STRC16 Lateral Forces Bridges Wind Seismic 0716

Structural Engineering Review Course

Lateral Forces: Bridges (Seismic Design)

Lateral Forces: Bridges Structural Engineering Review Course

STRC ©2016 Professional Publications, Inc.

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Bridges: Lateral Forces

Exam Specifications NCEES Specifications Specifications

Brid ridges Lat Lateral eral Force orcess

6 ques uestion tionss (Br (Brid idg ge) / 4 que quesstion ions (Bu (Buil ild ding ing)

A. Lateral Forces (Wind) B. Columns C. Bridge Piers D. Footi Footings ngs


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Lesson Overview •

bridge wind loads

•

bridge columns

•

bridge piers

•

bridge footings


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Prerequisite Prerequisit e Knowledge You should already be familiar with •

statics

•

dynamics

•

structural analysis

•

AASHTO seismic forces


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Referenced Codes and Standards •

AASHTO LRFD Bridge Bridge Design Specifications (AASHTO 2012)


© 2016 Professional Publications, Inc.

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Bridge Wind Loads Typical Bridge Components



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Bridge Wind Loads Types of Wind Loading •

•

wind on structure (WS) •

superstructure

•

substructure

wind on live (WL)



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Bridge Wind Loads Load Combinations with Wind •

•

Strength III

•

Service I

•

1.4 wind on structure

•


•

0.0 wind on live

•

1.0 wind on live

Strength V

•

Service IV

•


•


•

1.0 wind on live

•

0.0 wind on live



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Bridge Wind Loads Height of Structure •

height above low ground or design water elevation of structure or component

•

if height ≤ 30 ft, use the simplified procedure ( V DZ = V B and PD = PB)

•

if height > 30 ft, use the detailed procedure (calculate V DZ and PD)



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Bridge Wind Loads Base Wind Velocity, V B •

100 mph

•

wind velocity for height above low ground or design water elevation ≤ 30 ft

•

if height ≤ 30 ft, use V DZ = V B •

skip equation (AASHTO Eq. 3.8.1.1-1) and a nd use AASHTO Tables Tables 3.8.1.2.1-1 and 3.8.1.2.2-1



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Bridge Wind Loads



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Bridge Wind Loads



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Example Examp le – Bridg Bridge e Wind Wind Loads For a truss bridge with a maximum height of 25 ft what is the design wind wind speed? (A) 50 mph (B) 75 mph (C) 100 mph (D) 125 mph



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Example Examp le – Bridg Bridge e Wind Wind Loads For a truss bridge with a maximum height of 25 ft what is the design wind wind speed? (A) 50 mph (B) 75 mph (C) 100 mph (D) 125 mph

Solution

The simplified procedure can be used. Following the simplified procedure, the design wind speed is equal to the base wind speed of 100 mph. The answer is (C).



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Example Examp le – Bridg Bridge e Wind Wind Loads For a truss bridge with a maximum height of 25 ft, what are the design wind pressures for wind loading perpendicular to the bridge? (A) 0.050 kips/ft 2 (windward), 0.025 kips/ft2 (leeward) (B) 0.050 kips/ft2 (windward), NA (leeward) (C) 0.040 kips/ft2 (windward), NA (leeward) (D) 0.750 kips/ft2(windward), 0.000 kips/ft2 (leeward)



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Example Examp le – Bridg Bridge e Wind Wind Loads For a truss bridge with a maximum height of 25 ft, what are the design wind pressures for wind loading perpendicular to the bridge? (A) 0.050 kips/ft 2 (windward), 0.025 kips/ft2 (leeward) (B) 0.050 kips/ft2 (windward), NA (leeward) (C) 0.040 kips/ft2 (windward), NA (leeward) (D) 0.750 kips/ft2(windward), 0.000 kips/ft2 (leeward)

Solution

According to AASHTO Table 3.8.1.2.1-1 for trusses, the windward design pressure for loading perpendicular to the bridge is 0.050 kips/ft 2. The leeward design pressure for loading perpendicular to the bridge is 0.025 kips/ft2. The answer is (A).



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Bridge Wind Loads Extra Simplified Procedure

If all of the following are true, apply 0.05 kips/ft 2 transverse transverse and 0.012 kips/ft2 longitudinal simultaneously. simultaneously. •

girder and slab bridge

•

no individual span length > 125 ft

•

maximum height ≤ 30 ft



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Bridge Wind Loads Wind Load Application •

•

•

•

•

Multiply design wind pressure, PD, by the exposed area.

Areas that do not contribute to extreme force effect may be neglected.

Wind pressure is uniformly distributed on exposed area. Exposed area includes area of sound barriers, regardless of pressure used to design the sound barrier itself. Exposed area is shown in elevation view perpendicular to wind direction.



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Bridge Wind Loads Design Wind Velocity, V DZ •

design wind velocity at design elevation in mph, from AASHTO Table Table 3.8.1.1-1 •

•

•

 V 30   Z   ln    V B   Z 0 

V DZ  2.5V 0  V0 , Z 0  V 30  •

•

surface condition coefficients

wind velocity at 30 ft above low ground or design water elevation elevation (mph) If not provided, assume V30  V B  100 mph

V B  100 mph

Z = height of component above low ground or design water elevation (ft)



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Bridge Wind Loads



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Bridge Wind Loads Design Wind Pressure, PD 2  V DZ  P D  P B  3.8.1.2.1-1  AASHTO Eq. 3.8.1.2.1-1  V B  PB base wind pressure from AASHTO Table 3.8.1.2.1-1 •

•



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Example Examp le – Bridg Bridge e Wind Wind Loads A truss bridge in open country conditions has an average height of 50 ft. What are the design wind pressures for wind loading perpendicular to the bridge? Use average height for calculation of all wind loading. (A) 0.050 kips/ft 2 (windward), 0.025 kips/ft2 (leeward) (B) 0.055 kips/ft2 (windward), 0.030 kips/ft2 (leeward) (C) 0.061 kips/ft2 (windward), 0.031 kips/ft2 (leeward) (D) 0.075 kips/ft2 (windward), 0.000 kips/ft2 (leeward)



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Example Examp le – Bridg Bridge e Wind Wind Loads Solution

Step 1: Use surface condition coefficients from AASHTO Table Table 3.8.1.1-1. For open country conditions, V 0 = 8.23 mph and Z 0 = 0.23 ft Step 2: Calculate design wind velocity using AASHTO Eq. 3.8.1.1-1.  V 30   Z   ln   V  B   Z 0   100 mph   50 ft  V DZ   2.5   8.23 mph    ln    110.73 mph  100 mph   0.23 ft  V DZ  2.5V 0 



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Example Examp le – Bridg Bridge e Wind Wind Loads Step 3: Calculate windward wi ndward design wind pressures using AASHTO Equation Equation 3.8.1.2.1-1.  V  P D  P B  DZ   V B 

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kips kips   110. 110.73 73 mph mph   2 P D   0.050   0.061 kips/ft 2  f t 1 0 0 m p h   

Step 4: Calculate leeward design wind pressures using AASHTO Equation 3.8.1.2.1.  V  P D  P B  DZ   V B   

P D   0.025

2

kips kips   110. 110.73 73 mph mph 



2

2   0.031 kips/ft

ft 2   100 mph 

The answer is (C). STRC ©2016 Professional Publications, Inc.


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Bridge Wind Loads Minimum Wind Load •

for truss and arch components, 0.30 kips/ft (windward) or 0.15 kips/ft (leeward)

•

for beam and girder spans, 0.30 kips/ft



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Example Examp le – Bridg Bridge e Wind Wind Loads A truss bridge in open country conditions has an average height of 50 ft. If the exposed height is 3 ft, what is the minimum wind loading on the bridge? (For windward, use   0.061 kips/ft 2; for leeward use   0.031  0.031 kips/ft 2.)



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Example Examp le – Bridg Bridge e Wind Wind Loads Solution

Step 1: Convert pressures from previous problem to line loads. Windward: Using P D  0.061 kips/ft 2

 

LL  P D h   0.061

kips 

 (3 ft)  0.183 kips/ft

ft2 

2 Leeward: Using P D  0.031 kips/ft

 

LL  P D h   0.031

kips 

 (3 ft )  0.093 kips/ft

ft 2 



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Example Examp le – Bridg Bridge e Wind Wind Loads Solution (continued)

Step 2: Compare calculated wind loading to minimum wind loading. Windward:

0.183 kips/ft  0.30 kips/ft  minimum governs  Leeward:

0.031 kips/ft  0.15 kips/ft

 minimum governs 

The minimum wind load governs.



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Bridge Wind Loads Vertical Wind Pressure •

0.020 kips/ft2

•

upward on full width of deck, including parapets and sidewalks

•

acts as a line load at the quarter point of the deck width

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acts in conjunction with horizontal wind load

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only applies: •

in Strength III and Service IV load cases (no wind on live load (LL))

•

when wind loading is taken perpendicular to longitudinal axis of bridge



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Bridge Wind Loads Vertical Wind Pressure (continued)



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Bridge Wind Loads Wind on Substructure •

0.040 kips/ft2

•

substructure = everything below the bearings

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acts in conjunction with superstructure wind load

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if applied at angle, break into parallel and perpendicular components

•

only applies: •

in Strength III and Service IV load cases (no wind on LL)

•

when wind loading is taken perpendicular to longitudinal axis of bridge



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Bridge Wind Loads Wind on Substructure



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Bridge Wind Loads Wind on Live Load •

loaded areas that do not contribute to extreme force effect may be neglected

•

acts 6 ft above roadway roadway surface

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only applies in Strength V and Service I load cases

•

if normal to structure, taken taken as 0.010 kips/ft kips/ft line load

•

if taken at an angle, see AASHTO Table 3.8.1.3-1



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Bridge Wind Loads



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SEISMIC FROM HERE ON OUT !!!!


Bridge Seismic Loads For Seismic Zone 1 with SD1 < 0.1 •

design force per AASHTO Sec. 3.10.9.2 •

•

if As < 0.05, design superstructure to substructure connection for 15% of dead load + live load (DL+LL) if As ≥ 0.05, design superstructure to substructure connection for 25% of DL+LL



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Bridge Wind Loads For Seismic Zone 1 with SD1 ≥ 0.1 •

design force per AASHTO Sec. 3.10.9.2 •

if As < 0.5, design superstructure to substructure connection for 15% of DL+LL

•

if As ≥ 0.5 design superstructure to substructure substructure connection for 25% of DL+LL

•

transverse transverse reinforcement detailing per AASHTO AASHTO Sec. 5.10.11.4.1d

•

transverse transverse reinforcement spacing per AASHTO Sec. 5.10.11.4.1e



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Examp Ex ample le – Bri Bridg dge e Win Wind d Lo Load adss A bridge is located in seismic zone 1 with a design spectral response acceleration at a period of 1.0 sec of 0.1. The dead load is 200 kips and the live load is 300 kips. The acceleration coefficient is 0.05 0.05 and the load factor factor is 0.5. What is the design seismic force?



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Examp Ex ample le – Bri Bridg dge e Win Wind d Lo Load adss Solution

For a bridge in seismic zone 1 with SD1 = 0.1, design forces are per AASHTO Sec. 3.10.9.2. From AASHTO Sec. 3.10.9.2 for As = 0.5, design superstructure to substructure connection for 25% of DL+LL



EQ  0.25 DL    eq   LL 



  0.25  200 kips   0.5  300 kips ips   87.5 kips The answer is 87.5 kips.



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Bridge Wind Loads Spiral detailing per AASHTO Sec. 5.10.11.4.1d •

•

•

column core is confined in expected plastic hinge region. maximum yield strength on spiral reinforcement = yield strength of longitudinal reinforcement volumetric ratio of spirals, ρs, must satisfy both: strength design per AASHTO Sec. 5.7.4.6  f c '   AASHTO Eq. 5.10.11.4.1d-1 seismic design per  s  0.12  5.10.11.4.1d-1  y  within hinge zones, all splices are made by full-welded splices or by full-mechanical connections •

•

•



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Bridge Wind Loads Hoop Detailing per AASHTO Sec. 5.10.11.4.1d •

•

•

column core shall be confined in expected plastic hinge region maximum yield strength on spiral reinforcement = yield strength of longitudinal reinforcement total gross sectional area of hoop reinforcement, Ash, must satisfy either  f '   A g  AASHTO Eq. 5.10.11.4.1d-2 5.10.11.4.1d-2 A sh  0.30shc  c    1  f y  Ac     •

•

 f c '   f   y 

A sh  0.12shc 

AASHTO Eq. 5.10.11.4.1d-3 5.10.11.4.1d-3



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Bridge Wind Loads Hoop Detailing per AASHTO Eq. 5.10.11.4.1d •

s = vertical spacing of hoops, not exceeding 4.0 in

•

Ac = area of column core (in 2)

•

Ag = gross area of column (in 2)

•

reinforcement, including supplementary cross ties Ash = total cross section area of tie reinforcement, having a vertical spacing of s and a nd crossing a section having a core dimension of 2 hc (in )

•

hc = core dimension of tied column in the direction under consideration (in)

•

Ash is determined for both principal axes



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Bridge Wind Loads Confinement Transverse Reinforcement Spacing per AASHTO Sec. 5.10.11.4.1e •

•

provide at top and bottom of column over length not less than the great greatest est of •

•

•

•

maximum cross-sectional column dimension 1/6 clear height of column •

•

extend into top and bottom connections a distance not less than

15 in from face of column connection into adjoining member

spacing not to exceed

18 in •

¼ minimum dimension

•

4 in on center-to-center center-to-center



½ maximum column dimension

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Bridge Wind Loads For Seismic Zone 2 •

same as seismic zones 3 and 4, except •

0.01 Ag ≤ area of longitudinal reinforcement reinforcement ≤ 0.06 Ag

•

column design based on modified design desi gn forces only (per AASHTO Sec. 3.10.9.3)



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Bridge Wind Loads For Seismic Zones 3 & 4 •

•

•

•

•

column requirements per AASHTO Sec. 5.10.11.4.1 longitudinal reinforcement 0.01 Ag ≤ long. reinf reinf.. ≤ 0.04 Ag

•

plastic hinging detailing requirements for confinement

•

transverse reinforcement reinforcement spacing

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splice requirements requirements

•

wall pier requirements

•

column connection requirements requirements

flexural design requirements shear design requirements column shear and transverse reinforcement requirements



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Bridge Wind Loads Column Requirements per AASHTO Sec. 5.10.11.4.1 •

•

•

A vertical support should should be considered a column if the ratio of clear height height to maximum plan dimension ≥ 2.5. Piers typically behave like columns in the weak axis but behave differently in the strong axis. For supports with ratios < 2.5, follow the provisions for piers in AASHTO Sec. 5.10.11.4 5.10.11.4.2. .2.



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Bridge Wind Loads Longitudinal Reinforcement •

0.01 Ag ≤ longitudinal reinforcement reinforcement ≤ 0.04 Ag



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Bridge Wind Loads Flexural Design Force •

flexural resistance factor = 0.9 for either spiral or tie reinforcement

•

modified design forces = elastic seismic forces divided by R-factor •

calculate transverse (trans.) and longitudinal (long.)

•

combine trans. and long. using load combos per AASHTO AASHTO Sec. 3.10.8 combination 1 = 100% long. + 30% trans. combination 2 = 30% long. + 100% trans.



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Bridge Wind Loads Elastic Seismic Forces

Forces resulting from •

simplified seismic forces (15% or 25%)

•

single mode spectral analysis

•

uniform load method analysis



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Bridge Wind Loads



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Bridge Wind Loads Shear Design Force

Take the lesser of •

elastic seismic forces divided by R-factor

•

column plastic hinging forces



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Bridge Wind Loads Plastic Hinging •

size column with modified design forces

•

Procedure •

Calculate Mn.

•

Multiply Mn by the overstrength overstrength factor. factor. (Overstrength (Overstrength factors per AASHTO Sec. 3.10.9.4.3b taken taken as 1.3 for reinforced concrete columns or 1.25 for steel columns.)

•

Calculate shear force corresponding to plastic hinging moment.



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Bridge Wind Loads

V p 

T

 M B

L B

M T  M B   M n



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Bridge Wind Loads Concrete Shear Capacity •

•

If Pu ≥ 0.1 f’ f ’ c Ag, then V c per AASHTO Sec. 5.8.3 (strength design) if Pu ≤ 0.1 f’ f ’ c Ag, then V c varies linearly from the AASHTO Sec. 5.8.3 value to 0 (at 0 compression force)



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Bridge Wind Loads End Regions •

•

•

end regions at top and bottom of column must extend from face of cap/girder/footing cap/girder/footing into the column a distance equal to the greater of •

maximum cross-sectional column dimension

•

1/6 clear height of column

•

18 in

end region at top of pile bent must be the same as for columns (above) bottom of pile bent must extend three times the pile diameter below point of max moment but not less than 18 in above mud line



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Bridge Wind Loads Plastic Hinge Detailing Requirements for Confinement •

same requirements as discussed for seismic zone 1 per AASHTO Sec. 5.10.11.4.1d

Transverse Reinforcement Spacing •

same requirements as discussed for seismic zone 1 per AASHTO Sec. 5.10.11.4.1e



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Bridge Wind Loads Splice Requirements (AASHTO Sec. 5.10.11.4.1f) •

follow AASHTO Sec. 5.11.5

•

lap splices not allowed

•

•

maximum transverse transverse reinforcement spacing around splice is 4 in or ¼ the minimum member dimension full-welded and full-mechanical splices are allowed as long as •

•

adjacent longitudinal bars are not both spliced distance between splices of adjacent bars is greater than 24 in measured along the longitudinal axis



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Bridge Wind Loads Wall Pier Requirements (AASHTO Sec. 5.10.11.4.2) •

•

pier weak direction designed as a column (following column provisions with column response modification factor to determine design forces) for strong axis: •

minimum reinforcement ratio horizontal, ρh, and vertical, ρV , must be ≥ 0.0025 and the vertical must not be less than the horizontal.

•

minimum reinforcement reinforcement spacing (horizontal and vertical) = 18 in

•

shear reinforcement reinforcement continuous and distributed uniformly



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Bridge Wind Loads Wall Pier Requirements (AASHTO Sec. 5.10.11.4.2) (continued) •

Factored shear resistance in pier, V r , is lesser of •

Vr  0.253 f 'c bd

•

Vr  V n •

•



AASHTO Eq. 5.10.11.4.2-1 5.10.11.4.2-1

AASHTO Eq. 5.10.11.4.2-2 5.10.11.4.2-2



Vn  0.063 f 'c   h f y bd

AASHTO Eq. 5.10.11.4.2-3 5.10.11.4.2-3

  0.9

•

Horizontal Horizontal and vertical layers of reinforcement reinforcement are provided on each face.

•

Splices in horizontal horizontal reinforcement must be staggered.

•

Splices in 2 layers must not occur at the same s ame location. STRC ©2016 Professional Publications, Inc.


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Bridge Wind Loads Column Connection Requirements (AASHTO Sec. 5.10.11.4.3) •

•

•

design forces per AASHTO Sec. 3.10.9.4.3 (same design forces as footing) development strength of longitudinal steel = (1.25)(full yield strength) per AASHTO Sec. 5.11 end region transverse reinforcement reinforcement extend from the column into the adjoining member a distance not less than •

half the maximum column dimension

•

15 in



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Bridge Wind Loads Column Connection Requirements (AASHTO Sec. 5.10.11.4.3) (continued) •

Nominal shear resistance, V n , of joint must satisfy •

•

Vn  0.380bd

f c' [for normal weight concrete (AASHTO Eq. 5.10.11.4.3-1)]

Vn  0.285bd

f c' [for lightweight concrete (AASHTO Eq. 5.10.11.4.3-2)]



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Examp Ex ample le – Bri Bridg dge e Win Wind d Lo Load adss Which seismic zones require the calculation of plastic hinging forces for column design? (A) 1 and 2 (B) 2, 3, and 4 (C) 3 and 4 only (D) 4 only



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Examp Ex ample le – Bri Bridg dge e Win Wind d Lo Load adss Which seismic zones require the calculation of plastic hinging forces for column design? (A) 1 and 2 (B) 2, 3, and 4 (C) 3 and 4 (D) 4 only

Solution

Columns in seismic zone 1 are a re designated for a percent of the dead load. Columns in seismic zone 2 are a re designated for the modified design forces only. Columns in zones 3 and 4 are designed for the lesser shear force of the modified design forces or plastic hinging. The answer is (C).



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Multicolumn Bridge Piers Weak Axis

Modified and plastic hinging force calculated in the same manner as columns (per AASHTO Sec. 3.10.9.4.3b). Strong Axis

Use plastic hinging forces, calculated through an iterative procedure (per AASHTO Sec. 3.10.9.4. 3.10.9.4.3c). 3c).



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Example Examp le – Multi Multicolumn column Bridg Bridge e Piers Typically, Typically, the weak axis of a bridge pier corresponds with which bridge axis? (A) longitudinal (B) lateral (C) vertical (D) diagonal



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Example Examp le – Multi Multicolumn column Bridg Bridge e Piers Typically, Typically, the weak axis of a bridge pier corresponds with which bridge axis? (A) longitudinal (B) lateral (C) vertical (D) diagonal

Solution

On a typical bridge without skew, the weak axis of the pier corresponds with the longitudinal bridge axis. The strong axis of the pier will typically correspond with the lateral bridge axis. If a bridge is skewed, then the weak and strong axis of the pier will not correspond with either the longitudinal or the lateral bridge axis. The answer is (A).



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Multicolumn Bridge Piers Strong Axis (hinging forces) forces) per AASHTO Sec. 3.10.9.4.3c 3.10.9.4.3c

Step 1 •

•

Determine column overstren overstrength gth moment resistances. resistances. Determine initial axial load using extreme event load combination with EQ = 0. (This is typically DL only.) only.)

Step 2 •

Calculate shear forces corresponding corresponding to overstrength overstrength moments.



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Multicolumn Bridge Piers Strong Axis (hinging forces) per AASHTO Sec. 3.10.9.4.3c

Step 3 •

Sum column plastic shear forces and reapply to CG of superstructure. superstructure.

Step 4 •

Calculate axial forces in the columns due to overturning overturning with overstrength overstrength plastic moments developed.



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Multicolumn Bridge Piers Strong Axis (Hinging forces) per AASHTO Sec. 3.10.9.4.3c

Step 5 •

Compare step 4 axial forces to step 1 axial forces. •

•

If they are within 10%, stop iterating. If not, recalculate column hinging forces with new axial forces and repeat steps 2–5.



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Determine the design plastic hinging forces at the base of the columns for the following multicolumn pier given the following information. 250 k All dead load: DL  1250  eq

0



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Solution

Calculate the factored dead load corresponding to the extreme event seismic limit state.  P

 1.25

 p DL 

[from AASHTO Table 3.4.1-2]

1.25 1250 1250 k   1563 1563 k 1.25

Determine the overstrength overstrength moment. w M n

 2200 k -ft

Φ M n  1.3  2200 k -f -ft   2860 k -f t



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Solution (continued)

Determine the plastic shear. Φ M n , top  ΦM n , bottom 2860 2860 k -ft -ft  2860 2860 k -ft -ft   381 k V p  hc 15 ft

Calculate the change in axial load due to applied plastic shear. Δ P 

(20 ft)(1143 ft)(1143 k )  (3)(2860 (3)(2860 k ) 30 ft

 476 k



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Solution (continued)

Check within 10%. (10%)(1563 10%)(1563 k )  156 k  476 k  therefore, iterate

Repeat this process P  Po  P  1563 k  476 k  2039 k



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Multicolumn Bridge Piers Design Process •

determine design forces

•

size footing (based on bearing resistance) (AASHTO Sec. 10.6)

•

•

determine thickness based on shear (typically punching shear) (AASHTO Sec. 5.13.3.6) 5.13.3.6) design reinforcement (AASHTO Sec. 5.13.3)



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Multicolumn Bridge Piers Design Forces for Footings

Seismic Zone 1 •

Seismic Zones 3 & 4

same forces as columns

•

Seismic Zone 2 •

elastic seismic forces divided by half of the R-factor from AASHTO Table 3.10.7.1-1 for the the substructure substructure component to which it is attached

lesser of •

elastic seismic forces

•

column plastic hinging forces



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs What dead load + live load (DL+LL) forces are columns in seismic zone 1 designed for? (A) 15% of DL+LL (B) 20% of DL+LL (C) 25% of DL+LL (D) either 15% of DL+LL or 25% of DL+LL



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs What dead load + live load (DL+LL) forces are columns in seismic zone 1 designed for? (A) 15% of DL+LL (B) 20% of DL+LL

Solution

Column design forces in seismic zone 1 vary based on the peak seismic ground acceleration coefficient, As. For As < 0.05, 15% DL+LL.

(C) 25% of DL+LL (D) either 15% of DL+LL or 25% of DL+LL

For As ≥ 0.05, 25% DL+LL. DL+LL. The answer is (D).



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Multicolumn Bridge Piers Elastic Seismic Forces •

•

Use R = 1 for foundations per AASHTO Sec. 3.10.9.4.2. If taking modified forces used for column design, remember to remove the column design R-factor.



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Multicolumn Bridge Piers Elastic Seismic Forces (continued)



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Multicolumn Bridge Piers Column Plastic Hinging Forces •

Use overstrength overstrength hinging forces from from base of columns.

•

Use iterative approach for multicolumn pier in strong axis direction.

Phi Factors •

For extreme event limit state use phi factor of 1, except for uplift resistance of piles and shafts which should be taken as 0.8.



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Multicolumn Bridge Piers Size Footing Based on Bearing Resistance •

•

Take design forces and resolve into an axial load with an eccentricity. eccentricity. Using equations from foundation design lectures determines the footing size that works for allowable bearing pressure (which will be provided in the problem statement).



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Multicolumn Bridge Piers Equivalent Square Column Width (AASHTO Sec. 5.13.3.1) •

•

Circular columns may be treated as square columns with the same area, where 0.886 6D. h = 0.88 Used for •

location of critical sections for moment

•

shear

•

development of reinforcement in footings



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Most nearly, nearly, what is the equivalent diameter of a square column for a 24 in diameter circular column? (A) 21 in (B) 23 in (C) 24 in (D) 26 in



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Examp Ex ample le – Mu Mult ltic icolu olumn mn Bri Bridg dge e Pie Piers rs Most nearly, nearly, what is the equivalent diameter of a square column for a 24 in diameter circular column? (A) 21 in (B) 23 in

Solution h  0.886D   0.886  24 in

 21.26 21.26 in (21 (21 in) in) The answer is (A).

(C) 24 in (D) 26 in



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Multicolumn Bridge Piers Determine Thickness (Based on Shear) •

Check both flexural and punching shear.

•

Typically, Typically, punching shear governs the footing thickness.

•

Critical section is the same as in AASHTO concrete lectures (AASHTO Sec. 5.13.3.6).


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Multicolumn Bridge Piers Design Reinforcing Reinforcing Steel (Based on Moment) M oment) •

Check moment in both directions.

•

Critical section is the same as in AASHTO concrete lecture (AASHTO Sec. 5.13.3.6).

•

Minimum Reinforcement Reinforcement is the same as in AASHTO concrete lecture.

•

Temperature and shrinkage reinforcement minimums apply as well (per AASHTO Sec. Sec. 5.10.8 5.10.8). ).


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Lesson Overview •

bridge wind loads

•

bridge columns

•

bridge piers

•

bridge footings


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STRC16 Lateral Forces Bridges Wind Seismic 0716

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