 3 , also, concrete cover is located 80 mm from the centroid of the nearest reinforcement bar . Design the square footing having 25 mm bars. Assume f’c = 27.5 MPa and fy = 414 MPa. The column is to carry a service dead load of 1700 kN and live load of 1100 kN.

38. THANKS! Raymond Jay G. Severo [email protected] Please keep this slide for attribution. Questions? Sir sev CE-tutorials Mond Severo