Double Tee Section
Qualitative Assessment of your Floor before you Start Step 1 - Qualitative Review • Before you start your detailed capacity calculations • Carry out a qualitative review of the floor and potential damage areas obtained from your building analysis • Check the drawings and details for what information you need to get from the site investigation Step 2 - Visit the building • Review the parameters and details on site. • Site measure and audit against drawings • Is there some additional invasive investigation needed • The better the information the more accurate the result Step 3 Detailed Check • Now start your detailed calculations This example is for a Hollowcore floor building – but the same principles apply for all precast floors including Double Tees, Rib and Infill, and Flat Slab assessments. ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Design Drawings and Details ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Ductile Frame Example Parameters • Ductile frame building with 350 deep 2400 module Double Tee floors 10500mm (typ.) U R 500W x 800D • Beam span = 10,500 mm le = 3600mm • Beam depth = 800 mm 350 Double Tee 500W x 800D 500W x 600D 75mm topping • Column width = 800 mm 665 mesh 75mm seating • Floor unit seated 285 mm above the D12-300 D12-300 beam centreline 600 lap 600 lap • D12-300 starter bars • Elastic drift = 0.6% • Detailed example calculations provided in the Appendix ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Required checks Scenarios to check: A. Unit adjacent corner columns • Check using unrestrained hinge (U) B. Unit adjacent elongating beam, away from corners • Check using restrained hinge (R) C. Internal units away from elongating beam Note: Highlighted Units – the topping delaminates so is a special case A or B to consider ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Required checks Checks for each scenario: 1. Loss of support – Spalling – Elongation 2. Birds Mouth Failure – Review connection load transfer mechanism and probable failure modes of connection due to drift. – Strut and Tie check of flexural capacity and failure modes – Details on the assessment of the failure modes of flange hung double tees units is provided in Hare et al. (2009) ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Loss of Support Example
Double Tee Floor Example – Loss of Support Review ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review • Tee Unit adjacent corner column Elongation + • Initial seating = 75 mm Rotation • Construction tolerance = 20 mm Double Tee Unit • Initial spalling = 10 mm Drift related spalling • Bearing (calc = 8mm) = 10 mm (example) • Therefore remaining seating to Initial spalling permit elongation, rotation + further (example) Construction tolerance (portion of) spalling: 75 – 20 – 10 – 10 = 35 mm SUPPORT BEAM ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review Calculation of Elongation Plastic beam rotation (L p = 0.5*d/2) col * L / (L - h col - L p ) θ p = θ p = (1.39-0.6)*10500/(10500-800-335) = 0.89% Beam elongation d el = 2.6 * θp/2 * (d - d’) ≤ 0.036hb (For reversing plastic hinges ) = 2.6*0.0089/2*(800-65) (U) = 8.5 mm => 9 mm ≥ 0.005h b (R) = 4.2 mm => 4 mm (Restrained taken as ½ of reversing) ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – 1. Loss of support review Calculation of Beam Rotation Similar to the hollowcore design example the support rotation is the maximum { S d el + d r1 or d r2 + d el unit } q beam = 1.39% d r1 = ((h b /2)-h l ) *θ beam = (800/2-(115))*0.0139 = 4.2 mm d r2 = h l θ beam =115*0.0139 =1.4 mm ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review How is the spalling calculated ? COVER TO FIRST BAR XX mm + 10mm COVER TO FIRST BAR 25mm + 10mm • Initial Spalling = 10 mm 27mm + Additional spalling at limiting drift of 1.39% • Unit (27 -10) = 17 mm 10mm • Ledge = 5 mm • Total spalling = 32 mm 5mm L.O.S = 1.39% Figure C5E.26 Spalling depths to be considered for Flange hung and web supported double tees ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review Through iteration 35mm of seating is exceeded at a total inter-storey Initial seating = 75mm drift of 1.4% components of loss Construction Tolerance = 20mm Bearing = 10 mm • Unit Spalling = 17 mm Initial Spalling = 10 mm • Ledge Spalling = 5 mm • Beam elongation = 9 mm • Beam rotation = 4 mm 35 mm L.O.S = 1.4% ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review Through iteration 45mm of seating is exceeded at a total inter-storey Initial seating = 75mm drift of 1.7% components of loss Construction Tolerance = 20mm Bearing = 10 mm • Unit Spalling = 17 mm Initial Spalling = 10 mm • Ledge Spalling = 21 mm Extra investigation – tolerance reduced • Beam elongation = 11 mm • Beam rotation = 5 mm L.O.S = 1.7% 45 mm With suitable site investigation 20mm tolerance may be able to be reduced to actual site tolerance e.g. for the above example with a site measured 10mm variance on seating (75mm +/- 10mm) ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review Now repeat the L.O.S for rest of the floor • The L.O.S support calculation is easily put into a spreadsheet and once set up with the project specific geometry and spalling parameters can be repeated for the rest of the floor and levels relatively quickly ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Floor Example – Loss of Support Review Results for L.O.S for Building Summary A* Results repeated for the other units gives; Case A Adjacent Unrestrained Hinge • L.O.S = 1.4% drift Case B Adjacent Restrained Hinge • L.O.S = 1.5% drift Case C Internal Unit • L.O.S. = 1.7% drift These are the limiting drifts for the loss of support due to spalling or elongation Now you need to investigate the Birdsmouth Support detail and possible failures to see if these are critical for limiting drift *Note: Highlighted Units – the topping delaminates so is a special case A or B ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Loss of Support NBS Rating How to use Limiting Drift for Rating of %NBS Total required seating at calculated building drift demand 3.2% ULS design drifts are factored by 2.0 in the Yellow Book to get drift demand Summary [Note: this is different to NZBC = d ULS x 1/SP x 1.5] Elongation and Rotation 37.8 mm Building Drift Demand d ULS x 2 = 3.2% Unit Spalling 35.0 mm Ledge Spalling 29.5 mm Shrinkage 0.0 mm Limiting Drift Calculated = 1.4% Construction Tolerance 20.0 mm Bearing Length 10.0 mm Correct Rating Method Total 132.3 mm Limiting Drift / Building Drift Demand TOTAL SEATING REQUIRED 132 mm 1.4 / 3.2 = 44% NBS AS PER C5 GUIDELINES Incorrect Rating Method Seating Provided 75 mm Seating Provided / Seating Required Figure: Total required seating at calculated demand building drift using 75 / 132 = 57% NBS C5E Methodology The NBS rating is based on a limiting drift to loss of support i.e. the methodology assesses the amount of drift required before loss of precast support – this is a bi-linear function and gives a different answer than simply comparing against the total seating that would be required for design. ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Effect of L.O.S Due to Spalling and Elongation ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
No Spalling and After Spalling ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Birdsmouth / Loop Bar Review
Double Tee Example – Birdsmouth / Loop Bar 350x2400 Double Tee Example • Span = 10000mm Unit Width = 2400mm • Topping = 75mm f’c = 20MPa => f’cp = 30 MPa • Double Tee f’c unit = 45MPa • “Pigtail” Loop Bar Detail – 3 R12 Bars / fy = 275 MPa => fyp = 324MPa Loading Gravity [G unit = 2.5 KPa] + [Topping = 1.8 KPa] + [SDL = 0.5KPa] = 4.8KPa Live Load = Q= 3.0 KPa , Y E = 0.3 Total G + Y E Q + Eu = 5.7 KPa Load per Unit = 5.7*2.4*10 = 136 kN Load per web / Pigtail = 136/4 = 34 kN ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Birdsmouth / Loop Bar Loop Bar Hanger damage being investigated post Christchurch EQ ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Birdsmouth / Loop Bar Possible Failure Modes There are a number of possible failure modes / crack patterns that can occur at the birdsmouth connection. The failure in fig C5E 29 is a combination of – a, b, c, d, e & f. and the designer needs to be satisfied that they have investigated all likely modes in the failure analysis. Note: For the double tee supported in the elongation zone (immediately adjacent column) topping delamination is likely to occur and the check is for the precast portion (t-t topping ) only C5E 29 Example demonstrated in fig C5E.29 ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
Double Tee Example – Birdsmouth / Loop Bar Tension Shift ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS
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