Seismic Design Policy 1.0new - Mississippi

SEISMICDESIGNPOLICYMANUALFOR

HIGHWAYBRIDGES

Version1.0December2013

SeismicDesignPolicyManualforHighwayBridges

MDOT Bridge Division Seismic Design Manual (Version 1.0) Page | 2

TableofContents

1.0 General .......................................................................................................................... 3

2.0 Software for Seismic Analysis and Design ...................................................................... 4

3.0 MDOT Modifications to AASHTO Guide Specifications for LRFD Seismic Bridge Design .. 6

3.1 Definitions .................................................................................................................. 6

3.2 Earthquake Resisting Systems (ERS) Requirements for SDCs C and D ....................... 6

3.3 Seismic Ground Shaking Hazard .............................................................................. 11

3.4 Selection of Seismic Design Category (SDC) ............................................................. 11

3.5 Temporary and Staged Construction ....................................................................... 16

3.6 Load and Resistance Factors .................................................................................... 16

3.7 Selection of Analysis Procedure to Determine Seismic Demand .............................. 16

3.8 Local Displacement Capacity for SDCs B and C ........................................................ 16

3.9 End Bents.................................................................................................................. 17

3.10 Foundation – General .............................................................................................. 17

3.11 Foundation – Spread Footing ................................................................................... 17

3.12 Foundation – Pile Foundations and Drilled Shafts ................................................... 18

3.13 Analytical Procedures ............................................................................................... 18

3.14 Procedure 3: Nonlinear Time History Method ......................................................... 18

3.15 Foundation Rocking ................................................................................................. 18

3.16 Drilled Shafts ............................................................................................................ 18

3.17 Longitudinal Direction Requirements ...................................................................... 19

3.18 Liquefaction Design Requirements .......................................................................... 19

3.19 Reinforcing Steel ...................................................................................................... 19

3.20 Concrete Modeling ................................................................................................... 19

3.21 Splicing of Longitudinal Reinforcement in Columns Subject to Ductility Demands for

SDCs C and D ............................................................................................................ 19

3.22 Requirements for Capacity Protected Members ...................................................... 20

3.23 Joint Shear Design .................................................................................................... 20

4.0 Computer Analysis Verification .................................................................................... 24

FiguresFigure 3.2‐1 Permissible Earthquake‐Resisting Systems (ERSs) ...................................................... 8

Figure 3.2‐2 Permissible Earthquake‐Resisting Elements (EREs) .................................................... 9

Figure 3.2‑3 Permissible Earthquake‑Resisting Elements That Require Owner’s Approval ........ 10

Figure 3.23‐1 Joint Shear Principal Stress Diagrams ..................................................................... 21



1.0 General

Seismic design of new bridges shall conform to AASHTO Guide Specifications for LRFD

Seismic Bridge Design except for as modified by Sections 2 and 3. For nonconventional

bridges, bridges that are deemed critical or essential, or bridges that fall outside the

scope of the Guide Specifications for any other reasons, project specific design

requirements shall be developed and submitted to the MDOT Director of Structures,

State Bridge Engineer for approval.

The importance classifications for all highway bridges in Mississippi are classified as

“Normal” except for special major bridges. Special major bridges fitting the

classifications of either “Critical” or “Essential” will be so designated by the MDOT

Director of Structures, State Bridge Engineer. The performance object for “normal”

bridges is life safety. Bridges designed in accordance with AASHTO Guide Specifications

are intended to achieve the life safety performance goals.

The AASHTO Guide Specifications for LRFD Seismic Bridge Design employs a

performance‐based approach to seismic design. This design approach requires the

structure system and its individual components be designed to have enough capacity to

withstand the deformations imposed by the design earthquake.

Displacement based design is used instead of the traditional force based design

approach to overcome the drawbacks of the latter design approach which:

Does not directly address the inelastic nature of a structural system.

Requires the use of a somewhat arbitrary force‐reduction factor.

Provides little insight into actual structural behavior.

Does not provide a consistent level of protection against reaching a specified

limit state.



2.0 SoftwareforSeismicAnalysisandDesign

The following software is typically used by MDOT for seismic analysis and design:

LPILE:

o Soil‐structure interaction

o Pile/Shaft design

o Laterally loaded piles and shafts

CSiBridge:

o General purpose structural analysis software with steel design, concrete

design, AASHTO LRFD analysis and design, seismic response spectrum

analysis, sectional analysis of reinforced concrete members, pushover

analysis, time history analysis, staged construction…

RC‐PIER:

o Analysis and design of reinforced concrete bridge substructures and

foundations. Bent design for multi‐column and hammerhead piers, bent

caps, rectangular or circular columns, footings and drilled shafts.

o Considers slenderness effects through optional P‐delta analysis or moment‐

magnification.

o Plastic hinging moment in columns can be considered in the pier cap design.

o Generates pile forces in pile‐supported footings due to plastic hinging.

In addition to the above mentioned software packages, the following programs can be

used to aid in the seismic evaluation of a bridge. This previous list and the following list

are not intended to be all inclusive.

WINSEISAB

o Seismic response analysis of bridges

CAPP

o Pushover capacity of bridges

Response 2000: (http://www.ecf.utoronto.ca/~bentz/home.shtml)

o Section analysis of reinforced/prestressed concrete members

o Modified compression field theory



o Concrete model does not account for confinement effects (confinement

effects can be included by user‐specific input of the concrete stress‐strain

behavior)

o Can store templates for many different sections

CONSEC: (http://www.structware.com)

o Section analysis of reinforced concrete and structural steel sections

o Confining effects may be included for concrete sections

o Voids may be modeled

XTRACT:

o Section analysis of reinforced/prestressed concrete, steel, and composite

members

o Templates for common structural shapes



3.0 MDOTModificationstoAASHTOGuideSpecificationsforLRFDSeismicBridgeDesignMDOT amendments to the AASHTO Guide Specifications for LRFD Seismic Bridge

Design are as follows:

3.1 Definitions

Guide Specifications Article 2.1 – Add the following definitions:

Owner – Person or agency having jurisdiction over the bridge. For MDOT

projects, regardless of delivery method, the term “Owner” in the Guide

Specifications shall be the MDOT Director of Structures, State Bridge Engineer

or/and the MDOT Geotechnical Engineer.

3.2 EarthquakeResistingSystems(ERS)RequirementsforSDCsCandD

Guide Specifications Article 3.3 – MDOT Global Seismic Design Strategies:

Type 1 – Ductile Substructure with Essentially Elastic Superstructure. This category is permissible.

Type 2 – Essentially Elastic Substructure with a Ductile Superstructure. This category is not permissible.

Type 3 – Elastic Superstructure and Substructure with a Fusing Mechanism Between the Two. This category is permissible with MDOT Director of Structures, State Bridge Engineer’s approval.

Type 3 ERS may be considered only if Type 1 strategy is not suitable and Type 3 strategy has been deemed necessary for accommodating seismic loads. Isolation bearings shall be designed per the requirement of the AASHTO Guide Specifications for Seismic Isolation. The use of isolation bearings shall be approved by the MDOT Director of Structures, State Bridge Engineer.

MDOT preferences and limitations on the use of ERS and Earthquake Resisting Elements (ERE) are presented below. MDOT prefers to design the bridge without any contribution from the end bents in the ERS. This ensures that in the event end bent resistance becomes ineffective, the bridge will still be able to resist the earthquake forces and displacements. In such a situation, the end bents provide an increased margin of safety against collapse. The use of the end bents in the ERS requires approval from the MDOT Director of Structures, State Bridge Engineer.



Instances exists where the use of the end bent foundation and/or passive pressure of the backfill will be appropriate such as continuous bridges with integral or semi‐integral end bents or instances where the shear keys and/or anchor bolts are not designed to fuse due to the seismic loading. The design objective when end bents are relied on to resist either longitudinal or transverse loads is either to minimize column sizes or reduce the ductility demand on the columns. Even though the end bents are not assumed to provide energy dissipation during the design event, the end bent foundation capacity should be greater than the demand allowed by any connection of the superstructure to substructure including shear keys, anchor bolts, bearings and backwall to cap (capacity protected). The horizontal design connection force shall be addressed from the point of application through the substructure and into the foundation elements. If each bearing supporting the bridge superstructure is an elastomeric bearing, there may be no fully restrained directions due to the flexibility of the bearings. However, the forces transmitted through these bearings to substructure and foundation elements should be determined in accordance with this Article and with Article 14.6.3 of the AASHTO LRFD Bridge Design Specifications.

ERSs 1 and 3 in Figure 3.2‐1 represent typical conditions for MDOT bridges. These ERSs are permissible and preferred. ERSs 2 and 4 include the use of isolation bearings which is atypical for MDOT bridges and requires approval from MDOT. ERS 5 includes the resistance from the end bent including passive soil pressure. MDOT prefers to exclude the end bent resistance unless an integral end bent is employed. This approach requires approval from MDOT. ERS 6 is permissible but not preferred; MDOT frequently uses a bridge type with a multi‐span continuous unit(s) followed by a longer simple span and then a multi‐span continuous unit(s). This bridge configuration is permissible. If this configuration is used, the effects of the joints closed (compression model) as well as the joints open (tension model) shall be considered. ERE Types 3, 5, 6, 9, 10 and 13 in Figure 3.2‐2 are atypical details for MDOT bridges and require approval from MDOT. ERE Types 1, 2, 7, 8, and 12 represent the preferred options for MDOT bridges. ERE Type 4 is not permissible. ERE Types 11 and 14 include the resistance of the end bents in the ERS; therefore, these types require approval from MDOT. The ERE Type 8 shown in Figure 3.2‐3 is permissible. For ERSs and EREs requiring approval, the MDOT Director of Structures, State Bridge Engineer’s approval is required regardless of contracting method (i.e., approval authority is not transferred to other entities).



FIGURE 3.2‐1 PERMISSIBLE EARTHQUAKE‐RESISTING SYSTEMS (ERSS) (GUIDE SPECIFICATION FIGURE 3.3‐1A)



FIGURE 3.2‐2 PERMISSIBLE EARTHQUAKE‐RESISTING ELEMENTS (ERES) (GUIDE SPECIFICATION FIGURE 3.3‐1B)



FIGURE 3.2‑3 PERMISSIBLE EARTHQUAKE‑RESISTING ELEMENTS THAT REQUIRE OWNER’S APPROVAL(GUIDE SPECIFICATION FIGURE 3.3‐2)



3.3 SeismicGroundShakingHazard

Guide Specifications Article 3.4 – For bridges that are considered critical or essential or normal bridges with a Site Class F, the seismic ground shaking hazard shall be determined based on the MDOT Geotechnical Engineer recommendations.

3.4 SelectionofSeismicDesignCategory(SDC)

Guide Specifications Article 3.5 ‐ The following maps depict the Seismic Design Category for bridges in Mississippi for different site classifications. The maps are based on 1‐second period design response spectrum accelerations for the 7% probability of exceedance in 75 years.



SiteClass“B”



SiteClass“C”



SiteClass“D”



SiteClass“E”



3.5 TemporaryandStagedConstruction

Guide Specifications Article 3.6 – For bridges that are designed for a reduced seismic demand, the contract plans shall either include a statement that clearly indicates that the bridge was designed as temporary using a reduced seismic demand or show the Acceleration Response Spectrum (ARS) used for design.

3.6 LoadandResistanceFactors

Guide Specifications Article 3.7 – Use load factors of 1.0 for all permanent loads. Use a load factor of 0 for live load unless the bridge is located in a large metropolitan area where there is a high probability of large live load being on the bridge during an earthquake. The inclusion of live load in the load combination should be discussed with MDOT on a case‐by‐case basis. If live load is included it should be without impact and the inertia effects of live loads need not be included in the seismic analysis. Unless otherwise noted, all ϕ factors shall be taken as 1.0. The effect of scour on the soil surrounding the substructure of bridges needs to be taken into consideration. Scour is treated as an extreme event in the AASHTO Specifications. Typically, two extreme events are not considered simultaneously. However, since the timing of a seismic event is not predictable, the effect of long term scour in conjunction with the design seismic event should be discussed with MDOT on a case‐by‐case basis.

3.7 SelectionofAnalysisProceduretoDetermineSeismicDemand

Guide Specifications Article 4.2 – Analysis Procedures:

Procedure 1 (Equivalent Static Analysis) shall not be used.

Procedure 2 (Elastic Dynamic Analysis) shall be used for all “regular” bridges with two through six spans and “not regular” bridges with two or more spans in SDCs B, C, or D.

Procedure 3 (Nonlinear Time History) shall only be used with MDOT Director of Structures, State Bridge Engineer’s approval.

3.8 LocalDisplacementCapacityforSDCsBandC

Guide Specifications Article 4.8 – Push‐over analysis is not required for SDCs B and C. If the displacement demand is greater than the implicit capacity, the designer may reevaluate the capacity based on a push‐over analysis. In lieu of a push‐over analysis for SDC B bridges, the SDC C implicit equation may be used along with SDC C detailing. The pushover analysis typically results in a larger capacity.



Implicit equations were developed primarily for determining capacities of bridges with single and multi‐column reinforced concrete bents. They are also applicable for bents comprising single or multiple drilled shaft columns or prestressed‐concrete pile bents in which plastic hinging may occur below ground such that the clear height dimension would begin at the point of fixity in the soil. The implicit equations can also be used for steel pile bents. The results of the equations will be conservative and a push‐over analysis can be performed if demand exceeds capacity. For different bent types, Guide Specifications Article 4.8.2 (push‐over analysis) shall be used. This includes the following bent types: columns founded on oversized drilled shafts, when steel construction casing is used, and bents with struts at mid‐height. The implicit displacement capacity equations generally correspond to a limit state of initiation of concrete cover spalling for SDC B and an equivalent column member ductility of 3 or less for SDC C. Where in‐ground hinging is used in the ERE, the demand displacement shall be limited to the capacity for SDC B or the methods and limits for SDC D.

3.9 EndBents

Guide Specifications Article 5.2 – For typical MDOT bridges with seat‐type end bents, MDOT’s preference is to not include the end bent contribution in the earthquake‐resisting system. With MDOT Director of Structures, State Bridge Engineer's approval, the end bent and wingwalls may be considered and designed as part of earthquake resisting system (ERS) in the longitudinal direction of a straight bridge with little or no skew and with a continuous deck. The wingwall resistance shall be governed by the wingwall capacity or the applicable soil resistance.

3.10 Foundation–General

Guide Specifications Article 5.3.1 – The use of spread footings for MDOT bridges is atypical. MDOT prefers to model pile or shaft foundations with an estimated depth to fixity or p‐y curves. Soil springs with secant stiffness may be used to provide a better representation based on p‐y curves for the foundation and soil. Bi‐linear springs may be used in the pushover analysis if there is particular concern with depth of the plastic hinge and effective depth of fixity. The required foundation modeling method (FMM) and the requirements for estimation of foundation springs for spread footings, pile foundations, and drilled shafts shall be based on the MDOT Geotechnical Engineer’s recommendations.

3.11 Foundation–SpreadFooting

Guide Specifications Article 5.3.2 – The use of spread footings for MDOT bridges is atypical and requires approval by the MDOT Director of Structures, State Bridge Engineer.



3.12 Foundation–PileFoundationsandDrilledShafts

Guide Specifications Articles 5.3.3 and 5.3.4 – Group reduction factors should be considered in the response to seismic loading. Section 10 of the AASHTO LRFD Bridge Design Specifications provides guidance on the determination of group reduction factors for horizontal loading.

3.13 AnalyticalProcedures

Guide Specifications Article 5.4 – Procedure 2: Elastic Dynamic Analysis (EDA) shall be used to estimate the demand displacements for all typical MDOT bridges.

3.14 Procedure3:NonlinearTimeHistoryMethod

Guide Specifications Article 5.4.4 – Procedure 3: Nonlinear Time History Method is only used with approval by the MDOT Director of Structures, State Bridge Engineer. This type of analysis should be considered for irregular bridges, critical or essential bridges, SDC D bridges or cases in which seismic isolation is included. The time histories of input acceleration used to describe the earthquake loads shall be selected in consultation with the MDOT Geotechnical Engineer and the MDOT Director of Structures, State Bridge Engineer.

3.15 FoundationRocking

Guide Specifications Article 6.3.9 – Foundation rocking shall not be used for the design of MDOT bridges.

3.16 DrilledShafts

Guide Specifications Article C6.5 – It is cautioned that the scaling factor for diameter effects should not be used blindly without a sound mechanistic basis. A significant amount of pile load test data have been accumulated within the offshore industry on large diameter driven steel pipe piles including tests on 5 ft (1.5 m) piles. The diameter effects for offshore piles have either been concluded not valid or considered insignificant within the offshore industry. Juirnarongrit and Ashford (2005) performed vibration tests and lateral load tests on drilled shafts ranging from 16 in (0.4 m) to 4 ft (1.2 m) installed in dense weakly cemented sand. Data from the tests for each shaft

diameter were used to back‑calculate p‐y curves. Their analyses indicate that the shaft diameter has insignificant effect on the p‐y curves at the displacement level below the ultimate soil resistance. Beyond this range, the ultimate soil resistance increased as the shaft diameter increased. It has been found that the pile diameter effect depend on the pile head moment‐to‐shear ratio and the distribution of soil modulus with depth (Pender, 2004). For MDOT bridges, the scale factor for p‐y curves for large diameter shafts shall not be used unless approved by the MDOT Geotechnical Engineer and MDOT Director of Structures, State Bridge Engineer.



3.17 LongitudinalDirectionRequirements

Guide Specifications Article 6.7.1 – Typical MDOT bridges employ seat‐type (free‐standing) end bents. The seismic design of the these end bents should consider the transfer of seismic forces from the bridge superstructure to the end bents as well as force imparted on the end bent due to earthquake‐induced active earth pressures.

3.18 LiquefactionDesignRequirements

Guide Specifications Article 6.8 – For bridge sites where liquefaction occurs at the bridge foundations, the bridge should be analyzed for a non‐liquefied condition and a liquefied condition unless ground modification will be performed to mitigate the liquefaction. The liquefied soil parameters shall be based on the MDOT Geotechnical Engineer’s recommendation.

3.19 ReinforcingSteel

Guide Specifications Article 8.4.1 – The Guide Specification requires the use of A706 reinforcing for SDC D structures in areas where hinging is expected. In lieu of restricting the column bars to A706 material, permit the use of either of the following for column longitudinal bars:

1. A615 reinforcing with a maximum yield strength of 78 ksi, or 2. A706 reinforcing

This means that a lower over‐strength factor, λmo, of 1.2 may be used, but that the reduced ultimate tensile strain, εsuR, corresponding to A 615 steel must also be used in the moment curvature analysis. The net effect will be a reduced over‐strength plastic shear, but without the added displacement capacity achieved from the use of A706 transverse steel. (See Sections 8.4.2 and 8.5 of the Guide Specification). The reduced ultimate tensile strain for steel, εsuR, shall be used instead of the theoretical maximum, εsu. Guide Specifications Article 8.4.3 – Where in‐ground hinging is anticipated for a prestressed concrete pile bent as part of the ERS, the ultimate prestress strain limit shall be reduced to 0.015.

3.20 ConcreteModeling

Guide Specifications Article 8.4.4 – Where in‐ground plastic hinging is part of the ERS, the confined concrete core shall be limited to a maximum compressive strain of 0.008.

3.21 SplicingofLongitudinalReinforcementinColumnsSubjecttoDuctilityDemands

forSDCsCandD



Guide Specifications Article 8.8.3 – The splicing of longitudinal column reinforcement outside the plastic hinging region shall be accomplished using mechanical couplers that are capable of developing a minimum tensile strength of 85 ksi. Splices shall be staggered at least 2 ft. lap splices shall not be used. The design engineer shall clearly identify the locations where splices in longitudinal column reinforcement are permitted on the plans.

3.22 RequirementsforCapacityProtectedMembers

Guide Specifications Article 8.9 – For SDCs C and D where in‑ground hinging is part of ERS, the confined concrete core should be limited to a maximum compressive strain of 0.008 and the member ductility demand shall be limited to 4. Bridges shall be analyzed and designed for the non‐liquefied condition and the liquefied condition in accordance with Article 6.8. The capacity protected members shall be designed in accordance with the requirements of Article 4.11. MDOT may elect to use capacity protected oversized pile shafts to preclude hinging underground. If this approach is taken, oversized pile shafts shall be designed for an expected nominal moment capacity, Mne, at any location along the shaft, that is, equal to 1.25 times moment demand generated in the shaft by the overstrength column plastic hinge moment and associated shear force at the base of the column. The safety factor of 1.25 may be reduced to 1.0 in the liquefied condition with the MDOT Director of Structures and State Bridge Engineer’s approval. Design moments below ground for non‐oversized drilled shafts may be determined using the nonlinear static procedure (pushover analysis) by pushing them laterally to the displacement demand obtained from an elastic response spectrum analysis. The point of maximum moment shall be identified based on the moment diagram. The expected plastic hinge zone shall extend 3 diameters (3D) above and below the point of maximum

moment. The plastic hinge zone shall be designated as the “no‑splice” zone and the transverse steel for shear and confinement shall be provided accordingly.

3.23 JointShearDesign

Guide Specifications Article 8.13.1 – Bent caps/footings and the moment resisting connection with the column shall be designed to resist seismic forces as an essentially elastic element. Additional guidance on joint design may be found in the following:

1. Priestley, M. J. N, et al, Seismic Design and Retrofit of Bridges, 1996, Wiley and Sons, pages 348‐388.

2. Sritharan, Sri, Improved Seismic Design Procedure for Concrete Bridge Joints, ASCE Journal of Structural Engineering, September, 2005, pages 1334‐1344.



Guide Specifications Article 8.13.2 – Moment‑resisting joints shall be proportioned so that the principal stresses satisfy the requirements of the following equations. Refer to Figure 3.23‐1 for joint shear principal stress diagrams. The permissible principal stress levels shall be established using the expected concrete strength.

FIGURE 3.23‐1 JOINT SHEAR PRINCIPAL STRESS DIAGRAMS



For principal compression, pc:

0.25 (8.13.2‐1)

For principal tension, pt:

0.38 ′ (8.13.2‐2)

In which:

(8.13.2‐3)

(8.13.2‐4)

Where: fh = Average axial horizontal stress (ksi) fv = Average axial vertical stress (ksi) vih = Average joint shear stress (ksi)

If either criterion is not met, the member size (column and/or cap) must be increased until the limits are met. Typically, it is preferable to increase only the cap dimensions since changing the column dimensions would necessitate recalculating the plastic shear and the displacement capacity.

The average horizontal axial stress is based on the mean axial force at the center of joint, including the effects of prestress. For most projects, fh can be taken as zero due to lack of prestress in the cap.

Where: Pb = Beam axial force at the center of the joint including the effects of prestressing (kips) Bcap = Bent cap width (in) Ds = Depth of superstructure at the bent cap for integral joints under longitudinal response and depth of cap beam for nonintegral bent caps and integral joint under transverse response (in)

In the vertical direction, the average axial vertical stress in the joint is provided by the axial force in the column, Pc. Assuming a 45° spread away from the boundary of the

column to a plane at mid‑depth of the bent cap, the average axial stress is calculated by the following equation:



Where: Pc = Column axial force including the effects of overturning (kips) Beff = Effective width of joint (in)

Dc = Diameter or cross‑sectional dimension of column parallel to bent cap (in)

The average axial stress in the joint, fv, shall be modified if the cap beam does not extend beyond the column exterior face greater than half the bent cap depth by modifying Ds in the equation above to be 0.5Ds+ the cantilever length if less than 0.5Ds. As an alternate to the average joint stress, vjv, given in the Guide Specifications Eq. 8.13.2‐7, the average joint shear stress, vjh, can be approximated with the following equation with adequate accuracy:

Where:

M = The column overstrength moment, Mpo (kip‑in)

Dc = Diameter or cross‑sectional dimension of column in the direction of loading (in) hb = The distance from c.g. of tensile force to c.g. of compressive force on the section (in) This moment arm may be approximated by Db. Beff = Effective width of joint (in)

The effective width of joint, Beff, depends on the shape of the column framing into the joint and is determined using the following equations.

For circular columns:

√2

For rectangular columns:

For transverse response, the effective width will be the smaller of the value given by the above equations or the cap beam width. Guide Specifications Article 8.13.5 – The Guide Specifications do not specifically address knee joints for non‐integral bent caps. Refer to Sritharan (2005) for additional guidance on joint proportioning for knee joints of non‐integral bent caps.



4.0 ComputerAnalysisVerification

The computer results will be verified to ensure accuracy and correctness. The designer should use the following procedures for model verification:

Using graphics to check the orientation of all nodes, members, supports, joint, and member releases. Make sure that all the structural components and connections correctly model the actual structure.

Check dead load reactions with hand calculations. The difference should be less than 5 percent.

Calculate fundamental and subsequent modes by hand and compare results with computer results.

Check the mode shapes and verify that structure movements are reasonable.

Increase the number of modes to obtain 90 percent or more mass participation in each direction. CSiBridge directly calculates the percentage of mass participation.

Check the distribution of lateral forces. Are they consistent with column stiffness? Do small changes in stiffness of certain columns give predictable results?

Documents

Seismic Design Policy 1.0new - Mississippi