AASHTO Design and Material Specification

Changes2016

LRFD BDS Section 6, Various Articles

Karl H. FrankChief Engineer

Hirschfeld Industries

Bolts

2

ASTM SpecificationsHigh Strength Bolts

• New Specification Combines 4 Specifications into 1 for both buildings and bridges-F3125– A325 Standard Hex Bolt– F1852 (A325 Tension Control)– A490 Standard Hex Bolt– F2280 (A490 Tension Control)– + Metric

• The old names become Grades

3

F3125 Significant Changes• Grade A325- Fu = 120 ksi for all diameters

(results in an increase in shear capacity for bolts >= 1 in.)

• Annex A1- Table gives permitted coatings and over tapping required for nuts– No hot dip or mechanical galvanizing of Grade

A490 bolts– F1136 and F2833 Zinc/Aluminum Allowed on all

Grades A325 and A490• Rotational Capacity Test in Appendix A2

4

AASHTO LRFD Changes

• Bolt Shear Strength• Slip Critical Categories• Standard Hole Sizes• Girder Field Splice Design

KHF

Shear Strength 6.13.2.7

• Initial Length Reduction– Changed from 0.8 to 0.9– Long Joint from 50 to 38 in.

• Bolts with threads in the shear plane: (web bolts)( old value 0.38)

• Bolts with threads excluded from the shear plane:(old value 0.48)

• The nominal shear resistance of a bolt in lap tension connections greater than 38 in. in length shall be taken as 0.83 times the values above.KHF

Slip Critical Connections

Class Typical SurfaceSlip Coefficient

Old Specification

New Specification

A Mill Scale 0.33 0.30

B Zinc Rich Paint and Blasted 0.50 0.50

C Galvanized 0.33 0.30*

D Organic Zinc Rich - 0.45

KHF

*Do not wire brush the surfaceRequired tension for A325 > 1 in. diameter increased

Footnote on Bolting• New Hole Size

– 1 inch and greater: Standard hole = diameter of fastener +1/8 in.

• Miss drilled holes- fill with fully tensioned high strength bolt (Category B fatigue strength)

• New electric wrenches can be programmed for required turn of the nut

New Connection Design Criteria and Methods

Remove applicability of the 75 percent and average rules in Article 6.13.1 to the design of bolted and welded splices for flexural members.

75 percent rules are applicable to connections and splices for primary members subject to axial tension or compression only.

Clarify application of rules to primary members subjected to force effects acting in multiple directions due to combined loading.

Bolted Field Splices of Flexural Members

Revise general article on design of bolted splices for flexural members implementing new simplified bolted splice design procedure

Removal of check for slip of bolts during erection of steel

• The purpose is implementation of simplified design procedure and more economical field splice designs.

Expensive and Slow to Erect Field Splice

Field Splice92 bolts in each web32 bolts each flangeTotal 312 bolts936 holes

Bolts: 312x$20= $6,240Labor: 312x10 min= 52 field hours each splice

The Problem: Tub Girder Splice

Field Splice36 each top flange80 bolts in each web85 bolts bottom flange634 bolts1,902 holes

Bolts: 634x$20= $12,680Labor: 634x10 min= 106 field hours each splice

Splice Design Procedure1. Design Flange Connection to Develop the Smallest

Design Yield Resistance of the Connected Flanges

Design Yield Resistance:

2. Design Web Connection to Develop the Smallest Factored Shear Resistance of the Connected Webs

Two Rows of bolts minimum on each side of splice.

13

gnyfy

uue AA

FFA

nvr VV

eyffy AFP

Positive Moment Capacity CheckBottom Flange in Tension

Moment Capacity:Pfy for the Bottom Flange x Moment Arm to Mid- Depth of Deck

= (Fyf x Ae ) x A

14

2tt

2tDA s

haunchft

eyffy AFP

Negative Moment Capacity CheckIgnore Tensile Strength of Reinforcement

Moment Capacity:Smallest Value of Pfy x Distance Between FlangeCentroids

= (Fyf x Ae ) x A

15

2t

2tDA fcft

eyffy AF)top(P

eyffy AF.)bot(P

Additional Moment Provided by Web:Factored Design Moment – (Pfy x A) (flange moment)Web Contribution: Hw x Aw (web force at mid depth)

Positive MomentTake Moments about mid thickness of deck

16

2tt

2DA s

haunchw

wH

eyffy AFP

If Moment From Flanges is Not Sufficient

Negative MomentTake Moments about larger flange

2t

2DA f

w

wH

eyffy AFP

Flange with Largest Yield Capacity

Hw = (Additional required web moment) / Aw

17

Number Bolts Required = R / Bolt Capacity

Minimum of Two Rows each side of splice

Normally Maximum Spacing and 2 Row Requirement Controls Web Bolts

2w2

nv2

w2

r HVHVR

Resultant Web Force Moment and Shear

Design Comparisons

18

Girder Depth in.

Required Number of Bolts OneSideNumber of Top Flange

BoltsNumber of Web Bolts

Number of Bottom FlangeBolts

Difference Present-Proposed

7212 36 24

616 22 28

11124 102 28

5624 50 24

80Tub Girder

16 34 54-7

20 28 63

Validation Finite Element Analysis

10

9

916 T H K .

58 T H K.

58 T H K.

916 T H K .

716 T H K .

10

2

33

Sp

a.

@3

inc

h

204 Web Bolts

10

9

916 T H K .

58 T H K .

58 T H K .

916 T H K .

716 T H K .

10

2

24

Sp

a.

@

4in

ch

100 Web Bolts“Simplified Design of Bolted Splice Connections for Steel Girders” – Frank, Ocel, and Grubb 19

20

o Shell element models in Abaquso Adapted fastener models from NCHRP 12-84o Five loading scenarios

– Pure positive moment– Pure negative moment *– High shear (as little moment as possible) *– Proportion design positive moment/shear– Proportional design negative moment/shear *

*= deck not present

FEA Model Description

Results – High Shear

0

500

1,000

1,500

0.00 0.50 2.50

Left

Supp

ortR

eact

ion

(kip

)

1.00 1.50 2.00Left Support Displacement (inches)

Current Method Proposed Method

Vu

3,000

Step 26

21

2,000

2,500Step 26

CURRENT“Simplified Design of Bolted Splice Connections for Steel Girders” – Frank, Ocel, and Grubb

22

PROPOSED

Results – High ShearVon Mises Stresses @ Step 26

CURRENT“Simplified Design of Bolted Splice Connections for Steel Girders” – Frank, Ocel, and Grubb

17

PROPOSED

Results – High ShearBolt Shear Forces @ Step 26

Results – Prop Neg Mom & Shear

0

20

500

1,000

1,500

3,000

2,500

2,000

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Left

Supp

ortR

eact

ion

(kip

)

Left Support Displacement (inches)

Current Method Proposed Method

Vu

Results – Prop Neg Mom & ShearVon Mises Stresses @ Vu

CURRENT PROPOSED21

Results – Prop Neg Mom & ShearBolt Shear Forces @ Vu

CURRENT PROPOSED24

– Application of the new proposed design provisions for bolted field splices will typically result in a few more bolts in the flange splices and significantly fewer bolts in the web splices than under the current design provisions.

– The overall simplification in the design procedure should result in easier interpretation of the provisions, faster and more efficient design of field splices, and more consistent and cost-effective designs.

– Clarifications to the application of the 75 percent and average rules to the design of connections and splices in primary members at the strength limit state subject to combined force effects should also be beneficial to designers.

Anticipated Effect on Bridges:

Result of Changes to Field Splice Design

• Reduced Design Effort and Cost, Lower Connection Costs, & Faster Erection

KHF

A New Day- Another Bridge

29

Tub Girder to End Diaphragm Connection

KHF

Insufficient Hole Spacing to Clear Weld

End Section of Skewed Dapped Tub Girder

KHFThe Problem Area

80 bolts each flange!

2 in. Hole Spacing From Center of Web to Center of Hole

KHF

2 -1/2 in. Spacing

KHF

3-1/2 in. SpacingNow there is Clearance

KHF