Bridge design in SCIA Engineer
2012.0
Brief introduction
A bridge design requires specific calculation rules which are only needed for the analysis and design of this type of civil engineering structures. Take for example the input of combinations. A bridge is subjected to specific types of loads like traffic loads, foot travel by pedestrians, conduits used for the movement of goods and materials, and so on. But next to this, also standard loads like wind and snow may be present. The combination rules are described in Eurocode 0. To facilitate the user, bridge combinations are automatically generated and the decomposed combinations are set out for having sufficient transparancy during this process.
These combinations are available for 3 types of bridges: Road bridges, Footbridges and Railway bridges.
Next to this, design rules for prestressed and reinforced concrete bridges are implemented. This includes new concrete grades, several checks (redistributed bending moment, min and max characteristic strength, reinforcement classes for ductility, detailing provisions, decompression check and crack width) and extension of the National Annex library.
Detailed Technical information
Short description
Combinations
The module Bridge Load combinations allows the user to define load combinations for bridges according to EN 1990 and EN 1991-2. Load combinations can be defined for 3 types of bridges:
- Road bridges
- Footbridges
- Railway bridges
With the enhanced combination generator in SCIA Engineer, Decomposed EN combinations are obtained from original combinations and from these, bridge envelope combinations are automatically generated.
To prevent mistakes during the input of the combination, a filter has been added which facilitates the user to create either a combination for buildings or for bridges.
It is known that the bridge combinations are rather complex due to the big range of Psi factors (safety factors) and the numerous combination rules for the specific types of loads. To make this more transparant for the bridge designer, a dialog has been added to SCIA Engineer which shows the decomposed EN combinations.
Click here to watch the short introduction to Bridge combinations in SCIA Engineer
Concrete
- New concrete grades according to EN1992-2 have been added to the material library
- Time dependent analysis is enabled for concrete grades with strength fck ≥ 55MPa according to EN1992-2, Annex B.
- The National Annex library has been extended with numerous National Annexes according to EN 1992-2
- Checks on minimal and maximal characteristic cylinder strength (fck,min and fck,max)
- Check on allowed reinforcement class for adequate ductility: the allowed class for nonprestressed reinforcement are B and C. If the low ductility class A is used, an error message is given.
- Formulas for the redistribution of the bending moment according to clause 5.5.4(104) are implemented
- For a crack check, it's important to know the maximum crack width for each exposure class. These values are integrated in SCIA Engineer.
- The decompression distance of prestressed concrete is now also checked for bridge design: i.e. the required perpendicular distance from the edge of the prestressing strand or duct to the neutral axis where the concrete has to be in compression.
- Check of minimal diameter of transversal reinforcement for columns.
Detailed information

The new functionality allows the user to define, in addition to the standard combinations, combinations for 3 types of bridges according to the rules of EN 1990 and EN 1991-2, namely for Road bridges, Footbridges and Railway bridges.
First of all the combination setup has been split up into two types of structures: Bridges and Buildings.
The setup for bridges contains the combination rules which can be activated or deactivated by the user and both the psi and load combination factors.
After defining the load cases and load groups, the user can create a code combination and choose for the type of bridge to be analyzed.
During the creation of the combinations a check is performed on the content of the load cases. For example: if a load case belongs to a load group of Category A (Domestic), then it is not shown in the list of available load cases.
Once the user has created the bridge combinations, optionally a textfile with the decomposition of the combinations may be consulted.


New material classes are introduced in this project according to EN1992-2. The performed checks are based on the chosen type of material. For example: if C35/45 (EN 1992-2) is used, then checks are done according to the concrete code for bridges. The following classes will be introduced:
- C30/37(EN 1992-2)
- C35/45(EN 1992-2)
- C40/50(EN 1992-2)
- C45/55(EN 1992-2)
- C50/60(EN 1992-2)

Time dependent analysis according to EN 1992-2 annex B is provided only for concrete with strength fck ≥55MPa (C55/67 and higher). For the concrete classes with fck <55MPa even if these are assigned as EN1992-2, TDA will be done using EN1992-1-1 annex B.

To evaluate the design strength of construction elements with the new concrete grades, a check is performed on minimal and maximal characteristic cylinder strength. A message will appear when the characteristic cylinder strength is exceeded. This message appears in the following checks:
- Design As
- Design of nonprestressed reinforcement tin prestress members
- Check response
- Prestress check response

Another performed check is the check on reinforcement class for adequate ductility. According to article 3.2.4(101) the recommended classes of non-prestressed reinforcement for concrete bridges are B and C. If class A is used, following message appears: "The used non-prestressed reinforcement is not allowed due to low ductility." The warning appears in the following services:
- Design As
- Design of nonprestressed reinforcement tin prestress members
- Crack width
- Check response
- Detailing provisions
- Check capacity
- Prestress crack width
- Prestress check response
- Prestress check capacity

To calculate the ratio of the redistributed moment to the elastic bending moment, the formula for the redistribution of the bending moments has been implemented according to clause 5.5.4 (104) of EN 1992-2.

When a concrete surface is subjected to abrasion caused by ice or solid transportation in running water the cover should be increased by a 10mm. A new option has been added to the Design defaults in the concrete service. Delta_cabr: value of deviation for the concrete cover for members exposed to abrasion by ice or solid transportation in running water
Related to this a new item is inserted in the Concrete 1D member data, called Abrasion deviation.
When the checkbox above is activated, then the minimal calculated cover is increased by the value set in Design defaults:

When a member consists of a material from EN 1992-2 then the global imperfection is calculated in a different way than for a standard EN 1992-1-1 material.
The factor alfa_m is not considered in the formula for theta_i and the factor alfa_h has no lower limit:
The factor theta_i is used in the for the calculation of recalculated internal forces:

When a member consists of a concrete material from EN1992-2 then the adequate rotation capacity for plastic analysis is achieved if the depth of compression zone is restricted to the depth as follows:
This is applied when checking the calculated redistributed bending moment My (around y axis of LCS of the member) by using method 5.6.2.

The bonded prestressing reinforcement in the tensile zone of concrete can be taken into account for the calculation of shear check too according to 6.2.2(1)(101). Currently the prestressing reinforcement is not taken into account for calculation of d. Value Asl is needed for calculation of rho_l.
Shear check according to formula 6.2.

A new factor is implemented to take into account the maximum compressive stress during execution for a quasi-permanent combination. This is according to EN 1992-2, clause 113.3.2(103).
It's available in the National Annex setup manager under Concrete > SLS > Stress limitation.
By default this factor k = 1.
It is applied for the calculation of the allowable concrete stresses. The verification is only done for members where a decompression check is required.

For members where the decompression should be checked (quasi-perm or frequent combination) the allowable stress has to be less than k*fctm(t) for a combination (quasi-perm) from construction stages (k=1,0). This verification is only done for:
- Combinations generated from construction stages (it means stages which are before or a stage marked as Last construction stage)
- Quasi-permanent combination
The procedure is the following:
1) A new value is introduced in the service Allowable concrete stresses - sigma_c,ex – allowable stresses in concrete during execution
2) The comparative values are sigma_clt,max
3) The check value is calculated
This value appears only if :
1) Concrete extension check is activated in the functionality
2) Type of model is set to Construction stages
Astrid Bastiaens
03/06/2012