Overhead Crane Loads, Additional Seismic Masses and Different R–D
This article covers three topics specific to precast industrial buildings: additional masses that participate in the structural system only as seismic mass, overhead travelling crane loads, and different R and D coefficients for the upper and lower parts in buildings with a mezzanine floor. For the modeling steps, see the article Modeling Precast Structures.
1. Façade panel and crane masses (additional seismic mass)
For masses that must participate in the structural system only as seismic mass and not as weight, such as façade panels hung on columns or the crane bridge:
- Place a predefined joint at the point where the mass acts (e.g. on top of the column at the gutter beam elevation).
- Select the joint and open the Predefined Joint Properties window; in the Additional Seismic Mass box on the General tab, enter the mass in tonnes (= kN·s²/m).
The additional mass is included in the seismic mass of the floor where the joint is located, in the modal analysis mass matrix, in the equivalent seismic load, in the second-order indicator and in the upper/lower part ratios. Since it is not weight, it is not added to the vertical loads or the foundation loads. The floor assignment is made according to the joint’s elevation.
When several joints are selected, if the box is left empty the existing values are kept. Typical use: half of the panel weight (the share carried by the upper connection) at the tops of the columns at the gutter beam elevation, and the share of the crane bridge per wheel at the crane columns.
2. Overhead travelling crane loads
The Macros → Overhead travelling crane loads... window calculates the crane girder design forces, the vertical and horizontal forces transferred to the columns and the seismic mass share from the wheel reactions; if you wish, it writes the loads into the model.
| Input | Description | Example value |
|---|---|---|
| Lifting capacity (kN) | Crane capacity | 200 |
| R1max, R1min, R2max, R2min (kN) | Wheel reactions from the manufacturer’s table: loaded side (R1) and opposite side; maximum and minimum for the two trolley positions | 153 / 49 / 134 / 30 |
| Wheel spacing a (m) | Distance between the two wheels on one crane girder | 3.84 |
| Dynamic factor ψ | Magnifies the vertical and horizontal crane forces | 1.40 |
| Crane girder span L (m) | Axis spacing | 7.5 |
| Girder + rail q (kN/m) | Self-weight of the crane girder and rail | 5.1 |
| Corbel top elevation Z (cm, absolute) | Elevation of the crane corbel; loads are written at this elevation on the column axis | 800 |
| Corbel eccentricity e (cm) | Distance of the wheel load from the column axis (produces the moment M = V·e) | 0 |
| Target load case | User-defined load case to which the forces will be written | — |
| Horizontal force | Bridge +/− (main frame direction, ψ·P/10), Rail +/− (rail direction, ψ·P/7) or none | — |
- Calculate: The bridge self-weight, the mass share per wheel, the crane girder moments and shears, the reaction envelopes transferred to the column and the horizontal forces are listed. The crane girder is designed separately with these values.
- Create 4 crane load cases: Defines the load cases KrenX+, KrenX−, KrenY+, KrenY− (crane X+, X−, Y+, Y−). The load case and combination scheme must be user-defined (General Project Settings → Load Combinations).
- Apply to selected columns: Creates or updates a load point on the axis of each selected column at the corbel elevation: the vertical force, the eccentricity moment and the selected horizontal force are written to the target load case. If Also write the bridge mass share is checked, the additional seismic mass is entered as well. When the command is run again, the values are overwritten (no double summation). For the columns of the opposite axis, apply a second time with the R1min/R2min values.
The combinations containing the crane load cases (e.g. 1.4G + 1.6Q ± Kren) are defined by the user in the Load Cases window; the program does not generate crane combinations automatically for reinforced concrete. The crane corbel (short corbel) is not modeled separately as a member and is not checked.
In reinforced concrete projects: In this edition the load case and combination scheme cannot be selected as user-defined in reinforced concrete and precast reinforced concrete projects; therefore the Create 4 crane load cases and Apply to selected columns buttons do not work in a reinforced concrete project. In these projects use the Calculate results in the crane girder and corbel design, and enter the bridge mass as Additional Seismic Mass (Section 1).
3. Different R and D for the upper and lower parts (TBDY 4.3.6)
For buildings in which the lower part (e.g. the mezzanine section) has a different structural system from the upper part, the Macros → Different R-D for upper/lower parts (TBDY 4.3.6)... window is used. The analysis is carried out for the whole building with the coefficients of the upper part (R, D in General Project Settings); the internal forces of the lower part members are corrected with the ν coefficient given in the code.
| Input | Description | Default |
|---|---|---|
| Apply different R and D coefficients for the upper/lower parts | Enables the option | off |
| Last floor of the lower part (floor no.) | This floor and the floors below it are considered the “lower part” | 1 |
| Lower part R (X), R (Y) | Response modification coefficient of the lower part structural system (TBDY Table 4.1) | 8 |
| Lower part D (X), D (Y) | Overstrength factor of the lower part (TBDY Table 4.1) | 3 |
| νupper (X), νupper (Y) | Ratio of the upper part base shear to the total base shear; if 0 is entered, it is calculated automatically from the floor weights and heights | 0 |
The Calculate / Show button displays the values νupper, νlower, ν, (R̄a)lower and D̄lower.
In the analysis: the ductile internal forces of the lower part members coming from the seismic load cases are magnified with ν, non-ductile behavior (shear force capacity calculations) is carried out with D̄lower, and the relative storey drift of the lower part is calculated with (R̄a)lower. The setting is saved with the project.
Example (book building): Upper part R = 3, D = 2; lower part R = 5, D = 2.5; last lower floor 1; νupper = 0.408 (X) / 0.406 (Y) → ν = 0.763 / 0.762, (R̄a)lower = 3.93, D̄lower = 1.81. If a one-to-one comparison is required, the νupper value can also be entered manually.
4. Frequently encountered situations
| Symptom | Solution |
|---|---|
| “Create 4 crane load cases” does not add load cases. | The load case and combination scheme must be user-defined; in a reinforced concrete project this scheme cannot be selected in this edition (Section 2). |
| Crane loads appear to be summed twice. | “Apply to selected columns” overwrites the values; select separate target load cases for different positions. |
| The additional mass does not appear in the foundation loads. | This is the expected behavior: the additional seismic mass is not weight and participates only in the seismic calculation. |