Design Optimisation of Pre-Cast Support Beams

Overview

• Work performed on behalf of Roger Bullivant Limited.
• Existing design for pre-cast pile and beam foundation system for housing
• Introduced in early 1990’s
• Used in poor to marginal ground conditions
• Proved successful, currently 50 houses per week
• Existing design not cost-effective for good ground conditions

System Components

Known Conservatisms

• Existing design known to be unduly conservative
• Design calculations based on assumption of simply supported beams
• Full load of walls, floor and roof imposed as a UDL
• Bending moment = WL2/8
• Shear Force = WL/2
• Work by Building Research Station in 1950’s identified this approach as conservative as no account is taken of stiffness of masonry
• Conservatisms therefore relate to load paths and the interaction between the masonry superstructure and the supporting sub-structure of support beams and pile caps

Loading

• Load is attracted to piles due to:
  • Arching effect of masonry
  • Greater stiffness of support points
• Hence load is less than UDL at centre span
• Some moment restraint is offered at beam supports
• Extent of these effects difficult to quantify

Project Objectives

• To reduce the reinforcement in the beams to eliminate the over-design previously identified.
• To demonstrate that piles supporting a masonry structure do not need to be restrained in two directions by beams even when placed out of position, and therefore to eliminate unnecessary tie beams.
• To eliminate reinforcement cast into the ends of the beams to form the connections.
• To investigate different bearing lengths between cap and beams to allow the beam to be cast in incremental stock lengths.

Project Stages

Initial Test House

Key Design Aspects

• Constructed prior to use of FEA
• Spans of up to 5m
• Substantial reduction in levels of reinforcement
• Maximum design loads applied
• Removal of steel at joints

Results

• Separation at Damp Proof Course level
• Cracking of brick and blockwork

Benefits of FEA

• A more accurate prediction of the true load paths.
• A reduction in the number of conservative approximations needed in design calculations.
• An examination of the effects of different designs, boundary conditions and loading situations with (comparative) ease.
• An ability to investigate the robustness of the design to potential “hazards” (for example, the unforeseen catastrophic failure of a supporting pile).

FEA Modelling Methods

• Double layers of shell elements for walls
• Wall ties using beam elements
• Support beams modelled with beam elements, with cross-section defined explicitly
• Embedded reinforcements in beam elements
• Multiple fixed crack model for concrete
• DPC represented with interface gap elements

Initial Results

• Prediction of cracks at DPC level with size and disposition showing good agreement with test
• Areas where stresses in masonry exceeded likely tensile strength showed good agreement with size and location of observed cracks
• Cracks predicted to occur on beams in locations and of an extent that corresponded well with cracks observed on test beams

Decision For Further Actions

• Use FEA to investigate further the effects of various possible design modifications
• Derive a new test house layout
• Analyse the new test house layout, with various sensitivity studies
• Construct a new test house, and monitor it fully during a comprehensive loading sequence.

Layout Features

• Large garage
• Typical door and window layout from detached and semi-detached houses
• Large party wall
• Large patio door
• Opening included post-construction
• Modified beam connections
• Omission of tie beams
• Reduction in maximum beam span

Sensitivity Studies

• Zero lateral restraint at pile locations, compared with full lateral restraint.
• Variations in material properties.
• The inclusion of future openings.
• Piles being placed away from their intended position.
• The connections between the beams being unable to develop sufficient hogging capacity, allowing a “plastic hinge” to be formed.

Analysis Results

• Beam forces and moments for design purposes
• Displacements
• Minimal cracking of beams
• No more than hairline cracks at DPC level
• Low probability of significant cracking in masonry

Comparison with Strip Footing

• Simplified analysis for comparison
• Same house superstructure supported on Winkler spring representation
• Likelihood of masonry cracking predicted to be at least as great as that from kit T-beam foundation system

Design Methods

• Design calculations as per BS8110: Part 1: 1997
• Design of suitable beam cross-sections arrived at through iterative process
• Final forces and moments ~40% of those based on UDL
• Hogging moment capability at joints by use of top steel

Testing & Monitoring

• Full scale test house constructed
• Loaded with full design load
• Load applied in phases to investigate all potential load imbalances etc.
  Behaviour of structure fully monitored by CERAM Building Technology
• Displacements monitored and crack surveys carried regularly during entire loading process and subsequently as structure left at full load
• Strain gauges used at potential crack locations
• Precision levelling

Comparison of Test & Analysis

• No significant cracks or displacements observed during testing
• Displacements of similar order to predictions but too small to make accurate comparisons possible
• No distress to structure despite significant loading

Conclusions

• FEA used in combination with test programme
• FEA substantially reduced amount of testing required
• FEA allowed design to be optimized resulting in significant cost savings
• Environmental benefits from reduced use of materials
• Unnecessary conservatisms substantially reduced
• Final design proven by testing
• BBA approval granted for revised system
• Design optimization of pre-cast components is an ideal application for advanced FE methods

October 1998