Double Tee Section Qualitative Assessment of your Floor before you - - PowerPoint PPT Presentation

Double Tee Section

Qualitative Assessment of your Floor before you Start

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

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.

Step 1 - Qualitative Review

• Before you start your detailed

capacity calculations

• Carry out a qualitative review of the

floor and potential damage areas

• btained 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
• n 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

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Design Drawings and Details

Double Tee Example – Ductile Frame Example Parameters

• Ductile frame building with 350 deep

2400 module Double Tee floors

• Beam span

= 10,500 mm

• Beam depth

= 800 mm

• Column width = 800 mm
• Floor unit seated 285 mm above the

beam centreline

• D12-300 starter bars
• Elastic drift

= 0.6%

• Detailed example calculations provided

in the Appendix

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

le = 3600mm D12-300 600 lap D12-300 600 lap

500W x 600D 500W x 800D 500W x 800D

10500mm (typ.)

U R

350 Double Tee 75mm topping 665 mesh 75mm seating

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

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Note: Highlighted Units – the topping delaminates so is a special case A or B to consider

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
• Initial seating

= 75 mm

• Construction tolerance = 20 mm
• Initial spalling

= 10 mm

• Bearing (calc = 8mm)

= 10 mm

• Therefore remaining seating to

permit elongation, rotation + further spalling: 75 – 20 – 10 – 10 = 35 mm

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Construction tolerance (portion of) Initial spalling (example) Drift related spalling (example) Elongation + Rotation

Double Tee Unit SUPPORT BEAM

Double Tee Floor Example – Loss of Support Review

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Calculation of Elongation Plastic beam rotation θp = θp

col * L / (L - hcol - Lp)

= (1.39-0.6)*10500/(10500-800-335) = 0.89% Beam elongation del = 2.6 * θp/2 * (d - d’) ≤ 0.036hb = 2.6*0.0089/2*(800-65) (U) = 8.5 mm => 9 mm ≥ 0.005hb (R) = 4.2 mm => 4 mm

(For reversing plastic hinges) (Restrained taken as ½ of reversing) (Lp = 0.5*d/2)

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 del + dr1 or dr2 + del unit} qbeam = 1.39% dr1 = ((hb/2)-hl)*θbeam = (800/2-(115))*0.0139 = 4.2 mm dr2 = hlθbeam =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 ?

• Initial Spalling

= 10 mm + Additional spalling at limiting drift of 1.39%

• Unit (27 -10)

= 17 mm

• Ledge

= 5 mm

• Total spalling

= 32 mm

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

L.O.S = 1.39%

5mm 27mm 10mm

Figure C5E.26 Spalling depths to be considered for Flange hung and web supported double tees

COVER TO FIRST BAR 25mm + 10mm COVER TO FIRST BAR XX mm + 10mm

Double Tee Floor Example – Loss of Support Review

Through iteration 35mm of seating is exceeded at a total inter-storey drift of 1.4% components of loss

• Unit Spalling

= 17 mm

• Ledge Spalling

= 5 mm

• Beam elongation

= 9 mm

• Beam rotation

= 4 mm 35 mm

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

L.O.S = 1.4%

Initial seating = 75mm Construction Tolerance = 20mm Bearing = 10 mm Initial Spalling = 10 mm

Double Tee Floor Example – Loss of Support Review

Through iteration 45mm of seating is exceeded at a total inter-storey drift of 1.7% components of loss

• Unit Spalling

= 17 mm

• Ledge Spalling

= 21 mm

• Beam elongation

= 11 mm

• Beam rotation

= 5 mm 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

L.O.S = 1.7%

Initial seating = 75mm Construction Tolerance = 20mm Bearing = 10 mm Initial Spalling = 10 mm

Extra investigation – tolerance reduced

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

• nce set up with the project

specific geometry and spalling parameters can be repeated for the rest of the floor and levels relatively quickly

Double Tee Floor Example – Loss of Support Review

Results for L.O.S for Building Summary 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

A*

Double Tee Example – Loss of Support NBS Rating

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

How to use Limiting Drift for Rating of %NBS

ULS design drifts are factored by 2.0 in the Yellow Book to get drift demand [Note: this is different to NZBC = dULS x 1/SP x 1.5] Building Drift Demand dULS x 2 = 3.2% Limiting Drift Calculated = 1.4%

Correct Rating Method

Limiting Drift / Building Drift Demand 1.4 / 3.2 = 44% NBS

Incorrect Rating Method

Seating Provided / Seating Required 75 / 132 = 57% NBS 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.

Figure: Total required seating at calculated demand building drift using C5E Methodology

Total required seating at calculated building drift demand 3.2% Summary Elongation and Rotation 37.8 mm Unit Spalling 35.0 mm Ledge Spalling 29.5 mm Shrinkage 0.0 mm Construction Tolerance 20.0 mm Bearing Length 10.0 mm Total 132.3 mm TOTAL SEATING REQUIRED 132 mm AS PER C5 GUIDELINES Seating Provided 75 mm

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

Birdsmouth / Loop Bar Review

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

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 , YE = 0.3 Total G + YE Q + Eu = 5.7 KPa Load per Unit = 5.7*2.4*10 = 136 kN Load per web / Pigtail = 136/4 = 34 kN

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Loop Bar Hanger damage being investigated post Christchurch EQ

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

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

• ccur and the check is for the

precast portion (t-ttopping) only in fig C5E.29

C5E 29 Example demonstrated

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Tension Shift

e’ mm tension shift

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Elongation Demand: Solve for d to find limiting drift M* = P (d + e + e’) + mPt < MBM.capacity Friction Static (typical ranges)

mf= 1.0 - 1.2 concrete to concrete mf= 0.8 - 1.0 on soft mortar mf= 0.8 - 1.0 on steel mf= 0.6 - 1.0 on bearing strip Note: Be careful some published values of m are lower bound or dependable values i.e. they have a f.o.s or reduction factor for design built in and may be un-conservative for calculation

• f demand.

75mm seating 15mm gap 10mm min seating

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Scenario Review

a) Flexure

a) Yes - check required

b) Topping delamination

a) at loop bar – Yes likely - check b) In unit – Yes - possible as the R6 spirals are unlikely to be sufficient

c) Shear Failure

a) Yes - check required

d) Diagonal tension failure

a) Yes - check required

e) Bond failure

a) At Leg – <600mm anchorage for plain R12 bar in tension – to short to develop b) At bottom hangar bar – ? may be OK

f) Separation of Flange from Web

a) 4 sets of R6 Stirups at 50mm crs at end of unit so failure is likely suppressed but should check

Qualitative Review of Detailing

C5E 29 Example demonstrated

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Qualitative Review of Detailing

30O 65 90 75 50

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Elongation Demand: Solve for d => to find limiting drift Iteration 1 => try max d = (75/2-10) = 27.5mm Design Actions for Review P* = 34 kN (G + Qe + E) P*m = 27 kN (assuming m=0.8) M* = P (d + e + e’) + mPt = 34*[(75/2-10)+(75/2+15)+65]mm + 0.8*34*125mm = 8.3kNm (at max d of 27.5mm) Iteration 2 => try min d = 0mm M* = 7.4 kNm (at d of 0mm) i.e. At initiation of movement / crack 75 P

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

But why can’t I just use simple flexural theory?

Solve for limiting d - take first trial as max = 27mm fMn = fAsfy (d-a/2) f = 0.85 = 11.0 kNm M* = P (d + e + e’) + mPt = 8.3kNm < 11.0 kNm => OK T*N = P / Cos qh = 34 /Cos 60o = 68kN < 110 kN => OK This check may be simple but is un-conservative and does not consider the actual failure modes of the connection. The birds mouth is a disturbed region and you must use an appropriate method i.e. Strut and Tie to assess the failure modes. Further details on the assessment of the failure modes is in Hare et al. (2009)

75mm seating 15mm gap 10mm min seating 65 mm tension shift => e’

q =60o qh =60o

30O 65 90 75 50

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Review the Flexural Mechanism

Horizontal Reinforcing Tie To develop Tjd of 11kNm the tension steel yeilds => for 3 R12 T* = C = Asfy = 110 kN Compression Strut Check Strut / Node Force = 110/Cos30 = 127 kN Node width = 200mm x 20mm (Pigtail bar width) Node Capacity fFn = f*bn*f’c = 0.75*0.80*200*20*30 = 72kN < 127kN =>Capacity Ratio = 0.56 Strut Capacity fFs = f*bs*f’c = 0.75*1.0*200*20*30 = 90kN => Ratio = 0.70 Therefore node failure and concrete crushing may occur at loop bar node before you can fully develop tension steel capacity required for flexure theory.

P T = ASfy C C Concrete Tension

30O 65 90 75 50

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Note: Caution Needed - Concrete Tension Capacity - NZBC

Unreinforced Tie Check Not permitted by B1 / VM1 - NZS3101 Cl 2.3.2.3 Equations for Concrete Tension – Use with Caution! Tension capacity in slab (NZS3101 5.2.4) gives ft = 0.38 l [f’c]1/2 = f x 2.08 MPa (for 30MPa) Cold joint interface - for tension across slab/unit interface (ACI) = 0.25 [f’c]1/2 = f x 1.36 MPa

P T C Concrete Tension

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Review the Flexural Mechanism

Review slab in tension Tension tie required = 127*Sin30 = 63.5kN Assume 50mm at node x 200mm wide Tie capacity = f * f’t*At = 15.6 kN << 63.5kN Ratio = 0.25 ……. Iterating gives a max capacity = 22kN / 0.35 Slab Interface capacity = 10.2 kN << 63.5 kN Ratio = 0.16……. Iterating gives a max capacity = 23kN / 0.36 By iteration you can find S&T mechanisms which may give better results than this initial review

• example. A key consideration is the capacity of the tie is limited by the geometry of the joint

including the pigtail compression node width + amount of spread of strut of 1:2.5 max. Further details on the assessment of the double tee failure modes is in Hare et al. (2009)

Slab Interface

Simplified S&T Model

Reinforcing for Pigtail Tension tie Other failure mode?

Double Tee Example – Birdsmouth / Loop Bar

Review the results for failure mode a) Flexure for this example

• Flexural Capacity derived by S&T < 40% of that provided by simple Flexure theory fMi= Tjd

=>this is quite sensitive to geometry of loop bar and joint.

• Large increase in design actions due to tension from elongation Pmt => actions almost double

– also quite sensitive to support load and friction coefficient chosen

• Tension Shift – once concrete cracks also accounts for another shift approximately additional

30% - 40% increase

• Offset of support due to elongation has less affect and accounts for about another 10% to 15%

increase in load

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Ideal Flexural Capacity fMi = 11kNm MoR Flange = 5.5kNm S&T Capacity = 4.5 kNm 2.7 kNm 27 mm Limit before L.O.S governs 1.8 kNm 7.1 kNm 8.3 kNm Crack may form due to tension Pmt from elongation 6.1 kNm 4.9 kNm

Birdsmouth / Loop Bar – Demand / Capacity Envelope

kNm mm

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Review - Outcome

Strut and Tie Review of Mechanism (a)

• Review of the flexural mechanism with an initial S&T model gives a lower bound limiting drift

sensitive to cracking with d = <5 mm for the birdsmouth after onset of cracking.

• The S&T review indicated however that the unit it may be able to (just) carry gravity loads after

elongation has finished i.e. (Pm loads from elongation no longer action).

• Could investigate geometry further and sliding capacity – important to understand site

constraints and geometry of loop bar and joint Other Failures – mechanism (b), (c), (d), (e), etc

• Need to now complete the review and check other modes to determine critical mode of failure

has been determined. The floor %NBS in this example (a) Flexure will be difficult to rate as a result of the birdsmouth / loop bar connection having insufficient capacity to accept movement that would result in a crack forming and tension (from elongation) across the birdsmouth connection. A detailed further investigation of the loop bar details should be undertaken to help determine the actual as built capacity.

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Scenario Review

a) Flexure

a) Completed – Probably OK

b) Topping delamination

a) at loop bar – possible - check b) In unit – possible as the R6 spirals are unlikely to be sufficient

c) Shear Failure

a) OK

d) Diagonal tension failure

a) Yes – further check required

e) Bond failure

a) At Leg – <600mm anchorage for plain R12 bar in tension – Ld x 2 = 610mm ~ 570 => OK b) At bottom hangar bar – Ldh OK

f) Separation of Flange from Web

a) 4 sets of R6 Stirups at 50mm crs b) Capacity > 34kN => OK

Qualitative Review of Detailing

C5E 29 Example demonstrated

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Review of Double Tees - Summary

Loss of Support – Elongation

• Likely to govern most situations for most 1980s and early 90s buildings which have typical

seating detailed between 30mm to 50mm.

• Failures observed to date Clarendon / Statistics were loss of support.

Birdsmouth Failure

• Likely to govern all buildings at PHZ where delamination of topping occurs
• Likely to govern newer buildings mid 1990s onwards using the loop bar detail that are provided

with a more generous seating of 75 mm or more.

• Be aware of double tees that have the flange trimmed back locally (more common on steel beam

support) with no distributed flange bearing means redundancy is reduced.

• Be aware of long span double tees – high shear load at the birds mouth is problematic
• It can be difficult to assess the capacity and point of failure of the failure of those modes relying
• n concrete in tension. Need to undertake a rational approach and look at likely scenarios.
• Hare et al 2009 covers this topic very well.
• No direct failures– but lots of in service evidence that birdsmouth cracking mechanisms are
• ccurring even under service loads so may be problematic with the right earthquake conditions.

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

What happens at the Plastic hinge regions and corners ?

Topping Delaminates

tf Section capacity is based solely on the double tee flange depth tf for these areas

Double Tee Example – Birdsmouth / Loop Bar

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Delamination of topping

40

40

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Can not hang the unit from the topping

Concrete Tension at Interface (ACI) = 0.25 [f’c]1/2 = 1.36 MPa Capacity 10,000 x 2,400 x 1.36 MPa = 32,640 kN Double Tee Weight 10,000 x 2,400 x 2.4 kPa = 60 kN F.O.S = > 500 Why did this fail ? Not permitted by B1 / VM1 - NZS3101 Cl 2.3.2.3 Assess Birdsmouth and other failure mechanisms and using concrete tension with caution Re-Cast Project => programmed to look at this loop bar issue in existing buildings in more detail with detailed FEM solid modelling and lab tests

Precast Rib and Infill

Failure of Rib and Timber Infill Floors

• Possible rib entrapment under

positive moments

– Casting of ribs into the beam – Haunching final vertical form Seating

• n mortar
• Weak section forms along rib
• Follow

42

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

Assessment of Rib and Timber Infill Floors

• Similar Process to hollowcore

example

• Checking for;
• Spalling and loss of support

due to elongation

• Displacement / drift induced

design actions on the precast member and floor

Flat Slab Floors

Assessment of Flat Slab Floors

ASSESSMENT OF EXISTING PRECAST CONCRETE FLOORS

• Similar Process to hollowcore

example

• Checking for;
• Spalling and loss of support

due to elongation

• Displacement / drift induced

design actions on the precast member and floor

Questions