© arema 2016® sdrdt site is characterized by challenging geotechnical and seismic conditions....

Application of Large Diameter Circular Reinforced Concrete Filled Tube (RCFT) Shaft Foundationsfor Seismic Design of Railroad Bridges

Authors

Ebrahim Amirihormozaki, Ph.D., P.E.Kleinfelder, Inc. 550 West C Street, Suite 1200San Diego, CA 92101(619) [email protected]

Kelly Burnell, P.E.Kleinfelder, Inc. 550 West C Street, Suite 1200San Diego, CA 92101(619) [email protected]

Nathan Johnson, Ph.D., P.E.Kleinfelder, Inc. 550 West C Street, Suite 1200San Diego, CA 92101(619) [email protected]

Number of Words3867

ABSTRACT

This paper presents a procedure to model, analyze, and design a railroad bridge with Large Diameter Circular Reinforced Concrete Filled Tube (RCFT) shaft foundations for seismic loads. A case study recently designed in Southern California is presented. The cross section of RCFT shafts consists of permanent steel casing filled with a reinforced concrete core that behaves as a composite section. The steel tube not only enhances ductility capacity, stiffness, flexural capacity, and shear capacity of the shaft,but also can aid in constructability and reduce hydraulic and environmental impacts. Despite extensive research studies on large diameter RCFT foundations, they have not yet been considerably implemented in the contemporary design of railroad bridge foundations. Application of RCFT shafts for the seismic design of this case study using AREMA performance-based design method alleviated the extensive seismic demand on the foundations due to the high seismicity of the area combined with liquefaction and scour hazard. RCFT shafts were found to have significant cost savings relative to the ground improvement alternative.

REINFORCED CONTRETE FILLED TUBE INTRODUCTION

RCFT consists of permanent steel tube/casing filled with reinforced concrete. Permanent steel casing serves to both provide concrete confinement and add strength capacity. The confined concrete core provides axial and flexural capacity. The steel casing provides capacity improvement which can lead to smaller diameter shafts compared to similar capacity conventional reinforced shafts. Permanent steel casing also acts as formwork for the shafts eliminating caving soil issues during construction. Provided bond is developed between the tube and the concrete; the section behaves as composite with improved flexural, buckling, shear, and ductility capacity. Small diameter RCFT have been implemented in structural skeletons to enhance lateral and axial capacity. Extensive research and experimental studies have been recently performed on large diameter RCFT and were found to provide outstanding

626 © AREMA 2016®

performance as columns/foundation shafts under lateral forces (Roeder et al., 2010, 2012; WSDOT Memorandum, 2012). Following application of steel jacketing and reinforced concrete steel tubes to the bridge piers damaged during the Hyogo earthquake in Japan, Endo et al. (2000) performed experimental studies to investigate their mechanical properties and found out that ribbed steel tubes produce higher strength and ductility relative to smooth conventional Concrete Filled Tubes (CFT). This was due to a better interaction between steel tube and concrete core in developing composite action.

Motenjo et al. (2012) demonstrated that moment curvature analyses based on strain compatibility and equilibrium can be used to predict the flexural response of RCFT members and also proposed strain-based limit states based on experimental results. Prior to availability of extensive RCFT testing, Roeder et al. (2012) gathered a large database of CFT component testing. A plastic stress method was developed calculate the flexural strength and combined axial-flexural strength of CFT and RCFT cross-sections. In addition, Roeder et al. (2012) concluded, even if the tube and internal reinforcing ratios are equal, the tube will contribute more than 50 percent of the strength.

Although this concept has been implemented in highway bridge foundations in several western states; it has also been implemented on a few railroad bridge foundations. This paper presents the challenges and site conditions that led to the application of RCFT on a railroad bridge crossing of the San Diego River in San Diego, California. It also describes the design procedure used for the performance-based design of this bridge.

PROJECT BACKGROUND

The San Diego River Bridge Double Track (SDRDT) Project will add a 0.9-mile segment of double track to the existing Los Angeles-San Diego-San Luis Obispo (LOSSAN) Corridor in San Diego. The centerpiece of the SDRDT project is an approximately 900-foot-long double track bridge crossing the San Diego River. The existing San Diego River Bridge (BR263.8) is an 11-span steel through girder bridge (date stamped 1914) with several seismic deficiencies (Figure 1). Existing piles do not have sufficient vertical capacity and may experience liquefaction induced settlement during an earthquake and may be affected by lateral spreading, leading to potential collapse. Based on available borings near the bridge, deep foundations are recommended to extend at least 100 feet deep to denser materials. The existing BR 263.8 is to be demolished and replaced by the double track bridge.

Figure 1: Photographs of (A) South Approach and (B) South End of Existing San Diego River Bridge

Replacement Bridge

The superstructure choice for the SDRDT bridge is comprised of twin, simply-supported 7-span steel through girder structures with a maximum span length of 142 feet and a total length of approximately 918

(B) (A)

© AREMA 2016® 627

feet. While the bents are skewed at 5 degrees to align with the river flow direction, the superstructure is not skewed; this provides a geometric skew of the piers but maintains a normal steel structure to simplify fabrication and erection.

The superstructure is supported on 6 two-column bents with a 5-degree skew. The columns and supporting piles are 9-foot Type I Cast-in-Drilled-Hole (CIDH) piles with 1½-inch-thick permanent steel casing on the pile and the short above-ground columns. They are connected to the bent cap through a 4-inch-deep, 7-foot-diameter reinforced concrete reduced section. The section is reduced to satisfyCaltrans Seismic Design Criteria (SDC) of minimum bent cap width for adequate joint shear (SDC 7.4.2.1) while minimizing the bent cap width impact to hydraulic flow to ensure a no-rise condition.

The superstructure is supported on reinforced concrete seat-type abutments. Each abutment is supported on 60-inch-diameter conventional CIDH piles. The abutment areas will receive ground improvement to mitigate lateral spreading concerns.

A new Mid-Coast Light Rail Transit (LRT) bridge is planned parallel to the replacement SDRDT bridge with a horizontal clear distance between the structures of 4’-3” (Figure 2). Coordination of design and construction concerns was required between these two projects. The main seismic design challenge was to ensure that the seismic movement of both bridges in the transverse direction does not result in impact during an earthquake event. All three foundation elements, the two columns under the rail bridge and the single column of the LRT bridge, are in line and, therefore, the group effects of the foundations significantly reduces the lateral soil resistance of the piles.

Figure 2: Bent Elevation of SDRDT and Mid-Coast LRT Bridges

Site Geotechnical Evaluation

The SDRDT site is characterized by challenging geotechnical and seismic conditions. Liquefaction triggering was evaluated for the three design earthquake levels 1, 2, and 3 peak ground acceleration

SDRDT (LOSSAN) Bridge Mid-Coast LRT Bridge

628 © AREMA 2016®

(PGA) values. The site is underlain by approximately 71 feet of liquefiable soil interbedded with soft cohesive soils, then competent layers of sand/gravel/cobble and other sedimentary formations (Figure 3).Site access is difficult due to environmental conditions and restrictions. The Rose Canyon fault, which is capable of generating an approximately M7 earthquake, is less than 1 km from the site, so the design ground motions are severe (Figure 4). The AREMA survivability earthquake PGA is 0.53g. It should be noted that ground motion levels I, II, and III with the average return period of 100, 500, and 2400 years, respectively, were defined and implemented in the seismic analysis of SDRDT bridge.

Soil Properties Layer Depth

Shaft Elevation

Non-liquefiable Sand (Reese) 5 ft

Liquefiable Soft Clay (Matlock) 23 ft

Non-liquefiable Stiff Clay 15 ft

Liquefiable Soft Clay (Matlock) 28 ft

Non-liquefiable Sand (Reese) 69 ft

Figure 3: Pier Elevation and Soil Properties for Survivability Limit State

Figure 4: a) Design Acceleration Response Spectrum; b) Design Displacement Response Spectrum

The site is also prone to seismically-induced liquefaction and lateral spreading, which can be severely damaging to foundations if left unmitigated. Large diameter CIDH piles are proposed that would need to penetrate into the competent sedimentary formation at depth to provide sufficient resistance to axial and lateral loads. Ground improvement to mitigate liquefiable and compressible soils is proposed beneath abutment slopes.

9ft Diameter

8ft Uncased

Spectral Displacement, Sd (inches)

© AREMA 2016® 629

CHALLENGES

As noted previously, the replacement bridge will be founded through a deep layer of liquefiable soil. For bridges, the severe potential consequences of liquefaction are bearing capacity loss, loss of lateral soil stiffness, and lateral loading on foundations from lateral spreading/slope instability. Post-installed ground improvement was originally proposed at all pier locations to mitigate liquefaction. Ground improvement was used for an LRT project on the east of SDRDT bridge in 1998. However, soil mitigation would be expensive due to the significant volume that would be required. Therefore, the design team extensivelyexplored structural solutions that could be advantageous and lower cost.

The geotechnical study for the project indicated that steel casing (permanent or temporary) should beused to prevent caving of the loose and soft soils below the ground water surface during construction. Based on seismic analysis and design of the bridge substructure, it was determined that a permanent steel casing could increase the stiffness and capacity of the foundation to the extent that no soil mitigation at piers was required to alleviate liquefaction hazard. Steel casing also reduced the friction on the outer surface of the piles and consequently, lowered seismic downdrag forces on the piles. This benefit was found to offset the reduced skin friction for service loading. Another benefit of the stiff cased foundation was to limit the longitudinal deflection of the superstructure due to longitudinal braking force to one inch as dictated by AREMA (2014). This limit would not have been achievable with the same diameter uncased pile.

Moreover, the permanent steel casing of the piles extended to the bent cap provides sufficient capacity protection for the high shear demands in the column. The steel casing will terminate just below the bent cap where the section is reduced. Extending the casing to the cap ensures that the shafts have enough shear capacity, while eliminating the need for a double rebar cage into the casing, which would be difficult to construct.

Steel casing leads to smaller diameter shaft which results in a more efficient hydraulic condition for the channel at the bridge location. The hydraulic design criteria was a minimum of one foot of freeboard for the design 100-year event flood, and no rise in water surface elevation compared to the existing bridge. In order to meet this criteria, the maximum diameter of the shafts and width of bent cap is limited to 9 feet.

The 9-foot diameter steel casing is constructible to be extended below ground to the top of the gravel layer which is around elevation -81 feet. An 8-foot diameter uncased reinforced concrete section will be used below this elevation (Figure 3). This section was the target plastic hinging which means no plastic hinging is anticipated to occur in the 9-foot diameter cased section.

Lastly, the cased foundation provides enough stiffness in the transverse direction to prevent any impact between the SDRDT bridge and the adjacent LRT bridge, which have an approximate horizontal clearance of four feet.

SEISMIC DESIGN CRITERIA

Design criteria for the SDRDT bridge was based on to the AREMA Manual for Railway Engineering, 2014Edition. The Caltrans SDC, 2013 Edition, was referenced for seismic design/analysis methodology and seismic detailing of concrete elements.

Based on AREMA, Ch. 9-1.3.3, the bridge was designed for multi-level seismic performance as outlined in Table 1.

Table 1: AREMA Performance Criteria

Limit State Performance CriteriaServiceability (Ground Motion Level I) Structure shall remain elastic.

Ultimate (Ground Motion Level II) Structure may yield, but should remain in use after event.

630 © AREMA 2016®

Damage readily detectable and accessible for repair.

Survivability (Ground Motion Level III) Structure may be extensively damaged, but should not collapse.

Under the serviceability limit state, bridge critical members should remain elastic and permanent deformation is not acceptable in the bridge foundation components. For the ultimate limit state, the bridge may suffer damage provided it is readily detectable and accessible for repair. For the survivability limit state, the bridge may experience significant irreparable structural damage and undergo inelastic deformation that renders the bridge unserviceable/obsolete.

AREMA performance criteria for shaft foundations can be formulated as follows:

, < , , < , , <where , , , , and , are the shaft displacement demand due to earthquake levels 1, 2, and 3, respectively, obtained from an Elastic Dynamic Analysis (Response Spectrum Analysis) for each earthquake level. represents the first shaft yield which is the plastic hinging at the top of shaft where it is accessible for repair; denotes the plastic hinging in the shaft below grade; and is the failure displacement capacity of the shaft. , , and are determined using an Inelastic Static Analysis (Pushover Analysis).

An example of the SDRDT bridge shaft deflection under longitudinal pushover analysis is shown in Figure 5. As seen in this figure, the shaft reaches its yield point at lateral deflection of 21 inches ( =21 ). At this deformation, a plastic hinge forms in the 8-foot diameter section (below the casing).Beyond this point, the displacement capacity increases until the rotational capacity of the plastic hinge is reached. The rotational capacity corresponds to the rebar failure or crushing of confined concrete. The failure point is at the deflection of 82 inches ( = 82 ).

Figure 5: Pushover Analysis of Foundation Shaft in Longitudinal Direction

It should be noted that in addition to the AREMA requirements, Caltrans performance-based design criteria (SDC, 2013) including (but not limited to) local displacement ductility capacity ( ), local displacement ductility demand ( ), and P-delta effects were met.

© AREMA 2016® 631

DESIGN CONSIDERATIONS

Design considerations and requirements for RCFT shaft foundations are presented in the following summary, including methodology for section analysis and section capacity determinations.

1) Section Composite Behavior

The shaft section considered is an RCFT consisting of the permanent steel casing filled with reinforced concrete. Bond stress between the steel casing and the concrete core is a key factor in determining the degree of composite behavior of full section. Per studies on concrete filled tubes with concentric loading, the bond stress approaches zero for diameter to thickness (D/t) of 80 (Zhang et al., 2012). However, eccentric loading of such section significantly increases the bond stress. Roeder et al. (2012) claims that modest bending moments and some structural connection details dramatically enhance bond stress transfer.

Concrete filled tube sections without any shear connectors can be assumed fully composite when spiral welded tubes are utilized as steel casing. This cannot be completely valid for the cases with straight seam welded tubes. Shear ring bars at the transition zone of the shaft develop enough bond between concrete and tube to ensure composite action. This transition zone must be taken at least equal to the shaft diameter from the top of shaft. Therefore, for SDRDT bridge, shear rings are attached to the interior face of the tube at the top 15 feet of shaft to produce full composite action of the section for its entire length.

2) Local Buckling Prevention

The ratio of the permanent casing diameter to thickness (D/t) must be limited to prevent any premature local buckling in the steel tube. Requirements for the D/t ratio from various references are as follow:/ 0.11 / AASHTO LRFD 2013, Article 6.9.4.2/ 0.22 / Roeder et al. (2012)/ 0.15 / Roeder et al. (2012), Section Subjected Primarily to Axial Loading

where D and t are the steel tube diameter and thickness, respectively; E and are the elastic modulus and minimum yield strength of steel tube.

3) Steel Tube Corrosion

Per AASHTO LRFD (2013), Article 10.7.5, the effects of corrosion and deterioration from environmental conditions shall be considered in the selection of the pile type and in the determination of the required pile cross-section. AREMA (2014), Article 8-4.4.5.7 indicates an additional steel thickness for corrosion shall be provided. Caltrans (Caltrans MTD 3-1) typically includes a corrosion allowance (sacrificial metal loss) for steel pile foundations as listed in Table 2.

Table 2: Corrosion Rate for Various ZonesCorrosion Zone Corrosion Rate

Soil Embedded Zone 0.025 mm per yearImmersed Zone (salt water) 0.100 mm per yearScour Zone (salt water) 0.125 mm per yearSplash Zone (salt water) 0.150 mm per year

632 © AREMA 2016®

For SDRDT bridge, corrosion protection is not applied to the outer surface of steel casing, therefore, the corrosion sacrificial thickness was calculated for the bridge life span of 100 years and was subtracted from the original thickness. This modified thickness was implemented in the entire analysis and design procedure.

4) Section Effective Stiffness, ( )The effective stiffness of the section was utilized to capture the lateral behavior and deflection of the pile. With railway bridges, this stiffness is also used in the model to obtain the displacement of superstructure due to longitudinal forces. Instead of following formulas, the effective stiffness of the section can be obtained from Section Analysis programs.( ) = + . AASHTO Seismic Guidelines (C 7.6) for Concrete Filled Tubes

( ) = 0.88 + . Alternative Formula by AASHTO Seismic Guidelines (C 7.6) for Concrete Filled Tubes( ) = + Roeder et al. (2010) for Concrete Filled Tubes= 0.15 + + 2 0.9( ) = + + WSDOT Memo (2012)= 0.15 + + + 0.9

In the above equations, the subscripts s, c, and g refer to properties of the steel, concrete, and gross concrete section, respectively, and E, I, and A are the elastic modulus, moment of inertia, and area of the respective materials and sections.

5) Section Analysis

Moment curvature analyses of the shaft sections at the potential locations of plastic hinge formation must be performed using the available cross section properties and the axial dead load including lateral overturning effects. Bents in SDRDT bridge are under double curvature in the transverse direction and under single curvature in the longitudinal direction. Double curvature bending forms plastic hinging at the top of shaft (reduced section) as well as in the shaft below finished grade. Single curvature leads to plastic hinging only in the shaft below finished grade. Approximate location of plastic hinging in the shaft was estimated since it provided the axial force and section properties as the input for moment curvature analysis.

Table 3 illustrates section analysis results of SDRDT bridge RCFT section versus its un-cased counterpart. The un-cased section has the exact reinforced concrete core properties of the RCFT section but with no steel casing. As shown in this this table, 1.5-inch thick steel casing increases the crack moment inertia and plastic moment of the section for more than 400% and 500%, respectively. This boost in stiffness and capacity mitigated the need for larger section or ground improvement of surrounding soil.It was also found that an un-cased 12-foot diameter section could approximately produce the same stiffness as the RCFT. However, larger diameter shafts could lead to a higher cost and also hydraulic issues.

© AREMA 2016® 633

Table 3: Comparison of Section Analysis of SDRDT Bridge RCFT Section and its Un-Cased Counterpart

Parameter RCFT Un-CasedCracked Moment of Inertia ( ) 320.9 75.6Yield Moment ( ) 72,289 21,374Plastic Moment ( ) 140,201 25,611

6) Pushover Analysis

To determine the displacement capacity of bridge bents, pushover analysis (Inelastic Static Analysis) is required. Nonlinear behavior of the elements including shaft section nonlinearity, soil effects, and geometry nonlinearity (p-delta effects) must be included. The SDRDT bridge shafts were analyzed in both longitudinal and transverse directions to develop the pushover curves which graphically represent the nonlinear lateral force-deformation of the substructure. Inelastic Static Analysis of the SDRDT bridge for a single shaft in the longitudinal direction is illustrated in Figure 6. Plastic hinging forms in the 8-foot section at a displacement of 21 inches. In general, the failure branch of the pushover curves is expected to be descending which is not the case for this bridge. That is because the point of maximum bending occurs in the 9-foot section whereas plastic hinging forms in the 8-foot section.

Pushover analysis of a 9-foot diameter section with no steel casing was also performed for comparison purposes. As shown in Figure 6, the shaft would have lower lateral stiffness which means a higher displacement demand would be expected. Higher displacement demand increases the chance of impactbetween the SDRDT and Mid-Coast bridges in the transverse direction. Also, the un-cased shaft reaches the failure point at a lower displacement.

Figure 6: Comparison of Pushover Analysis for the Proposed Single SDRDT Bridge Shaft versus 9-foot-diameter Section with No Casing in the Longitudinal Direction

7) Section Shear Capacity

634 © AREMA 2016®

The steel casing confining the reinforced concrete section significantly increases the shear capacity of the member. AASHTO LRFD (2013), Article 6.12.3.2.2 overlooks the shear capacity of the reinforced concrete section. Therefore, the shear capacity of the section is derived from the shear capacity of the steel tube: = = 0.58 0.5Roeder et al. (2012) presents the same approach to calculate the shear resistance on the section for both RCFT: = = 0.6 0.5whereas WSDOT Memo (2012) takes into account the shear capacity of the concrete section:= = ( + 0.5 )= 0.6 0.5 = 0.0316 if is compression

RECOMMENDATIONS AND CONCLUSION

Research performed on RCFT demonstrates outstanding vertical and lateral performance in terms of increasing the section stiffness, capacity, and ductility. Application of large diameter RCFT for a railroad bridge set on liquefiable soil during a seismic event was investigated and determined to be the most efficient approach. The RCFT concept replaced other more costly alternatives such as improvement of the soil around the deep foundations. RCFT could provide the same structural stiffness and capacity of much larger diameter un-cased shafts contributing less environmental impact and better hydraulic performance. RCFT had the advantage of greater resistance against longitudinal forces including braking and traction, a requirement for railroad bridges, than could be provided by a standard reinforced concrete section. The RCFT also minimized the substructure seismic displacement where less deflection is desirable. RCFT shafts also lower the downdrag force during seismic events because of their lower outer surface friction. As a summary, in the case of the SDRDT bridge RCFT shafts enhanced the bridge service and seismic behavior and performance while reducing the construction cost.

REFERENCES

AASHTO. (2009). AASHTO LRFD bridge design specification, 4th Ed., Washington, D.C.

AISC. (2005). “Specifications for structural steel buildings.” ANSI/AISC Standard 360-05, Chicago, Illinois.

American Concrete Institute (ACI). (2008). Building code requirements for structural concrete and commentary, Farmington Hills, Michigan.

Denavit, M. D. and Hajjar, J. F. (2012). “Nonlinear Seismic Analysis of Circular Concrete-Filled Steel Tube Members and Frames,” Journal of Structural Engineering, ASCE, Vol. 138, No. 9, pp. 1089-1098.

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Endo, T., Shioi, Y., Hasegawa, A., Wang, H.j. (2000). “Experimental study on reinforced concrete filled steel tubular structures,” 25th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 23 - 24 August 2000, Singapore

Motenjo, L., Gonzalez, L., Kowalsky, M. (2012). “Seismic Performance Evaluation of Reinforced Concrete-Filled Steel Tube Pile/Column Bridge Bents,” Journal of Earthquake Engineering, 16:3, 401-424

Roeder, C., Lehman, D., and Bishop, E. (2009). ”Composite Action in CFT Components and Connections,” Engineering Journal, AISC, Vol. 46, No. 4, pp. 229-242

Roeder, C., Lehman, D., and Bishop, E. (2010). ”Strength and Stiffness of Circular Concrete-Filled Tubes.” J. Struct. Eng., 136(12), 1545–1553.

Roeder C. and Lehman D. (2012). “Initial Investigation of Reinforced Concrete Filled Tubes for Use in Bridge Foundations,” Washington State Transportation Center (TRAC), Report WA-RD 776.1.

Zhang, J., Denavit, M. D., Hajjar, J. F., and Lu, X. (2012). “Bond Behavior of Concrete-Filled Steel Tube (CFT) Structures,” Engineering Journal, AISC, Vol 49, No. 4, pp. 169-185.

WSDOT Memorandum (2012). “Structural Design Recommendation of CFT and RCFT for Bridge Foundation,” Washington State Department of Transportation.

Wheeler, A. and Bridge, R. (2011) Flexural Behavior of Concrete-Filled Thin-Walled Steel Tubes with Longitudinal Reinforcement. Composite Construction in Steel and Concrete VI: pp. 225-236.

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© AREMA 2016® 637

AREMA 2016 Annual Conference & Exposition

Project Purposes

Carries Commuter, Amtrak and BNSF Freight rail lines

Construct 0.9-mile segment of second main track

Project Purposes

Carries Commuter, Amtrak and BNSF

-LOSSAN Project-


Project Purposes

11 mile Extension

9 new stations

pj pj p-Mid-Coast Lightrail Extension Project-


San Diego River Bridge

200 ft

Proposed Mid-Coast LRT Bridge

Existing LOSSAN BR 263.8

Existing LRT Bridge

N

Proposed LOSSAN Bridge Double Track

Ocean Beach Bike Path


Typical Section

Mid-Coast Lightrail Bridge New LOSSAN Bridge


Design Criteria for Different Structures

LOSSAN Project AREMA 3 Level Seismic Performance Criteria Site Specific RSA

Mid-Coast Lightrail

Caltrans Seismic Design Criteria

1000-yr Return Period

PGA – 0.42g

Collapse Prevention

PGA (g)

Return Period (years)

Performance

Serviceability 0.13 100 Minor Damage, Structure useable

Ultimate 0.27 500 Inspectable Damage

Survivability 0.53 2400 Collapse Prevention


Response Spectra

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 5 6

PGA

(g)

Period (sec)

ARS Curves

Lightrail AREMA - Level 1 AREMA - Level 2 AREMA- Level 3

638 © AREMA 2016®

AREMA 2016 Annual Conference & Exposition AREMA 2016 Annual Conference & Exposition

River Soil Conditions During Earthquake

Ultimate and Survivability Events – Liquefaction up to 80’ deep Scour is up to 20 feet Slope Stability and Lateral Spreading

50

0

-50

-100


Original Approach – Ground Improvement


Original Approach – Ground Improvement

Ground improvement 90 feet deep

Conflicts with existing foundations

Staging of ground improvement


Existing Trolley Bridge Approach– Ground Improvement

Ground Improvement around Shafts


Alternative Approach – Permanent Steel Casings

Liquefiable Alluvium

Dense Gravels

11’

9’

© AREMA 2016® 639


San Diego River

Approx. $4M Cost Savings


Permanent Steel Casing Advantages

Cracked Moment of Inertia



Cracked Moment of Inertia

0

50

100

150

200

250

300

350

400

Un-Cased W/ Perm Casing

Mom

ent o

f Ine

rtia

(ft^

4)


Permanent Steel Casing Advantages Flexural Capacity

Yield Moment (K-ft)

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000


Plastic Moment (K-ft)

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000



Permanent Steel Casing Advantages Pushover Analysis

Un-Cased

W/ Perm Casing



Shear Capacity

640 © AREMA 2016®



Shear Capacity

0

500

1000

1500

2000

2500

3000

3500

4000

4500


Shea

r Cap

acity

(kip

s)



Other Advantages

Soil caving prevention

Increased section ductility

Lower braking force displacement

Lower downdrag force on piles

Improved hydraulic bahvior


Permanent Steel Casing Concerns Corrosion

Exclude the corrosive sacrificial layer Use protective coating


Permanent Steel Casing Concerns Composite Action

Weld bar rings to inside face of casing


Conclusions Permanent steel casing for deep foundations provides additional cross sectional capacity and stiffness. Permanent steel casing minimize the substructure seismic displacement where less deflection is desirable. Implementation of RCFT into two adjacent project relieved the need for costly ground improvement.


Thank you!

© AREMA 2016® 641

© arema 2016® sdrdt site is characterized by challenging geotechnical and seismic conditions....

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