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Deck areas are a convenient tool for generating distributed loads associated with surface loads on members. For general information on modeling deck areas, please see Chapter 18 in the RAM Elements Manual. Additional modeling tips and common issues with deck areas are discussed below.
Deck area distribute load to members that border the deck area only. A load will not be distributed to any members that are inside but do not border the actual deck area.
Shells do not qualify as perimeter member. If a deck is bordered by a shell on one side as shown in the screen capture below, a error indicating that no member is connected between two nodes of the deck area will be displayed during the analysis (see error message below). To work around this problem, model a GEN section with very low stiffness between the nodes at the top of the shell.
Because loads are only distributed to members that border the deck area, a separate deck area needs to be modeled in each bay of framing. The fastest way to do this is to first select all members on a level and then click on one of the active spreadsheet tools shown below to generate the deck area:
This will generate a deck area in each bay as shown below:
In some cases, it may be convenient to use a deck area for both in-plane and out-of-plane pressures. One practical example is a perimeter wall that is subject to out-of-plane pressure due to wind load and in-plane pressure associated with the dead weight of the cladding. The current implementation of the deck area feature will not calculate distribute loads for some in-plane pressures. If the surface load vectors are defined so that they are parallel and in-plane of the deck area, then no distributed load will be calculated. However, if the surface load vectors are defined so that they are in-plane of the deck area but perpendicular to the deck span, then a distributed load will be calculated.
In the screen capture below, the deck area on the right is defined with Vector Y = 1 for the deck span and Y Dir = -1 for the surface load. Since the surface load is in-plane and in the same direction as the deck, no load is distributed to the members. The deck area on the left is defined with Vector X = 1 for the deck span and Y Dir. = -1 for the surface load. Since the surface load is in-plane but perpendicular to the deck, an in-plane distributed load is applied to the members.
There is also a bi-directional feature for deck areas that can be used to distribute loads in two directions. When using this feature to model two-way decks, it is important to note that the program calculates the tributary area of the member, determines the tributary load to the member, and applies a uniformly distributed load over the length of the member based on this load. The program will not distribute a triangular or trapezoidal load as may be expected.
For bi-directional deck areas, the resultant of the deck span vector (Vector X, Vector Y, and Vector Z) should always be a unit vector. The magnitude of the load that is applied is scaled by the resultant. In the screen capture below, a 1 ksf pressure is applied to a 20 ft x 20 ft deck area. The deck area on the left is defined with Vector X = 0.707 and Vector Z = 0.707. The resultant = [0.707^2 + 0.707^2]^0.5 = 1. The deck area on the right is defined with Vector X = 1 and Vector Z = 1. The resultant = [1^2 + 1^2]^0.5 = 1.414. Note that the total load for the deck area on the left is 1*(1 psf)*(20 ft)*(20 ft) = 400 k. However, the total load for the deck area on the right is 1.414*(1 psf)*(20 ft)*(20 ft) = 565.6 k.
No Distributed Loads Generated for Deck Area