Floor Load

Area Load/Oneway Load/Floor Load Specification

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Technical Reference Manual 5.32.4.3 Floor Load Specification Purpose Used to distribute a pressure load onto all beams that define a closed loop assuming a two way distribution of load. The Floor Load specification be applied to groups and can also use live load reduction per IBC or UBC codes.

General Format FLOOR LOAD { YRANGE f1 f2 GZ } (INCLINED)

FLOAD f3

(XRA

f4

f5

ZRA

f6

f7) { GX | GY |

or

YRANGE f1 f2 FLOAD f3 GZ } (INCLINED)

(XRA

f4

f5

ZRA

f6

f7) { GX | GY |

YRANGE f1 f2 FLOAD f3 GZ } (INCLINED)

(XRA

f4

f5

ZRA

f6

f7) { GX | GY |

or

or FloorGroupName FLOAD

f3 { GX | GY | GZ } (INCLINED) }

Where: f 1 f 2 = Global coordinate values values to specify Y, X, or Z range. range. The load will be calculated for all members lying in that global plane within the first specified global coordinate range. f 3 =

The value of the load (unit weight over square length unit). If the

global direction is omitted, then this load acts parallel to the positive global Y if command begins with YRA and based on the area projected on a X-Z plane. Similarly, for commands commands beginning with with XRA, the load acts parallel parallel to the positive global X and based on the area projected on a Y-Z plane. Similarly, for commands beginning with ZRA, the load acts parallel to the positive global Z and based on the area projected on a X-Y plane. f 4 - f 7 = Global coordinate values to to define the corner points points of the area on which the specified floor load (f3) acts. If not specified, the floor load load will be calculated for all members in all floors within the first specified global coordinate range.


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GX,GY,GZ = If a Global direction is included, then the load is re-directed to act in the specified direction(s) with a magnitude of the loads which is based on the area projected on a plane as if the Global direction was omitted. The Global direction option is especially useful in mass definition. FloorGroupName = Please see section 5.16 of this manual for the procedure for creating FLOORGROUPs. The member-list contained in this name will be the candidates that will receive the load generated from the floor pressure. Notes

a. The structure has to be modeled in such a way that the specified global axis remains perpendicular to the floor plane(s). b. For the FLOOR LOAD specification, a two-way distribution of the load is considered. For the ONEWAY and AREA LOAD specification, a one-way action is considered. For ONE WAY loads, the program attempts to find the shorter direction within panels for load generation purposes. So, if any of the panels are square in shape, no load will be generated for those panels. For such panels, use the FLOOR LOAD type. c. FLOOR LOAD from a slab is distributed on the adjoining members as trapezoidal and triangular loads depending on the length of the sides as shown in the diagram. Internally, these loads are converted to multiple point loads.

Figure 5. 25 - Members 1 and 2 get full trapezoidal and triangular loads respectively. Members 3 and 4 get partial trapezoidal loads and 5 and 6 get partial triangular load. d. The load per unit area may not vary for a particular panel and it is assumed to be continuous and without holes. e. The FLOOR LOAD facility is not available if the SET Z UP command is used (See Section 5.5.) f. If the floor has a shape consisting of a mixture of convex and concave edges, then break up the floor load command into several parts, each for a certain region of the floor. This will force the program to localize the search for panels and the solution will be better. See illustrative example at the end of this section. g. At least one quadrilateral panel bounded on at least 3 sides by "complete" members has to be present within the bounds of the user-defined range of coordinates (XRANGE, YRANGE and ZRANGE) in order for the program to successfully generate member loads from the FLOOR/ONEWAY LOAD specification. A "complete" member is defined as one whose entire length between its start and end coordinates borders the specified panel.


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The load distribution pattern depends upon the shape of the panel. If the panel is Rectangular, the distribution will be Trapezoidal and triangular as explained in the following diagram.

Figure 5.26 For a panel that is not rectangular, the distribution is described in following diagram. First, the CG of the polygon is calculated. Then, each corner is connected to the CG to form triangles as shown. For each triangle, a vertical line is drawn from the CG to the opposite side. If the point of intersection of the vertical line and the side falls outside the triangle, the area of that triangle will be calculated and an equivalent uniform distributed load will be applied on that side. Otherwise a triangular load will be applied on the side.

Figure 5.27 Example

The input for FLOOR LOAD is explained through the following example. Consider


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the following floor plan at y = 12:

Figure 5.28 If the entire floor has a load of 0.25 (force/unit area), then the input will be as follows:

… LOAD 2 FLOOR LOAD YRA 12.0 12.0 FLOAD -0.25 … If in the above example, panel A has a load of 0.25 and panels B and C have a load of 0.5, then the input will be as follows: Note the usage of XRANGE, YRANGE, and ZRANGE specifications.

… LOAD 2 FLOOR LOAD YRA 11.9 12.1 FLOAD -0.25 XRA 0.0 11.0 ZRA 0.0 16.0 YRA 11.9 12.1 FLOAD -0.5 XRA 11.0 21.0 ZRA 0.0 16.0 LOAD 3 … The program internally identifies the panels (shown as A, B, and C in the figure). The floor loads are distributed as trapezoidal and triangular loads as shown by the dotted lines in the figure. The negative sign for the load signifies that it is applied in the downward global Y direction.


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Illustration of Notes Item (f) for FLOOR LOAD The attached example illustrates a case where the floor has to be sub-divided into smaller regions for the floor load generation to yield proper results. The internal angle at node 6 between the sides 108 and 111 exceeds 180 degrees. A similar situation exists at node 7 also. As a result, the following command:

LOAD 1 FLOOR LOAD YRANGE 11.9 12.1 FLOAD -0.35 will not yield acceptable results. Instead, the region should be subdivided as shown in the following example

LOAD 1 FLOOR LOAD YRANGE 11.9 12.1 FLOAD -0.35 XRA -.01 15.1 ZRA -0.1 8.1 YRANGE 11.9 12.1 FLOAD -0.35 XRA 4.9 10.1 ZRA 7.9 16.1

Figure 5.29 h. The global horizontal direction options (GX and GZ) enables one to consider AREA LOADs, ONEWAY LOADSs and FLOOR LOADs for mass matrix for frequency calculations. i. For ONE WAY loads, the program attempts to find the shorter direction within panels for load generation purposes. So, if any of the panels are square in shape, no load will be generated on the members circumscribing those panels. In such cases, one ought to use the FLOOR LOAD type.

Applying FLOOR LOAD onto a Floor Group


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When applying a floor load using XRANGE, YRANGE and ZRANGE, there are two limitations that one may encounter: a. If panels consist of members whose longitudinal axis cross each other in an X type, and if the members are not connected to each other at the point of crossing, the panel identification and hence the load generation in that panel may fail. A typical such situation is shown in the plan drawing shown in the next figure.

Figure 5.30 b. After the load is specified, if the user decides to change the geometry of the structure (X, Y or Z coordinates of the nodes of the regions over which the floor load is applied), she/he has to go back to the load and modify its data too, such as the XRANGE, YRANGE and ZRANGE values. In other words, the 2 sets of data are not automatically linked. The above limitations may be overcome using a FLOOR GROUP. A GROUP name is a facility which enables us to cluster a set of entities – nodes, members, plates, solids, etc. into a single moniker through which one can address them. Details of this are available in section 5.16 of this manual. The syntax of this command, as explained earlier in this section is:

FLOOR LOAD Floor-group-name FLOAD

f3 { GX | GY | GZ }

Where: f3 = pressure on the floor To create equal loads in all 3 global directions for mass definition or other reasons, then enter direction labels for each direction desired; GY first then GX and/or GZ. Example

START GROUP DEFINITION FLOOR


_PNL5A

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21 22 23 28

END GROUP DEFINITION LOAD 2 FLOOR LOAD On Intermediate Panel @ Y = 10 Ft FLOOR LOAD _PNL5A

FLOAD -0.45 GY

_PNL5A FLOAD -0.45 GY GX GZ INCLINED - This option must be used when a FLOOR LOAD is applied on a set of members that form a panel(s) which is inclined to the global XY, YZ or ZX planes. Example

LOAD 5 LOAD ON SLOPING ROOF FLOOR LOAD _SLOPINGROOF FLOAD -0.5 GY INCLINED

Live Load Reduction per UBC and IBC Codes The UBC 1997, IBC 2000 and IBC 2003 codes permit reduction of floor live loads under certain situations. The provisions of these codes have been incorporated in the manner described further below. To utilize this facility, the following conditions have to be met when creating the STAAD model. 1. The live load must be applied using the FLOOR LOAD or ONEWAY LOAD option. This option is described earlier in this section of this manual, and an example of its usage may be found in example problem 15 of the Examples manual. 2. As shown in section 5.32, the load case has to be assigned a Type called Live at the time of creation of that case. Additionally, the option called Reducible, also has to be specified as shown.

LOAD n LOADTYPE Live REDUCIBLE Where: n is the load case number The following figures show the load generated on members for the two situations.


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Figure 5.31

Figure 5.32 Table - Details of the code implementation Code Name UBC 1997

Section of code which has been implemented 1607.5, page

Applicable Equations Equation 7-1 R = r(A-150) for FPS units

IBC 2000

1607.9.2, page 302

R = r(A-13.94) for SI units Equation 16-2 R = r(A-150) for FPS units

IBC 2003

1607.9.2, page 277

R = r(A-13.94) for SI units Equation 16-22 R = r(A-150) for FPS units R = r(A-13.94) for SI units

In the above equations, A = area of floor supported by the member R = reduction in percentage


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R = rate of reduction equal to 0.08 for floors. Notes

a. Only the rules for live load on Floors have been implemented. The rules for live load on Roofs have not been implemented. b. Since the medium of application of this method is the FLOOR LOAD or ONEWAY LOAD feature, and since STAAD performs load generation on beams only, the rules of the above-mentioned sections of the code for vertical members (columns) has not been implemented. The distributed load on those members found to satisfy the requirements explained in the code would have a lowered value after the reduction is applied. c. Equation (7-2) of UBC 97, (16-3) of IBC 2000 and (16-23) of IBC 2003 have not been implemented. d. In the IBC 2000 and 2003 codes, the first note says “A reduction shall not be permitted in Group A occupancies.” In STAAD, there is no direct method for conveying to the program that the occupancy type is Group A. So, it is the user’s responsibility to ensure that when he/she decides to utilize the live load reduction feature, the structure satisfies this requirement. If it does not, then the reduction should not be applied. STAAD does not check this condition by itself. e. In the UBC 97 code, the last paragraph of section 1607.5 states that “The live load reduction shall not exceed 40 percent in garages for the storage of private pleasure cars having a capacity of not more than nine passengers per vehicle.” Again, there is no method to convey to STAAD that the structure is a garage for storing private pleasure cars. Hence, it is the user’s responsibility to ensure that the structure satisfies this requirement. If it does not, then the reduction should not be applied. STAAD does not check this condition by itself. f. Because all the three codes follow the same rules for reduction, no provision is made available in the command syntax for specifying the code name according to which the reduction is to be done. Related Topics

Floor Load

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