Continuous Purlins

This mode enables to perform checks of continuous girders of a round or rectangular section. The dialog box of this mode contains two tabs: General and Loads.

The General tab is used to specify the number of spans and their lengths. You also have to specify information on the cross-section, timber, and service factors. The roof inclination can also be specified here (pitch force component will be taken into account). If it is necessary to perform the analysis for the serviceability limit state, check the Deflection limitation checkboxs (for characteristic and/or characteristic long-term loads) and select (or enter) the maximum ratio of the maximal deflection to the length of the span in the respective drop-down list.

The Loads tab is used to specify the loads acting on the purlin. The application is capable of analyzing several patterns of loading and each load case can in its turn consist of multiple loads.

To enter a new load case (including the first one), follow these steps:

• click the Create button in the Load case group;
• select a load case type (permanent, temporary long-term, temporary short-term, snow, or wind), which determines the combination factors according to SNiP to be used with the loads of this load case in a combination of loads;
• select a load type by clicking the respective button;
• enter values for the parameters of the load;
• click the Add button.

A few load components can be specified for each load case. It is assumed that the design values of loads are entered.

Depending on the load type, its parameters may include:

• for distributed loads — the load intensity;
• for a distributed load on a part of the span — the intensity of the load, its position and width of application;
• for a concentrated force — the value of the force and its position in the span;
• for a concentrated moment — the value of the moment and its position in the span;
• for a trapezoid load — value of the load at the beginning of the application area, distance from the beginning of the bar to the beginning of the application area, width of the load application area, value of the load at the end of the application area;
• no additional data is required to specify the dead weight.

For each load (except for the dead weight) you have to specify a span to which this load is applied (it is selected in the Span drop-down list).

The Delete button is used to delete a load case (not a separate load included in it).

To switch to the next load case, click the Create button again, and the number of loadings will be automatically increased by one. If you need to view or modify the data from any of the previously entered load cases, just select its number in the Load case list.

Once you click the Add button, an image of the current loading is displayed in the Diagrams field with the superimposed diagrams of the bending moments and shear forces underneath.

Once you have entered all the loadings, you can view the envelopes of moments and their respective shear forces, or the envelopes of shear forces and their respective bending moments. The criterion (maximal/minimal moment, maximal/minimal shear force) can be selected from the drop-down list. The envelope diagrams are plotted according to SNiP 2.01.07-85* "Loads and actions".

The All, Force, and Moment buttons enable you to select a mode for displaying the diagrams: diagrams of moments and shear forces together, only shear forces, and only moments respectively. Furthermore, clicking the button will display the design values of support reactions.

If you place the mouse pointer in the diagram field, values of the moment and shear force in a particular cross-section corresponding to the position of the pointer will be displayed.

If it is necessary to perform the analysis for the serviceability limit state, you should check whether the safety factor for load is specified correctly for each load case, because the calculation of deflections is based on the characteristic load values.

If you want to change the load value or delete the load from a load case, use the table of loads (the button in the Load case group). The Table of loads dialog box, which opens by clicking this button, displays the type, the value, and the position of the load. Changes made to the load parameters will be saved after you exit the table by clicking the OK button.

Calculation

To perform the calculation of the load-bearing capacity utilization factors, click the Calculate button.

The purlins are checked for the following factors:

If the Deflection limitation checkbox is checked in the General tab, the deflection envelopes will be calculated. The maximum value of the deflection will be displayed in the Maximum deflection group in the General tab. The button in the same group invokes a dialog box with deflection diagrams.

It should be noted that if the roof inclination is not zero, the purlins are subjected to biaxial bending (bent in two planes). Therefore, the deformed axis of a purlin is a spatial curve. Bending moment diagrams and deflection envelopes are plotted for the zero roof inclination (!). However, the values of the factors for strength of the element under the action of the bending moments My and Mz, the value of the factor for strength at the biaxial bending, and  the extreme value of the deflection and the respective factor are calculated precisely  taking into account the roof inclination specified by the user.

Limitations

Formula (50) of SNiP, KMK (formula (55) of SP 64.13330.2011, formula (61) of SP 64.13330.2017) is not used when calculating the maximum deflection because purlins are assumed to have a constant height, and Table 3 of Annex 4 of SNiP, KMK (Table F.3 of Annex F of SP 64.13330.2011, Table F.4 of Annex F of SP 64.13330.2017) requires that k=1, с=0 (the first line in the table is the most frequently used case).