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Background
Designing floor systems for the effect of a concentrated live load placed anywhere on the floor system such that it produces the maximum effect on the structural members is required by the the National Building Code of Canada 4.1.5.10.
The magnitude and area over which this load is applied is specified according to the building type, occupancy or use of the floor (e.g garage floors) The most common concentrated live load applicable to wood structures is for office floors, the application of office concentrated loads is mandated in the design codes and standards. Load magnitudes and load footprint areas are also specified and can be applicable for other wood buildings and floor uses (for instance, libraries, hospitals and passenger car garages). The load footprint area is always square.
Input of Load
When the Concentrated load checkbox is checked, the program automatically adds a partial area load to the load list, and disables all other loads. The partial area load is a moving load that will be applied at intervals over the length of the joist. You can modify the magnitude of the concentrated live load and the area over which it is applied.
Note that if this is the first load you place on the member with will not be able to add another loads on the member. Load the member with other loads first and then check the Concentrated Load option.
The default magnitude is 9.0 kN for floor joists over an area of 750 x750 mm, and 1.3 kN for roof joists, over an area of 200 x 200 mm. It is possible fo both floor and roof joists.
Internal Representation of Load
The specified concentrated live load is resolved from a partial area load to a partial line load acting on an individual joist. The line load can be located anywhere along the length of the joist. Two cases exist for determining the magnitude of the partial line load. If the load footprint extends more than halfway to the adjacent joists, then the area load intensity C / d2 times the spacing s is used. Otherwise, it is the area load intensity multiplied by width of load d:
For d < s : LC = C / d
For d > s: LC = C s / d2
d = dimension of the concentrated load footprint area (d x d), feet
s = joist spacing, feet or m
C = magnitude of the concentrated live load (lb or kN)
LC= resultant partial line load representing the effect of the concentrated load, (lb/ft, or kN/m)
Load Combinations
The concentrated load is treated as a live load that replaces the live load due to occupancy, storage, or controlled fluids in Sizer load combinations. It is applied only in combination with dead loads and other non-live loads such as snow and wind. In other words, it is considered to act in place of any live load for all load combinations (and patterns) specified by the design code.
The program verifies all of the load combinations it ordinarily does, plus those with the concentrated load. The extra combinations are comprised of the moving concentrated load combined with the self-weight and the other live loads present on the joist (dead, snow, wind, and impact and construction for the USA version and hydrostatic, earthquake, dead (soil), for the Canadian version).
Concentrated load combinations are not created for pattern loads - they themselves are a form of patterning.
Locations and Beam Length
Internally, the program creates a large number of "virtual" load combinations corresponding to positions on the beam of the concentrated load, for each Concentrated Load Combination. .
The interval for moving the concentrated load along the beam is 1 foot for beams under 60ft and to 2 feet for beams over 60ft. When pressing "Design", if the step is set at 2 feet, a warning message will inform the user. The check is limited to beams under 120ft. If the beam length is over 120ft, when pressing "Design" a warning message will inform the user that the check is not being performed and the design will be canceled.
In order to perform this check, the maximum number of virtual load combinations has been set at 1000. By limiting the beam's length to 120ft and setting the moving step to 2 ft, the worst-case scenario creates around 840 load combinations. If somehow the number of generated load combinations would pass 1000, the check will end at the 1000th load combinations and a message will warn you that the results are not accurate.
Analysis
The program determines the worst case for shear, bending and deflection from all load combinations, regular and concentrated, when performing the analysis. It will not perform separate analysis for concentrated and non-concentrated situations; you must run two separate designs for this.
Design Cycle
Design proceeds as usual - if any of the concentrated load virtual combinations is critical for shear, moment, or deflection, the program compares the resistance to the critical value. During full design, a section must pass the Concentrated Load Check along with the other checks in order to be included in the Suggested Sections list. During design check, a warning is issued if the section fails the Concentrated Load check.
Deflection Design
The program always checks total deflection for the Load Combinations which include the Concentrated Load, regardless of the user's choice for deflection check, and use a total deflection factor of 0.5 for these load combinations.
The regular load combinations are checked using the settings input by the user. The deflection limits for the Concentrated Load Combinations are the same as for the other Load Combinations.
Bearing Design
Concentrated load combinations are not used for bearing design.
Analysis Report
Only one Concentrated Load Combination corresponding to each of the ordinary load combinations for will be included in the Load Combinations (LC's) list. For example, if only dead loads are to be considered, there will be only one concentrated LC. Only the worst design ratio responses for all the Concentrated Load Combinations are shown in the Shear and Bending table for shear, bending, span bending and reactions. The Concentrated Load in Concentrated Load Combinations is symbolized as Lc.
Design Check Report
The responses associated with the Concentrated Load Check are shown only if this is the governing Load Combination case; otherwise the only mention of this check is in Design Notes. If the section fails this check, this is noted in the list of failed criteria.
Analysis Diagram
A Concentrated Load Combination appears only if the "Critical Results" load combination is selected by the user, and then only if the Concentrated combination governs for the diagram in question. Concentrated Load Combinations are not listed in the dropdown for users to select load combinations to view.
This topic is further discussed in - Load Combinations, Pattern Loads