CHAPTER 6 Serviceability Criteria

CHAPTER 6: SERVICEABILITY CRITERIA

Deflection and Fatigue Three elements of the design matrix that are related to the performance of a bridge structure: Deflection Slenderness and Flexibility of structure Fatigue Deflection and Depth-to-Span Ratios - are the key to providing bridges with durable riding surfaces as well as comfort to the occupants of moving vehicles. 1871- Phoenix bridge company, limited the deflection due to passage of train & locomotive at 30 mph to 1/1200 of the span. 1905, the AREA specifications provided:  Pony trusses and plate girders shall preferably have a depth not less than 1/10 of the span.  Rolled beams and channel used as girders shall preferably have depth not less than 1/12 of the span  When ratios decreased, flange should have a proper increase. Modified Depth-to-Span Ratios

2 reasons for limiting deflection for Railroad Bridges(ASCE): To avoid excessive vibration of the structure in resonance with the recurring hammer blows of the locomotive driving wheels. To avoid objectionable oscillation

of the rolling stock induced when the

deflections of the successive spans tended to set up a harmonic excitation of the sprung weight.

Reasons for limiting deflection for Highway Bridges(ASCE): 1. To avoid undesirable structure effects, including 

Excessive deformation stresses in secondary members or connections resulting either from the deflection itself or induced rotations at joints or supports.



Excessive dynamic stresses of the type considered in the design by the use of conventional “impact” factors.



Fatigue effects resulting from excessive vibration.

2. To avoid undesirable psychological reactions by 

Pedestrians, whose reactions are clearly consequence of the motion of the bridge alone.



Passengers in vehicles, whose reactions are affected as a result of the motion of the vehicle in the combination with the bridge, or by the motion of the bridge when the vehicle is at rest on the span.

Deflection-to-span and the depth-to-span ratios are not independent but are related by the ff. expression (Wright & Walker,1971):

Deflection requirements Calculated deflections of structures have often been found to be difficult to verify in field, because many sources of stiffness are not accounted for in calculations.

Depth-to-Span Ratio Requirements Steel Members Reinforced concrete members

Deflection due to Dead Load and Camber No limitation is set for the deflection caused by Dead Load in the AASHTO specifications. An acceptably level floor can be achieved in many ways: 1. Providing a slab of varying thickness over sagged beams 2. Providing stiffer beams to minimize deflections 3.Shoring the beams before concrete placement 4. Cambering the beams While specifying camber, the following rules should be observed(AISC1989): A single minimum value for camber, within the ranges shown for the length ordered, should be specified. Camber should be specified by the ordinate at the midlength of the portion of beam to be curved. Camber will approximate a simple regular curve nearly at full length of the beam or bet. any 2 specified pts. Camber is measured at the mill and will not necessarily be present in the same amount in the member as received. P ermissible variations for camber ordinates are given in table 5.5 Deflection Calculation Deflections are calculated for service load conditions by using the elastic design formulas that can be found in texts on strength of materials and handbooks. 1. Dead load Deflections •

Steel members

Composite members Steel –concrete

composite beams may involve shored and unshored construction.

Shored construction, the composite section resists both dead and live load. Reinforced concrete members Two types of deflections should be considered for serviceability of reinforced concrete members: (1) Immediate deflection are defined to be those that occur during the normal service life of the member as a result of sustained load. (2) Long-term deflection Prestressed concrete members Deflection calculations for prestressed members involve several steps owing to the presence of prestressing force. The beam would generally have a net initial upward deflection, generally referred as “camber”, resulting from a combined effect of dead load and the prestressing force, before the application of live load.

Live-load deflection •

Should be computed for the governing loading conditions – truck or lane- by the usual methods of computing deflections.

•

For composite beams: – n, modular ratio – Ic, Moment of inertia of composite sections

•

H20 loading: – Deflection should be calculated by placing the 16-kip wheel at the midspan, with the 4-kip wheel at 14 ft away from it.

•

HS20 loading: – Placing the two 16-kip loads symmetrically about the center line of the stringer: the 4-kip load can be placed at 14-ft away from one of the 16-kip loads.

For the two symmetrically placed loads,(case 9 of references AISC 1989):

For the 4-kip load placed away from the center,(case 8 of AISC.1989):

Fatigue in Steel Bridges Fatigue is a phenomenon of reduced material resistance under fluctuating stresses; a fracture of structural components caused by a number of load fluctuations or reversal. ASTM E206-62T defines fatigue as the process of progressive localized permanents structural change occuring in a material subjected to which produce fluctuating stresses and strain at some point or points. Mechanism of fatigue failure: Steel •

Fatigue is commonly classified as: High-cycle – as the fatigue caused by more than 10 000 cycles. Low-cycle – a fatigue that occurs within 10 000 cycles.

Cyclic Stresses are known to initiate and then to propagate cracks in structural members, eventually weakening them. Fatigue crack is characterized by the absence of apparent large deformation prior to failure, which also makes its timely discovery difficult.

Classification of Fatigue Fatigue Failure is not simply a behavior under fluctuating load alone: it is also highly influenced by such factors as initial discontinuities, residual stresses, variable stress cycle, and connection details. Secondary stresses - are caused by the interaction of various structural components in response to the live load. - have little effect, and therefore not considered in design. - they contribute to fatigue failure of main members, secondary bracing members, joints. Fatigue Strength of Steel: Test Results 1. Up to tensile strength of 200 ksi, the fatigue limit of steel is about 50% of tensile strength. 2. Fatigue strength of steel increases with tensile strength if steel; the higher the tensile strength, the higher the fatigue strength. 3. The Fatigue strength decreases with increasing number of loading cyckes 4. The presence of a weld in a specimen reduces its fatigue strength considerably. The decrease in fatigue strength is greater for steels with greater tensile strength. 5.

For the same maximum stresses, the number of stress cycles required to produce failure increases with increasing stress ratio.

Design Approach For the “ safe-life” design to be successful, sveral elements of the design matrix have to be considered: – The modes and frequencies of working loads during the whole operational life of the structure. – The size and shape of all components and their connections, which must be planned as to provide sufficient reserve strength. – Systematic fatigue test of various structural groups – Additional measures of environmental effects on the fatigue strength. For the Fail-Safe design approach, the elements of the design matrix that need to be considered are the ff:

– Timely inspection of the structure and its parts. – Arresting a crack before it can propagate completely through a life-important structural part – Making additional stress paths available & thus guarding against damage from failure of one structural member by providing structural redundancy. 4 Design approaches are currently in use to prevent fatigue failure: 1. Fracture mechanics approach 2. Strain-life approach 3. Stress-life reduction-factor approach 4. Stress-life detail-category approach 1. Limiting the stress range to acceptable levels. 2. Choice of an appropriate connection detail that would

introduce

minimum stress concentration effect and secondary stress. AASHTO Fatigue Design Approach 1940 – AREA &AASHTO use the AWS bridge specifications for welded structures, which specified maximum allowable stresses in relation to the stress ratio for three Load conditions: 100 000 cycles

600 000 cycles

2 000 000 cycles

1965 – new specifications for fatigue design were adopted. These specifications provided for maximum allowable stresses for various connection and fabrication details for 3 load cycle conditions: 100 000 cycles

500 000 cycles

2 000 000 cycles

Redundant-load-path structure is a bridge whose configuration has multiple load paths, such as a multigirder bridge or a truss bridge with multi-element eye bars. Nonredundant-load-path structure are structure configured such that failure of one structural element would cause the entire structure to collapse. Various kinds of Stress parameters: 1. Maximum Stress, Smax 2. Minimum Stress, Smin 3. Average Stress, Savg = ½(Smax + Smin)

4. Stress range, Sr = Smax - Smin 5. Stress ratio, R = Smax / Smin Stress Category: 1. Stress Category A consists of plain material: base metal with rolled or cleaned surfaces or good quality flame-cut edges with ANSI smoothness of 1 000 or less. 2. Stress Category B covers built-up members and such connections as a. Continuous longitudinal filled or groove welds b. Full penetration transverse grove welds ground flush c. Tapered splices with the weld reinforcement removed d. 24-inch-radius curved transition for flange plates or groove welded attachments e. Bolted Joints 3. Stress category B’ covers bare metal and weld metal in coverplated rolled or built-up members connected by full-penetration groove welds parallel to the direction of the applied stress. 4. Stress category C covers: a. transverse stiffness or attachments. b. full-penetration transverse groove welds with the reinforcement not removed. c. 6-in. radius curve transitionfor groove welded attachments. d. Stud-type shear connections. 5. Stress category D: a. 4-in attachments b. 2-in radius curved transitions for groove or filled welded attachments c. Riveted joints. 6. Stress category E a. Ends of cover plates fillet-welded to flanges not greater than 0.8-in thick b. Attachments longer than 4 in. c. Intermediate longitudinal fillet welds d. Fillet-welded lap joints 7. Stress category E’ 1. Ends of cover plates fillet-welded to flanges greater than 0.8 in. thick 2. Girder flanges greater than 1 in. thick that pierce through the web of another girder and are fillet-welded to each side of the web.

8. Stress category F covers only shear stress on the throat of fillet welds and applies to continuous or intermittent longitudinal or traverse fillet welds. Application of Fatigue Specifications to Design The loadings, lateral load distribution factors, and impact factors used for computing the stress range for fatigue check are the same as those used for the nominal strangth considerations. AASHTO 10.3.2 mandates the ff: 1. The number of cycles of maximum stress range to be considered from AASHTO table 10.3.2A unless traffic and loadometer survey or other considerations indicate otherwise. 2. Allowable fatigue stresses shall apply to those group Loadings that include live load or wind load. 3. The number of cycles of stress range to be considered for wind loads in combination with the dead loads is to be 100 000 cycles, except for structures where other considerations indicate a substantially different number of cycles. Fatigue in Reinforced and Prestressed Concrete Bridges: 3

components of Fatigue strength of concrete structures:

a) Concrete

b) Steel

c) Combination of the two

Concrete To determine for a fatigue life of ten million cycles, the ACI Committee on Fatigue recommends the ff. expression: Modified(Tepfer and Kutti)

Reinforced concrete members ACI Recommended the ff. formula for safe fatigue strength of reinforcing bars more than two million cycles: