After each construction stage, the finite element analysis produces results that can be utilized to ensure structural compliance with codes. To create the stages correctly, users must understand the underlying logic. At the start of the stage construction analysis, all structural elements are in an unconstructed state.
Constructing a typical steel bridge requires defining at least four stages: pier/foundation construction, girder and bracing construction, application of deck loads on non-composite girders, and deck stiffness gain. Introducing deck pouring stages will require additional stages for non-composite and composite stages. For example, if the deck is poured in three stages, six stages will be necessary (three for deck load application on girders and three for deck stiffness gain stages).
For permanent loads, such as wearing surface loads and barriers, two extra stages may be required.
Additional stages will be necessary for each transient load, such as wind loads, braking loads, live loads, and temperature loads.
Stage
Prior Stage: The continuity of stages is maintained using the prior stage parameter, which instructs the software on which stage to analyze next. For the first stage, the prior stage will be none. If users wish to apply temporary loads like wind or live load, they can select the final permanent load stage as the prior stage for all transient loads.
Construction Method[none/equal/match]: Usually, node coordinates are utilized as user inputs without any modification. However, certain scenarios require adjustments to the node locations based on prior stage deflections. For example, during the construction of girders and the application of deck load, girder nodes experience displacement while deck nodes do not. In construction site scenarios, the deck formwork also undergoes deflection due to girder displacement. To address this, the node coordinates of the deck must be adjusted based on girder displacements. The "equal" parameter modifies the initial node coordinates of the deck shell elements by applying the top node displacements of the girder to the closest nodes. For a typical steel I-girder bridge, pier construction, steel I-girder construction, load application on deck, and transient load stages will typically be defined with the "none" parameter. However, the stage that represents the deck construction (the deck hardens stage) should be defined with the "equal" parameter. The match cast technique is typically not used in the construction of steel I-girder bridges and is generally applicable to segmental bridges.(need screenshot)
Load Type: Selecting the appropriate load types, such as dead load, wearing surface load, and wind load, for each stage is crucial, and different load types should not be combined in a single stage. In the following steps, users must combine the results of each stage based on their load type using AASHTO load factors before code checks. Combining results becomes more challenging if more than one load type is applied in one stage or if load types are not selected correctly. In summary, load type allows users to filter results according to its load type, providing a way to organize and analyze results more effectively.
Active[Yes/No]: Users can deactivate specific stages to expedite the model's runtime, especially if they are interested in something from the earlier stages. However, if the deactivated stages negatively affect the continuity of stages, the staged construction analysis will fail to run successfully. Therefore, when users deactivate certain stages, they must ensure that all active stages' prior stages are still active.
Time Dependent
Typically, in a steel I-girder design project, time-dependent staged construction analysis is not necessary. However, if the bridge owner requests CEB-FIP 1990 creep/shrinkage computations for the substructure, or if post-tensioned tendons are employed for the pier cap or pier columns, time-dependent analysis can be conducted. In such cases, the following parameters can be used:
Construction Day:
Code[CEB-FIP 1990]: At present, only the CEB-FIP 1990 code is supported. If any other code is required, please contact the support team, and it can be added to the OpenBrIM Library.
Temperature [F]: Temperature during curing
Humidity [%]: Relative humidity during curing
Time Dependent Elastic Modulus[Include/Ignore]: The time-dependent modulus of elasticity is a parameter used to account for the change in elastic modulus over time due to creep and shrinkage for concrete. When the time-dependent modulus of elasticity is included, the modulus of elasticity values entered under basic tab are overridden based on the computed values. According to CEB-FIP 1990, the following parameters are used to compute the time-dependent modulus of elasticity:
Concrete compressive strength at 28 days (fcm)
Cement hardening type
Relative humidity and temperature during curing
Age of concrete at time of loading (t)
These parameters are used to calculate the elastic modulus at different times during the life of the concrete member. In OpenBrIM, all calculations are based on the secant modulus of elasticity. However, CEB-FIP computes the tangent modulus. Once the tangent modulus is obtained using the CEB-FIP procedure, OpenBrIM divides the E value by 1.05 before utilizing it in finite element analysis (FEA). To ensure that this effect is included in the analysis, both the material and stage settings should be selected to include it.
Concrete Creep Effect[Include/Ignore]: Concrete creep is a time-dependent deformation of concrete under sustained load. The CEB-FIP model considers several parameters to determine the creep coefficient, which is used to calculate the time-dependent deformation of the concrete. These parameters include the concrete compressive strength at 28 days (fcm), maximum aggregate size (Dmax), cement type and percentage of cement content, relative humidity and temperature during curing, age of concrete at time of loading (t), and applied stress level (σ). OpenBrIM monitors stress changes at each stage and calculates creep accordingly by utilizing these parameters. To ensure that this effect is included in the analysis, both the material and stage settings should be selected to include it.
Concrete Shrinkage Effect[Include/Ignore]: Concrete shrinkage refers to the reduction in the volume or size of concrete over time due to the loss of moisture. When concrete is first poured, it contains a lot of water which evaporates gradually over time causing the concrete to shrink. This can result in cracks or deformations in the concrete which can affect its structural integrity. The parameters used in CEB-FIP for concrete shrinkage are the age at loading, the age at unloading, the duration of loading, the humidity of the environment, and the type and composition of the concrete mix. Additionally, CEB-FIP also considers the influence of creep on shrinkage. To ensure that this effect is included in the analysis, both the material and stage settings should be selected to include it.
Steel Relaxation Effect[Include/Ignore]: To account for relaxation effects in tendons, time-dependent material property definitions are assigned to the relevant materials. To ensure that this effect is included in the analysis, both the material and stage settings should be selected to include it.
PT Losses from Structure[Include/Ignore]: Elastic shortening losses refer to the reduction in the length of a prestressed concrete member caused by the initial stress generated during the pre-tensioning process or other external loads that could alter the length of the member. To account for this effect in the analysis, it is essential to select both the material and stage settings that include it.
AASHTO N-3N
Material: Choose the deck material to override for short-term and long-term properties
Composite Section[NA/Short Term/Long Term]: If the short-term option is selected, this parameter will override the modulus of elasticity values of the selected material. The short-term modulus of elasticity is computed by dividing the selected deck material's modulus of elasticity by 3. As per AAHSTO standards, the deck's modulus of elasticity and section modulus must be adjusted for design purposes when calculating stresses. OpenBrIM already computes stresses during the design phase based on long-term/short-term section properties. However, if an analysis beyond the design phase is required with modified E for short-term and long-term loads, this parameter can be used.
Add Comment