1. 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 FLOAD f3 (XRA f4 f5 ZRA f6 f7) { GX | GY |
GZ } (INCLINED)
or
YRANGE f1 f2 FLOAD f3 (XRA f4 f5 ZRA f6 f7) { GX | GY |
GZ } (INCLINED)
or
YRANGE f1 f2 FLOAD f3 (XRA f4 f5 ZRA f6 f7) { GX | GY |
GZ } (INCLINED)
or
FloorGroupName FLOAD f3 { GX | GY | GZ } (INCLINED) }
Where:
f1 f2 = Global coordinate values to specify Y, X, or Z range. The load will be
calculated for all members lying in that global plane within the first specified
global coordinate range.
f3 = 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 beginning with XRA, the load acts 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.
f4 - f7 = Global coordinate values to define the corner points of the area on
which the specified floor load (f3) acts. If not specified, the floor load will be
calculated for all members in all floors within the first specified global
coordinate range.
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2. 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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3. 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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4. 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:
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.
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.
…
LOAD 2
FLOOR LOAD
YRA 12.0 12.0 FLOAD -0.25
…
…
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
…
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5. 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:
will not yield acceptable results. Instead, the region should be subdivided as
shown in the following example
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
LOAD 1
FLOOR LOAD
YRANGE 11.9 12.1 FLOAD -0.35
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
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6. 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
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7. 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
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.
_PNL5A 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
LOAD 5 LOAD ON SLOPING ROOF
FLOOR LOAD
_SLOPINGROOF FLOAD -0.5 GY INCLINED
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8. Figure 5.31
Figure 5.32
In the above equations,
A = area of floor supported by the member
R = reduction in percentage
Table - Details of the code implementation
Code Name
Section of code which
has been implemented
Applicable Equations
UBC 1997 1607.5, page Equation 7-1
R = r(A-150) for FPS units
R = r(A-13.94) for SI units
IBC 2000 1607.9.2, page 302 Equation 16-2
R = r(A-150) for FPS units
R = r(A-13.94) for SI units
IBC 2003 1607.9.2, page 277 Equation 16-22
R = r(A-150) for FPS units
R = r(A-13.94) for SI units
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9. 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
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