optimisation of pre-cast support beams

Transcription

1 optimisation of pre-cast support beams Design Optimisation of Pre-cast Support Beams Investigation into pile and beam systems for a client in the civil engineering industry with the following 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. T: +44 (0) E: info@wildefea.co.uk Wilde FEA Ltd. 1998

2 Case Study: Design Optimization of Pre-Cast Support Beams Tim Morris - Wilde & Partners

3 Problem Description Background 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

4 Background System Components

5 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 = WL 2 /8 Background 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

6 Background 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

7 Project Objectives To reduce the reinforcement in the beams to eliminate the overdesign previously identified. Background 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.

8 Background Pile caps Beams directly under walls Tie beams for additional stability (no wall above)

9 Project Objectives To reduce the reinforcement in the beams to eliminate the overdesign previously identified. Background 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.

10 Front Elevation Background Plan View

11 Project Objectives To reduce the reinforcement in the beams to eliminate the overdesign previously identified. Background 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.

12 Project Stages Construction of Test House Evaluation of Behaviour FEA and Comparison with Test Initial Test House Background Derivation of New Layout FEA of New Layout Design Optimization Construction and Monitoring of Test House Submission to BBA for Approval Second Test House

13 Initial Test House 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

14 Benefits of FEA A more accurate prediction of the true load paths. Initial Test House 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).

15 Initial Test House FE Model

16 Initial Test House FE Model

17 Modelling Methods Wall Ties Beam Elements Initial Test House Blockwork Brickwork T Beam Floor Reinforcement Schematic Layout Shell Elements Shell Elements Gap Elements Beam Elements FE Representation

18 Modelling Methods Initial Test House 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

19 Results Initial Test House 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

20 Decision For Further Actions Initial Test House 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.

21 Layout Features Second Test House 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

22 Second Test House Layout

23 Second Test House FE Model

24 Sensitivity Studies Second Test House 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.

25 Analysis Results Second Test House 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

26 Bending Moments FEMGV Wilde & Partners 24 JUN 1998 Model: NBM2 LC1: Load case 1 Step: 1 LOAD: 1 Element EL.MX..L MY Max/Min on model set: Max =.115E8 Min = -.208E8 Second Test House Y Z X

27 Second Test House

28 Beam Displacements Second Test House FEMGV Wilde & Partners 17 SEP 1998 Model: NBM LC1: Load case Step: 1 LOAD: 1 Nodal TDTX...G RESTDT Max/Min on model set: Max = 3.3 Min = 0 Factor = 238 Y Z X

29 Second Test House

30 Second Test House

31 Comparison With Strip Footing Second Test House 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

32 Second Test House

33 Second Test House

34 Design Methods Second Test House 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

35 Second Test House Beam Cross-Sections

36 Testing / Monitoring Second Test House 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

37 Second Test House Testing / Monitoring

38 Second Test House Testing / Monitoring

39 Second Test House Testing / Monitoring

40 Comparison of Test & Analysis Second Test House 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

41 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