Load Combinations

Directly enter the following basic items necessary to define load combinations in the Load Combination List.

No : Numbers are sequentially and automatically assigned in the order of the load combination entries.
Name : Enter the load combination name.

In the case of a construction stage analysis for a bridge structure, the loads applied in the construction stage analysis are integrated into the construction stage loads in the analysis. The analysis results are separately stored as the cases below. We can then obtain the analysis results for the load cases and their combinations for each construction stage. Separate load combinations must be specified for the Post CS using the following load cases and/or general load cases. We cannot however verify the auto-generated load cases below.

Dead Load (CS) : Dead load including Self Weight included in construction stages
Erection Load (CS): Erection load defined by Load Cases to be Distinguished from Dead Load for CS Output of Construction Stage Analysis Control
Tendon Primary (CS): Analysis results due to prestressing forces in tendons
Tendon Secondary (CS): Indeterminate forces resulting from indeterminate condition of the structure
Creep Primary (CS): Results the (imaginary) loads causing creep strain
Creep Secondary (CS): Real member forces resulting from creep strain due to indeterminate structure
Shrinkage Primary (CS): Results for the (imaginary) loads causing shrinkage strain
Shrinkage Secondary (CS): Real member forces resulting from shrinkage strain due to indeterminate structure
Summation (CS): Summation of the results of all the cases above

The results for creep and shrinkage are separated to check the results individually.

Active : Specify Active if the corresponding load combination is applied in design.

Inactive : The corresponding load combination is not applied in the post-processing mode.
Active : The corresponding load combination is applied in the post-processing mode. (General tab)
Strength/Stress : The corresponding load combination is applied in the post-processing mode (Concrete Design tab) except for the serviceability check (crack and fatigue checks).
Serviceability : The corresponding load combination is applied in the post-processing mode (Concrete Design tab) except for the auto-design and strength check of beam members.

Type : Assign the combination types for analysis results.

Add : Linear combination of analysis results

L1 + L2 + ... + M1 + M2 + ... + S1 + S2 + ...+ (R1 + R2 + ...) + T + LCB1 + LCB2 + ... + ENV1 + ENV2 + ...

Envelope : Maximum, minimum and maximum of absolute values for individual analysis results

CBmax : Max (L1, L2, ..., M1, M2, ..., S1, S2, ...,R1, R2, ..., T, LCB1, LCB2, ..., ENV1, ENV2, ...)
CBmin : Min (L1, L2, ..., M1, M2, ..., S1, S2, ...,R1, R2, ..., T, LCB1, LCB2, ..., ENV1, ENV2, ...)
CBall : Max (|L1|, |L2|, ..., |M1|, |M2|, ..., |S1|, |S2|, ..., |R1|, |R2|, ..., |T|, |LCB1|, |LCB2|, ..., |ENV1|, |ENV2|, ...)

The CBall condition produces the maximum of absolute values; the results are produced in non-directional positive (+) values.

ABS : Linear combination of the absolute values of analysis results

|L1| + |L2| + ... + |M1| + |M2| + ... + |S1| + |S2| + ...+ (|R1| + |R2| + ...) + |T| + |LCB1| + |LCB2| + ... + |ENV1| + |ENV2| + ...

SRSS : Linear combination of the SRSS (Square Root of Sum of the Squares) of response spectrum analysis results and other analysis results

[L12 + L22 + ... + M12 + M22 + ... + S12 + S22 +...+(R12 + R22 +...) + T2 + LCB12 + LCB22 + ... + ENV12 + ENV22 + ...]½

where,

L : static analysis results for a unit load case × scale factor
M : static analysis results for a moving load case × scale factor
S : static analysis results for a settlement load case × scale factor
R : dynamic analysis results for a Response Spectrum case × scale factor
T : dynamic analysis results for a Time History Analysis case × scale factor
LCB : Analysis results for a predefined load combination × scale factor
ENV : Analysis results for a predefined envelop conditions × scale factor

1
Among the methods of load combinations, Envelope, ABS and SRSS can be applied in General only.

2
We can combine 150 load cases or load combinations in a load combination.

3
For the method of calculating principal stresses, effective stresses and maximum shear stresses for each load combination type, refer to the explanations at the bottom of the page.

Description : Short descriptions for the load combinations
Enter the unit load cases and the corresponding scale factors included in the relevant load combination as many times as desired.
Load Case : Select the unit load cases from the load case list or the load combinations defined earlier.
Factor : Enter the load (scale) factors corresponding to the selected unit load cases or load combinations.

Option

Add : Add the generated load combinations to the previously defined load combinations
Replace : Replace the previously defined load combinations with the generated load combinations

Add Envelope : Envelope of load combinations is auto-generated. This is applicable in the General tab only.

Code Selection

Steel : Design code for structural steel
Concrete : Design code for reinforced concrete
SRC : Design code for steel-reinforced concrete composite sections

Design Code : Select the design code to be used for the auto-generation of load combinations (Refer to for design load combinations for different codes)

Manipulation of Construction Stage Load Case : Select a method of combining construction stage analysis results and PostCS results when auto-generation of load combinations is used for construction stage analysis model.

ST Only : This option generates load combinations using only Post CS load cases.
CS Only : This option generates load combinations using only construction stage load cases.
ST+CS : This option generates load combinations using combined Post CS and construction stage load cases.

Click the Spread Sheet Form button in the load combinations dialog box to convert the data into a spreadsheet form table arranging the unit load cases in rows. Enter or modify the items mentioned in Method 1 to add or modify unit load cases.

Click the Import button in the load combinations dialog box to display the dialog box that imports the load combination files. Select a file containing previously entered load combinations.

The fn.LCB file type is as follows:

Sequential number, combination method, unit load case i, load factor i, unit load case j, load factor j, ...

Examples)

1, , 1, 1.0, 2, 1.0
2, , 1, 1.4, 2, 1.7

Combination methods

blank,  0: Add
           1: Envelope
           2: ABS
           3: SRSS

The Copy into button: Select desired load combinations from the General tab and a design tab to the right of the Copy into button, and click the Copy into button. Only the selected load combinations are copied to the corresponding design tab. This functionality can be used only under the General tab.

Make Load Combination Sheet: The load combinations in the currently activated tab can be output in a text file (*.lcp).

AASHTO-Std2K (AASHTO Standard Specifications for Highway Bridges : 2000 Interim Revisions, 1996 16th Edition)
AASHTO-LRFD02 (American Association of State Highwqy and Transportation Officials, LRFD Bridge Design Specification Section 6,Steel Structures,2002)
|Load and Resistance Factor Design of the American Institute of Steel Construction (AISC-LRFD2K, Load & Resistance Factor Design Specification for Structural Steel Buildings, 2000)
Load and Resistance Factor Design of the American Institute of Steel Construction (AISC-LRFD93, Load & Resistance Factor Design Specification for Structural Steel Buildings, 1993)
Allowable Stress Design of the American Institute of Steel Construction (AISC-ASD89, Specification for Structural Steel Buildings: Allowable Stress Design, 1989)
Taiwan, Specification of the Highway Bridge Design, Chapter 8: Steel Structure Design, MOTC Standard (90).

AASHTO-Std2K (AASHTO Standard Specifications for Highway Bridges : 2000 Interim Revisions, 1996 16th Edition)
AASHTO-LRFD02 (American Association of State Highwqy and Transportation Officials, LRFD Bridge Design Specification Section 6,Steel Structures,2002)
RC Structures Design Codes of the American Concrete Institute (ACI318-02, Building Code Requirements for Reinforced Concrete, 2002)
IRC:6-2000 (Indian Standard, Plain and Reinforced Concrete Code of Practice (Fourth Revision), 2000)
Taiwan, Specification of the Highway Bridge Design, Chapter 6: Reinforced Concrete Structure Design, MOTC Standard (90).

SSRC79, Structural Stability Research Council, A Specification for the Design of Steel-Concrete Composite Columns, 1979

AASHTO LRFD 2012 Bridge Design Specifications 6th Edition (Table 3.4.1-1 )
AASHTO LRFD 2007 Bridge Design Specifications 4th Edition

Dead Load(D)
Dead Load of Component and Attachments(DC)
Dead Load of Wearing Surfaces and Utilities(DW)
Downdrag(DD)
Live Load(L)
Trailer or Crawler Induced Live Load(LC)
Overload Live Load(LP) / AASHTO
Roof Live Load(LR)
Wind Load on Structure(W)
Earthquake(E)
Temperature(T)
Snow Load(S)
Rain Load(R)
Live Load Impact(IL)
Overload Live Load Impact(ILP) / AASHTO
Earth Pressure(EP)
Horizontal Earth Pressure(EH)
Vertical Earth Pressure(EV)
Live Load Surcharge(LS)
Trailer or Crawler Induced Surcharge(LSC)
Earth Surcharge Load(ES)
Buoyancy(B)
Ground Water Pressure(WP)
Fluid Pressure(FP)
Stream Flow Pressure(SF)
Ice Pressure(IP)
Wind Load on Live Load(WL)
Longitudinal Force from Live Load(BK)
Centrifugal Force(CF)
Rib Shortening(RS)
Shrinkage(SH)
Creep(CR)
Prestress(PS)
Collision Load(CO)
Vehicular Collision Force(CT)
Vessel Collision Force(CV)
Construction Stage Load(CS)
Erection Load(ER)
Locked in Erection Stresses(EL)
Braking Load(BRK)
Settlement(STL)
Wave Pressure(WPR)
Crowd Load(CRL)
Temperature Gradient(TPG)
Friction(FR)
Response Spectrum(ESP)

EspX: Response Spectrum
EspY: Response Spectrum

SFx: Scale up factor of X-Direction
SFy: Scale up factor of Y-Direction

AASHTO LRFD Bridge Design Specifications 6th Edition (2012)

Load Modifier
Load Factors for Permanent Loads (Yp)

Component and Attachments
Downdrag
Wearing Surfaces and Utilities
Horizontal Earth Pressure

Active : Active Earth Pressure
At-Rest : At-Rest Earth Pressure

Vertical Earth Pressure

Overall Stability
Retaining Walls. Abutments
Rigid Buried Structure
Rigid Frames
Flexible Buried Structures (Non Metal Box Culverts)
Flexible Metal Box Culverts

Earth Surcharge

Load Factor for Settlement : Load factor corresponding to the support settlement load case
Structural Plate Box Structures (Metal Box Culverts)
: Check on for steel box culverts
Condition for Temperature, Creep, Shrinkage Factor

Deformation Check
All Other Effects

Load and Resistance Factor Design

Total factored force
: Load modifier User Input, default : 1..0

TU, CR, SH : The larger value - when checking deformation, The smaller value - when checking other cases than deformation
TG : 1.0: when live load is not considered, 0.5: when live load is considered
SE :YSE: project specific basis
Structural Plate Box Structures (metal box culverts) LL, IM factors : 2.0

: AASHTO Standard Specifications for Highway Bridges : 2000 Interim Revisions, 1996 16th Edition
Service Load Design

Condition for Earth Pressure Factor

Rigid Frames

Lateral Earth Pressure Load Factor: 1
Vertical Earth Pressure Load Factor: 1

Culverts

May be user defined

All Other Structures : All the structures other than rigid frame structures and culverts

Lateral Earth Pressure Load Factor: 1
Vertical Earth Pressure Load Factor: 1

Earth Pressure Factor for Culverts : The user must manually enter the coefficient of earth pressure in the case of a culvert

Lateral : Lateral Earth Pressure Load Factor
Vertical : Vertical Earth Pressure Load Factor

Load Factor Design

Condition for Earth Pressure Factor

Lateral Earth Pressure for Retaining Walls and Rigid Frames Excluding Rigid Culverts : Apply 1.3
Lateral at-Rest Earth Pressure : Apply 1.15
Lateral Earth Pressure for Checking Positive Moments in Rigid Frames : Apply 0.5
Rigid Culverts : Apply 1.0
Flexible Culverts : Apply 1.5

In Load Factor Design, Vertical Earth Pressure is always 1.0.
Condition for Dead Load Factor

Checking for Minimum Axial Load and Maximum Moment or Maximum Eccentricity : Apply 0.75
Checking for Maximum Axial Load and Minimum Moment : Apply 1.0
Flexural and Tension Members : Apply 1.0

For Service Load Design

βE = 1.0 for vertical and lateral loads on all other structures

For Load Factor Design

βE = 1.3 for lateral earth pressure for retaining walls and rigid frames excluding rigid culverts. For lateral at-rest earth pressure, βE =1.15
βE = 0.5 for lateral earth pressure when checking positive moments in rigid frames
βE = 1.0 for vertical earth pressure
βD = 0.75 when checking member for minimum axial load and maximum moment or maximum eccentricity
βD = 1.0 when checking member for maximum axial Column load and minimum moment
βD = 1.0 for flexural and tension members
βE = 1.0 for Rigid Culvertso
βE = 1.5 for Flexible Culverts

Canadian Highway Bridge Design Code, CSA-S6S1-2010

Load Factors for Permanent Loads

Dead Load
Wearing Surfaces
Earth Pressure

Passive Earth Pressure
At-rest Earth Pressure
Active Earth Pressure
Backfill Pressure

Hydrostatic Pressure
Secondary Prestress Effects

• Load Factor for Ultimate Limit State and Serviceability Limit State Load Combinations

For the static load type for D1, D2, ...etc., refer to the table below.

• Static Load Case and Special Analysis Load Case Type in midas Civil

*[Define Earth pressure Load Type] button is activated when Earth Pressure (EP) type static load case has been defined in Static Load Cases. Classify the earth pressure load cases to the proper sub types in order to generate load combinations including earth pressure load cases automatically.

Canadian Highway Bridge Design Code, CSA-S6-14

1. Ultimate Limit States

Combinations of actions for persistent or transient design situations are automatically generated. Combinations of actions for accidental/seismic design situations need to be separately created by the user.

(EN1990:2002 Eq 6.10)

Where, ψ factor is based on Table A2.1 Recommended Values of ψ factors for road bridge in EN1990:2002 Annex A2.

γ factor is based on Table A2.4(B) Design values of actions(STR/GEO) in EN1990:2002 Annex A2.

For Road traffic loads, ψ factor must be entered by the user in the Define Standard/User Defined Vehicular Load dialog. Ψ factor in the Define Standard/User Defined Vehicular Load dialog represents ψ1 in EN1990:2002 Annex A2 Table A2.1. Thus in case of Road Traffic loads, ψ1 will be applied for ψ0 since the value of ψ0 and ψ1 are the same. ψ Factor for variable actions other than traffic loads is automatically entered during load combinations.

In case of Rail Traffic Loads, the value of ψ1 is considered in the definition of the Moving Load case with Railway Bridge Load model and the value of ψ0 is automatically entered when the load combinations are generated. If the user wishes to neglect ψ factor, the Ignore ψ factor option must be checked on in the Define Moving Load Case dialog. If the traffic load is considered as Leading Variable Action in the load combination as per Eurocode 0, the moving load case for which the Ignore ψ factor option is checked on will be used.

When other actions such as Wind forces, Thermal actions, Snow loads or Construction loads is considered as Leading Variable Action and the traffic load is considered as Accompanying Variable Action in the load combination, the moving load case for which Ignore ψ factor option is checked off will be used. That is, the user must define two different moving load cases (one considering the Ignore ψ factor option, the other one not considering the Ignore ψ factor option) in order to generate load combinations for both traffic load cases for Leading Variable Action and Accompanying Variable Action.

Example of Load Combination as per Eurocode 0

1. When traffic load is considered as Leading Variable Action,

2. When wind force is considered as Leading Variable Action and traffic load is considered as Accompanying Variable Action

2. Serviceability Limit State

For serviceability limit states, load combinations will be automatically generated based on Table A2.6 in EN1990:2002 Annex A2.

EN1990:2002, Annex A2, Table A2.6

Characteristic Combination

It is the same as load combination for Ultimate Limit States. However the γ factor will not be applied.

Frequent Combination

(1) When traffic load is a Leading Variable Action,

When traffic load is a Leading Variable Action, the moving load case(Q) for which Ignore ψ factor

option is checked off needs to be used in order to apply ψ factor.

Revision of Civil 2013 (v2.1)

When generating frequent load combinations, only frequent values of traffic loads should be used. Characteristic and frequent values can be classified in the following dialog box.

(2) When traffic load is not a Leading Variable Action and wind force is a Leading Variable Action,

When traffic load is not a Leading Variable Action, traffic load need not be included in load combinations since ψ2 factor for traffic load is zero

(1)
Here, the ψ0 for the Road Traffic Loads value entered in Define Standard/User Defined Vehicular Load will be used.
For the Rail Traffic loads the value of ψ1 is entered in the Moving Load Case with Railway Bridge Load model. The value of ψ0 is considered when the Load combinations are generated and is reflected in the factor to the Traffic load as a multiple of γ (Gamma).
The γ value = 1.00 or 1.35 for Permanent actions is used depending on the option of Prestress or Settlement action.

(2)
The load combinations can be generated corresponding to the Rail Traffic and Road Traffic by checking the required option After Auto generating the Load combinations and specifying the Eurocode 0 as design Code.

:Standard Specifications and Code of Practice for Road Bridges

Conditions for Load Combinations

Besides the load combinations for strength check, add the load combinations for serviceability or construction stage.

Consider Service Condition : Add the load combinations for serviceability.
Consider Construction Condition : Add the load combinations for construction stage.

1. Ultimate Limit State

Combinations of actions for basic/accidental/seismic design situations are automatically generated based on Table B.2
in IRC: 6 LSD (Limit State Design).

2. Serviceability Limit State

For serviceability limit states, load combinations will be automatically generated based on Table B.3in IRC: 6 LSD (Limit State Design).

If load combination consists of Special Vehicle, loads such as wind, seismic, braking, impact, temperature gradient will not considered in that combination. For the load combination with special vehicle the partial safety factor on SV load for verification of equilibrium and structural strength under ultimate Limit state (Basic Combination) and for verification of Serviceability Limit State (Rare Combination) shall be taken as 1.15 and 1.0 respectively.

When generating load combinations considering the Special Vehicle, the moving load case with the special vehicle will be classified in the following dialog box.

Indian Railway Standard Design Loads

Concrete structure only

Australian Standard Bridge Design Part 2: Design loads

Concrete structure only

For the steel composite girder design, the load factors for the dead load of structure are different between steel and concrete as per AS 5100.2:17.

The auto-generation of load combination cannot differentiate between the dead load cases of steel and concrete. The same load factors will be assigned to the dead load cases. Thus, it is necessary for the user to modify the load factors after the auto-generation of load combinations.

Here are the steps how to do this.

1. Apply the dead load of steel of the composite girder using the Self Weight function, and assign this to Dead Load (CS) of the construction stage.
2. Apply the dead load of concrete of the composite girder using the Beam Element Load function, and assign this to Erection Load (CS) of the construction stage.
3. Run the auto-generation of load combinations and the load factor of 1.2 will be assigned to the two load cases by default.
4. Modify the load factor of Dead Load (CS) from 1.2 to 1.1.

British Standard Bridge Design Part 2: Design loads

Concrete structure only