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Concrete Raft Design 2020

Posted on July 8th, 2020 in Analysis

Key Steps in Raft Foundation Design:

1. Modeling and Supports: The foundation is modeled as a 200mm slab using C28/35 concrete. Loading includes surface loads (2.5 kN) and alternate line loads for walls. Crucially, the vertical restraints are applied across the entire surface using a vertical spring support defined by a subgrade modulus of around 10,000, suitable for soft clay or loose sand. Alternate load patterns are utilized to determine maximum "hugging" moments (inverted sagging moments).

2. Analysis and Initial Reinforcement: Static analysis involves six loading cases. Initial design settings include 30mm top cover and 50mm bottom cover. A basic layer of reinforcement (e.g., 10mm bars at 200mm spacing) is applied.

3. Identifying Deficiencies: While the top reinforcement (hugging) generally proved satisfactory, checking the bottom reinforcement revealed over-stressing in certain areas, particularly in the y-y direction.

4. Enhancement: To address the over-stressing, additional strip reinforcement was added to the required zones. This enhancement involved applying extra bars (e.g., 10mm at 400mm spacing) set as bottom steel perpendicular to the strip.

5. Final Result: After adding the necessary strip reinforcement, all reinforcing areas passed the unity checks. The final design, consisting of the basic mesh plus additional strips in specific zones, is then ready to be exported as a detailed drawing file (DWG). The slab, not being a columned slab, required no punching shear rebar.

Step	Description of Raft Foundation Design Process
1. Modeling and Material Setup	The raft foundation is modeled as a 200mm slab using C28/35 concrete. The modeling is done using Master Frame Finite Element Analysis and Master Case Lab Design.
2. Defining Loads and Supports	The foundation carries surface loads (2.5 kN) and alternate line loads for walls (partition/internal). Alternate load patterns are utilized to determine the maximum "hugging" moments (inverted sagging moments). Vertical spring support is applied across the entire surface using a subgrade modulus of around 10,000, suitable for soft clay or loose sand.
3. Analysis and Initial Design	Static analysis is conducted, resulting in six loading cases. Initial design settings include a 30mm top cover and a 50mm bottom cover. A basic layer of reinforcement (e.g., 10mm bars at 200mm spacing) is applied.
4. Reviewing Top Reinforcement (Hugging)	After reviewing force capacities, the top reinforcement (hugging) generally proved satisfactory. Unity checks confirmed the top reinforcement (10mm at 200mm) was sufficient in both X and Y directions, with ratios all below one.
5. Identifying Deficiencies in Bottom Reinforcement	Checking the bottom reinforcement, particularly in the Y-Y (vertical) direction, revealed areas of over-stressing that needed enhancement.
6. Adding Strip Reinforcement	To address the deficiencies, additional strip reinforcement was added. This strip was defined as bottom steel, set to be perpendicular to the strip, and involved applying extra bars, such as 10mm bars at 400mm spacing.
7. Final Design Check and Conclusion	After adding the necessary strip reinforcement to the required zones, a final unity check confirmed that all reinforcing areas passed (nothing came up red). The final design combines the basic mesh plus the additional strips. The design is then exported as a detailed DWG file. No punching shear rebar was required as the slab is not a columned slab.

Video Summary

The design process for the raft foundation of a semi-detached house, utilizing Master frame finite element analysis and master key slab design, involves several key stages, focusing on material properties, complex loading patterns, spring supports, and iterative reinforcement refinement.

I. Foundation Setup and Material Properties

The specific raft foundation detailed in the excerpts is for a semi-detached house.

Slab Specification: The slab is 200 thick. The material used is codified concrete, specifically C28/35. For analysing deflection cases, a short-term creep crack sack is selected.
Loading: The loading includes line loads applied around the edges, across the middle (for the partition), and for an internal wall. A uniform 2.5 kilonewton load is applied to the surface, in addition to dead and live line loads on all members.
Loading Cases and Analysis: All line loads are designated as alternate loading cases. This pattern loading approach is employed to achieve an idealized maximum hugging moment in specific areas. Since the line loads effectively act as inverted supports, the structure spans between them. The analysis focuses on calculating the maximum hugging moment, which is the equivalent of a sagging moment when considering the slab is in reverse. Static analysis (space frame) is performed, yielding 6 loading cases due to the use of alternate loading patterns .

II. Support Conditions

The supports are designed to simulate the interaction between the raft and the ground:

Static Restraints: Two static supports are included to prevent the slab from moving sideways or twisting horizontally.
Vertical Restraints (Spring Support): The slab has no vertical restraints in the static support sense. Instead, all restraints are applied to the surface. The entire surface is given a vertical spring support.
Subgrade Modulus: A subgrade modulus of around 10,000 is used, which is characteristic of loose sand or soft clay .

III. Concrete Slab Design and Initial Reinforcement

The concrete slab design begins by setting basic parameters :

Steel Type and Cover: 500H steel is specified . The top cover is set at 30 mil (mm), and the bottom cover is 50 mil (mm) . Direction one is designated as the outer layer .
Reinforcement Strategy: Basic rebar is set at 10s at 200 spacing . Initial settings for peak zones and strip zones are set to 1 metre wide . The auto design bars feature limits additional or replacement reinforcement to 200 spacing or a double of 200 (400) .

After adding a basic set of reinforcement (10s at 200 spacing) , force capacities are assessed .

Top Steel Requirements (Hugging): The top reinforcement requirements are generally small in the horizontal (XX) direction (around 252) . In the vertical (YY) direction, requirements are around 300, suggesting a basic 393 mesh might be needed . Unity checks confirm that the initial 10s at 200 on the top is satisfactory for both hugging X and Y directions (all results below 1) .
Bottom Steel Requirements (Sagging/Inverted Hugging): The horizontal (YXX) direction requires very little bottom reinforcement . However, the vertical (YY) direction requires enhancement above the 393 baseline mesh in specific areas, showing areas of overstressing .

IV. Adding Supplementary Strip Reinforcement

To resolve the areas of overstressing in the bottom reinforcement (YY direction), supplementary reinforcement strips are added .

First Strip: A strip line is drawn over the length of the critical area . Its width is adjusted from 1 metre down to 750 each side (1500 overall) . The steel direction is crucial: the default (top steel) is changed to bottom steel, and the placement direction is set to perpendicular to the strip . This strip provides additional strength using 10s at 400 (meaning every other bar is added to the basic 10s at 200) .
Second and Third Strips: The process is repeated for the other overstressed zones . The subsequent strips are extended by one metre to cover the return . The second strip uses bottom steel perpendicular and results in bars at 350 spacing . The third strip is also set as bottom steel perpendicular with a spacing of 400 .

V. Final Checks and Output

After adding the required strips, a final check on the unity ratios confirms that nothing is coming up red, indicating a satisfactory reinforcement design .

Final Reinforcement: The final design consists of the basic 10s at 200 spacing, supplemented by additional reinforcement (e.g., 10s at 400 or 350 spacing) .
Export: The design is exported to a drawing file, such as a DWG file opened using AutoCAD . The export includes four layouts of rebar (basic strip and peak reinforcement, bottom and top rebar) .
Punching Shear: No punching shear rebar is necessary because the design relates to a simple raft slab and is not a columned slab .

Summary with Timestamps

This is a summary of the design process for the semi-detached house raft foundation, presented in a table format with corresponding timestamps and citations.

Summary of Raft Foundation Design Process

Time Range	Summary of Event / Detail
00:00:07,280 – 00:00:20,640	The analysis uses Master frame finite element analysis and master key slab design for a raft foundation of a semi-detached house.
00:00:21,280 – 00:00:33,360	The structure includes line loads applied around the edges, across the middle (for the partition), and for an internal wall.
00:00:46,880 – 00:01:05,120	Material properties include a 200 thick slab made of codified concrete C28/35. For deflection cases, a short-term creep crack sack is selected.
00:01:07,360 – 00:01:24,880	Loading includes a uniform 2.5 kilonewton load on the surface, along with dead and live line loads on all members.
00:01:28,400 – 00:01:46,479	All line loads are set as alternate loading cases to use pattern loading, aiming for an idealized maximum hugging moment because the line loads act as inverted supports.
00:01:58,160 – 00:02:23,440	Alternate load patterns (focused on two sets of loads) are applied to find the maximum hugging moment, which is the equivalent of a sagging moment since the slab is effectively in reverse.
00:02:25,480 – 00:02:46,240	Support conditions include two static supports (to prevent sideways movement or horizontal twisting). There are no vertical restraints in the traditional sense, as all restraints are on the surface.
00:02:47,040 – 00:03:06,400	The entire surface is given a vertical spring support. A subgrade modulus of around 10,000 is used, which characterizes loose sand or soft clay .
00:03:13,840 – 00:03:22,079	Static analysis (space frame) is performed, resulting in 6 loading cases due to the use of alternate loading patterns .
00:03:26,880 – 00:03:42,720	Concrete slab design parameters are set: 500H steel, top cover 30 mil (mm), bottom cover 50 mil (mm). Direction one is set as the outer layer .
00:03:49,360 – 00:04:12,720	Initial reinforcement is set at 10s at 200 spacing . Peak zones and strip zones are initially set to 1 metre wide . The auto design feature limits additional or replacement reinforcement to 200 spacing or a double of 200 (400) .
00:04:24,360 – 00:04:29,120	The basic set of reinforcement is added to the surface .
00:04:41,480 – 05:12,480	Top steel requirements (hugging moment): XX direction requirements are low (around 252). YY direction requires around 300, suggesting a 393 mesh might be needed .
00:05:17,600 – 05:31,200	Bottom steel requirements: YXX (horizontal) requires little reinforcement. YY direction shows areas that need to be enhanced above the 393 mesh .
00:05:57,040 – 06:04,960	Unity checks confirm the initial 10s at 200 spacing on the top is satisfactory for both hugging X and Y directions (all results below 1) .
00:06:14,800 – 06:31,280	The check on bottom reinforcement (YY direction) shows areas of overstressing which require supplementary reinforcement strips, suggested to be 2 metres overall wide .
00:06:45,440 – 07:11,280	First reinforcement strip added: Drawn over the full length; width adjusted from the 1 metre default to 750 each side (1500 overall) .
00:07:11,840 – 07:38,680	The strip setting is changed from the default top steel to bottom steel .
00:07:39,640 – 08:17,120	The placement direction is changed to perpendicular to the strip for additional strength; the supplementary rebar is set to 10s at 400 (meaning every other bar is added to the basic mesh) .
00:08:20,480 – 08:44,400	Second reinforcement strip added: Drawn across the surface and stretched by about a metre to cover the return .
00:09:07,200 – 09:33,640	The second strip is set as bottom steel perpendicular, resulting in bars at 350 spacing .
09:35,000 – 10:03,200	Third reinforcement strip added: Drawn, extended by one metre, and reduced to half a meter either side .
10:04,360 – 10:33,040	The third strip is confirmed as bottom perpendicular with a spacing of 400 .
10:35,600 – 10:44,400	Final unity check confirms that nothing is coming up red, indicating a satisfactory design after adding the supplementary strips .
10:45,280 – 11:05,600	The final reinforcement consists of the basic 10s at 200 spacing supplemented by additional reinforcement (e.g., 10s at 400 spacing) .
11:14,000 – 11:27,600	The design is exported to a drawing file (1:100 scale), selecting four layouts: basic strip and peak reinforcement, and bottom and top rebar .
11:28,440 – 11:57,520	The design is exported as a DWG file (opened in AutoCAD), including four layouts of rebar. No punching shear rebar is required because the slab is not a columned slab .
12:03:040 – 12:24,480	The output details the rebar layouts for the top and bottom reinforcement, noting that while the slab is simple, the detailing capability is valuable for more complex rafts .