AASHTOWare BrD 6.8. BrR and BrD Tutorial. PS12 - Three Span Prestressed I Beam Stirrup Design Example

Transcription

1 AASHTOWare BrD 6.8 BrR and BrD Tutorial PS12 - Three Span Prestressed I Beam Stirrup Design Example

2 PS10 - Three Span Prestressed I Beam Stirrup Design Example (BrDR 6.8) #4 12" 6" 110'-0" 120'-0" 110'-0" 6" Eleation 6" 6" 1'-0" 1'-0" 1'-3" 44'-6" 42'-0" 1'-3" 8" Future Wearing Surace 2" thick, 150 pc AASHTO-PCI BT-72 4'-3" 3 12'-0" = 36'-0" 4'-3" Typical Section Last Modiied: 8/30/2016 1

3 Material Properties Beam Concrete: 'c = 7.0 ksi, 'ci = 5.5 ksi Deck Concrete: 'c = 4.0 ksi Prestressing Strand: 1/2" dia., 7 Wire strand, Fu = 270 ksi, Low Relaxation 9" 2" 4" 15.2" Draped Strands 7" 3" X X Weight = 300 pl Parapet Detail X = t (End Spans) X = t (Mid Span) Strand Pattern or all Spans Strand harped at 48.5' rom end o beam Strand Pattern at Mid-Span Strand Pattern at End o Beam Last Modiied: 8/30/2016 2

4 BrR and BrD Training PS10 - Three Span Prestressed I Beam Stirrup Design Example From the Bridge Explorer open the bridge PCITrainingBridge6 (BID 9). Expand the SUPERSTRUCTURE DEFINITIONS tree, and then expand the Structure Deinition #1. Expand MEMBERS, then expand MEMBER ALTERNATIVES and inally expand Member Alternatie #2. Last Modiied: 8/30/2016 3

5 Open the Shear Reinorcement Ranges to begin work on haing BrD design the shear stirrup ranges. Click on the Stirrup Design Tool button. Last Modiied: 8/30/2016 4

6 The Shear Stirrup Design Tool dialog comes up. Reerring to the dialog box, let us go through the options and explain. Using the design tool causes preiously entered stirrup data to be deleted and replaced by newly computed stirrup data. Distance to irst stirrup This is the distance to the irst stirrup rom the end o the beam. Typically, this will be 2 to 4 inches. Entering a number too high will proide a stirrup range that starts too ar rom the support. This will result in zero ratings and ailed end shear resistance. Not entering a alue will incorrectly put the irst stirrup at the end ace o the beam. In this example, we will enter 3 inches. Maximum jump in spacing between ranges The maximum jump in stirrup spacing that the module assumes rom one range to the next. I this alue is not input, the deault is taken as 24 (i.e. the maximum stirrup spacing). Last Modiied: 8/30/2016 5

7 The eect o leaing this ield blank and using the deault alue o 24 permits a change in spacing rom range to range o any amount (since the maximum allowable spacing is 24 ). This will proide the most eicient stirrup ranges. For our example, we will leae this blank. Oerride maximum allowable spacing Enter a alue or the desired maximum spacing o shear stirrups. I let blank, the sotware will use LRFD Article to compute max allowable spacing. We will let the sotware calculate the maximum shear steel spacing. Number o Stirrup Deinitions The user may set the design tool to use one or two stirrup deinitions. Some beams may beneit by haing more robust steel sizes at the beam-ends where shear resistance is required and less robust steel sizes within the span where the shear resistance is less necessary. Our example will use 1 stirrup deinition. Deinition 1 minimum range length This range is the minimum distance rom end o beam oer which Stirrup Deinition 1 is to be used. This is disabled when one stirrup deinition is being used. The design o the bars is based on the stirrup area entered or each 1/10th point location (and critical distance). When those alues are passed into the design module that calculate the inal ranges, the shear areas or the design are based on the minimum range length and are then carried along as the ranges are being deeloped. The range length is a minimum and the inal range length computed by the stirrup module will always be greater than or equal to this input. Since we are using one stirrup deinition, we need not consider this. Stirrup Entry Method Choose to either use existing stirrup deinition(s) or create new stirrup deinition(s). The user can deine stirrups in the bridge workspace tree or a new deinition can be created using the design tool. We will hae the design tool create a new deinition or us. Extends into deck Check this box i the reinorcing bars extend into the concrete deck. In our design, we will extend the stirrups into the deck. Stirrup Deinition (1 or 2) Under each o the Existing Stirrup Deinition sections, select an existing Vertical Shear Reinorcement Deinition rom the drop list. This is aailable when the "Use existing stirrup deinition" radio button is selected. Last Modiied: 8/30/2016 6

8 New Stirrup Deinition When using the design tool to create a stirrup deinition, this section will be aailable. The user will enter the material and a bar size rom the two drop-down lists. The number o legs is then entered. The inclination is always 90 degrees. In our example, we will use the proided 60 grade steel and will use #4 bars that hae 2 legs. Press Continue ater entering the data or the example. The analysis will run and computed the required stirrup spacing at each tenth point and critical d point. The Stirrup Design Tool Result dialog will appear. Last Modiied: 8/30/2016 7

9 The dialog aboe has a table showing the required stirrup spacing at each analysis location throughout the beam or each span. The user can select the span rom the drop-down list and see the results or each span. Using the results rom the analysis the user can construct the desired stirrup layout scheme by selecting the maximum number o ranges and the number o minimum number o stirrups per range. In our case, we will use the deault three ranges and require we hae at least three stirrups per range. For each span, press Create Ranges and hit Apply. Once completed, Close the dialog. The results are displayed in the PS Shear Reinorcement Ranges dialog. Last Modiied: 8/30/2016 8

10 Look at the schematic o the beam. Prestressed Concrete Control Options or BrR BrD - Consider Splitting Resistance Last Modiied: 8/30/2016 9

11 The user may select this option to consider the splitting resistance o the pretensioned anchorage zone as per LRFD For this example turn on the splitting resistance article in the control options or LRFD and run the analysis. In the spec check, open the LRFD article. As is the area o steel within the distance rom h/4 rom the end o beam. We need to increase the area o steel in this zone. Pru = kips = s x As. As 2.73 in². h/4 = 72 in / 4 = 18 in. We hae 3 #4 stirrups within the distance o h/2 at 6 in spacing. As = 3 x 0.20 in² x 2 legs = 1.2 in² As o each bar = 2.73 in² / 3 = 0.91 in² or 6 in spacing or use 3 #7 bars or As = 3.6 in². As o each bar = 2.73 in² / 5 = 0.55 in² or 4 in spacing or use 5 #5 bars or As = 3.1 in². We will use #5 bars at 4 inches oer 16 inches rom the irst stirrup. We will push the stirrups 1 inch closer to the end o beam to ensure the stirrups all within the h/4 distance. Last Modiied: 8/30/

12 Go back to the Stirrup design tool and enter the ollowing: Last Modiied: 8/30/

13 Set the minimum number o stirrups per range to 5. Create and Apply the ranges to all 3 spans. Last Modiied: 8/30/

14 Rerun the AASHTO LRFD analysis and check the speciication article LRFD Last Modiied: 8/30/

15 Ignoring tensile rating in top o beam Run the AASHTO LRFR analysis or the aboe Control Option not set and set and then compare the speciication LRFR 6A. to see the tensile ratings eliminated. Last Modiied: 8/30/

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17 Flowcharts or PS Stirrup Design Tool Compute PS Losses Compute lexural resistance at supports to compute d distance rom each support At each d point, at each 10 th point, and the location where the irst stirrup deinition ends, compute eectie stress in strands and Vp.Also ealuate the ollowing articles at these points. Vu V p 0.25 ' ( ) c b d Yes Section not adequate, issue message and end No Shear Computation Method? Go to A For General Procedure Compute Vc Go to B For Simpliied Procedure Compute Vc Go to C For Simpliied Vci, Vcw Procedure Compute Vc Go to D to continue the design Last Modiied: 8/30/

18 A. General Procedure to Compute Vc s xe s ( ) x a in <= s xe <= 80.0in g Determine β and θ Article s ( ) M d u 0.5N E s u A V s u E V p A p ps A ps po Where M u >= (V u V p )d The Yes is enorced by the smax equation on page 11. The Design Tool will always use the Yes path. A ' c ( ) b y s Yes No 4.8 (1 750 ( ) s ) (1 750 ) (39 ( ) s s xe ) s ( ) V c ( ) ' b d c Compute Vc Article Last Modiied: 8/30/

19 B. Simpliied Procedure to Compute Vc β=2.0 θ = 45.0 Determine β and θ Article V c ( ) ' b d c Compute Vc Article C. Simpliied Vci, Vcw Procedure to Compute Vc V ci 0.02 ( ) Vi Mcre ' c b d Vd 0.06 Mmax ' c b d Mcre Sc r ( ) Mdnc cpe S nc Article Vcw (0.06 ' c 0.30 pc ) b d Vp ( ) V c = min(v ci, V cw ) Compute Vc Article Yes V ci <= V cw? No pc cot θ = 1.0 cot ' c ( ) Last Modiied: 8/30/

20 D. Now that Vc is computed, continue the design V sreq Vu Vp Vc Vu V p u b d ( ) User input max allowable spacing? Yes smax = user input No A y s max min 0.4d,12.0in, ' cb No u < c Yes A y s max min 0.8d,24.0in, ' cb A yd cot sreq min s max, Vsreq Location at the CL o simply supported bearing? No Create ranges or sreq Yes Ealuate Article on next page Last Modiied: 8/30/

21 Pr 0.04( pbt * Aps) (Article ) ( A s )*( h / 4) sreq min sreq rom pre page, Pr / s s = serice stress in the stirrup steel, limited to 20ksi For I beams: As is the total area o ertical reinorcement within h/4 rom the end o member where h = height o beam. For box beams with no oids or circular oids: As is the total area o horizontal reinorcement within h/4 rom the end o member where h = oerall width o beam. Opis will assume that each ertical leg o stirrup contributes 1 horizontal leg o reinorcement. For box beams: As is the area o ertical or horizontal reinorcement within h/4 rom the end o member where h = min(oerall height, oerall width) o beam. Opis will assume that each ertical leg o stirrup contributes 1 horizontal leg o reinorcement. Pr is ealuated twice: once or the Aps in the bottom lange and the horizontal As and once or the Aps aboe the bottom lange and the ertical As For U beams: As is the area o ertical or horizontal reinorcement within h/4 rom the end o member where h = min(oerall height, oerall width) o beam. Opis will assume that 2 ertical legs o stirrup contributes 1 horizontal leg o reinorcement. Pr is ealuated twice: once or the Aps in the bottom lange and the horizontal As and once or the Aps aboe the bottom lange and the ertical As For T beams: As is the total area o ertical reinorcement within h/4 rom the end o member where h = oerall height o beam. pbt = stress in prestress steel at transer (= jacking stress without losses) Aps = area o prestressing steel, does not take into account the reduced area at the CL bearing due to transer and deelopment length Last Modiied: 8/30/

22 Article or LRFD Design Reiew Ealuate this article at each CL bearing o the simply supported condition s 20ksi No Rect box or U beam? Yes Pr 0.04( pbt * Aps) P r ert 0.04( pbt * Apsert) A sreq Pr s A sertreq P rert s A No Fail sactual A sreq A sertactual A sertreq No Fail Yes Pass P r horiz Yes 0.04( pbt * Apshoriz) A shorizreq P rhoriz s A shorizactual A shorizreq No Fail Yes Pass Last Modiied: 8/30/