| Product(s): | RAM Structural System; Ram Steel | ||
| Version(s): | All | ||
| Area: | Modeling; Analysis | ||
| Original Author: | Bentley Technical Support Group |
Are the applied surface loads cumulative?
No, in RAM Structural System only the top applied surface load counts. The underlying loads are not deleted however, so if you delete the top load you can see the original load underneath. If too many load layers are applied to a model, a polygon error can occur when processing the loads. For this reason it is always best to remove any existing surface loads before modeling new layers. If the order of the loads needs to be changed, this can be done using Layout - Loads - Surface Loads - Change Priority. The loads listed first in the list have priority over those below.
The same logic applies to deck polygons. If two or more decks overlap, it's the last or top deck that has priority.
Also note, this is different than the behavior in RAM Concept. In that program, overlapping surface loads are cumulative. Consequently, when Direct Gravity loads are imported from RAM SS into Concept, they are converted into equivalent separate polygons that do not overlap.
Is the structure self-weight included in the loads?
That depends on the settings under RAM Manager - Criteria - Self-Weight. Here the user can automatically include beam, column, wall or deck self-weight. Note, self-weight is never included for open web steel joists or vertical braces.
On the right hand side of the dialog box are the settings for self-weight as it applies to the building mass which is used for seismic loads, dynamic analysis and for P-Delta calculations.
For composite beam design, the self weight is always considered part of the Construction Dead Load. Hence, if all the self weight options are turned on, and there is no other load present during construction, the user applied CDL might be zero.
For open web steel joist systems it's important to note that no self-weight is applied for the steel joists. The reason for this is primarily that the unit weight of steel joists can vary depending on the manufacturer and the configuration of the diagonals. The user should always include extra dead load in the surface loads to account for estimated joist self weight.
In order for the steel gravity beam and column self weight mass to be considered in RAM Frame, it is imperative that those modules be run first, using the design-all process. So long as the RAM Manager indicates a green light next to each of those modules, RAM Frame should have the latest member self weight data available. Freezing the design of all gravity beams and columns is another way to ensure that member self weights are always considered.
Steel column self-weight is based on the column length from story to story, plus or minus any elevation offsets, times the section area and steel density. We do not increase the self-weight based on splices being a few feet above the story datum.
The self-weight of steel gravity columns is not included in the design of a steel gravity transfer beam that supports a column above. This is true because the design of the beams typically happens prior to the design of the steel columns. (In Ram Frame, the column self-weight is applied for the case of a steel gravity column setting upon a lateral beam, assuming that steel column design took place first (green light).
Note, in RAM Frame, under Loads - Masses, the program calculated diaphragm mass totals can be overridden with User Specified values, normally using calculated masses is advised. There is a similar dialog box for the total Gravity Loads which is used to determine program generated notional loads.
Is the additional weight of concrete due to beam sag or "ponding" considered?
No, the self-weight of the deck is based on the thickness and weight parameters set in the Modeler - Deck Properties. When beams sag under the weight of the deck it is a common practice for the topping concrete to be leveled off which adds additional weight to the system assuming it's not cambered or shored. This additional weight should be incorporated into the applied construction dead loads (and masses).
How is the self weight of a Concrete Beam determined?
The program calculates rectangular beam unit self-weight based on the area of the beam times the "Unit Weight of Self weight". The other "Unit Weight" parameter is only used in calculating the elastic modulus, E, of the member.
The Concrete slab can independently be included in the self weight, so in cases where there is a concrete slab and rectangular concrete beams the weight of the concrete times the thickness of the slab and the width of the beam is double counted.
To alleviate this problem, "T" shaped beam sections are handled differently. With T beams, it's only the area of the stem below the slab that is applied as the beam self weight.
This also applies to the material take off report in the Concrete Beam module.
How are partition loads handled?
The Partition Load is an additional Live Load; it is treated as an unreducible Live Load and will not be reduced. It is in addition to the loads specified as Live Load. Partition loads are defined variously by the Codes, some as Dead Load and some as Live Load. For those codes that define Partition loads as an unreducible Live Load, those should be specified here. For those Codes that define Partition loads as a Dead Load or as part of the regular Live Load, those should be included as part of the Dead Load or Live Load accordingly.
Unlike construction live load, the partition live load is not a portion of the total live load entered. You can apply 0 Live Load and still apply 15 psf Partition Live load, for example.
Partition loads are not automatically included in the seismic mass. The total Mass DL should be increased to account for partition weight as required by the code for seismic loads.
Why do the Roof and Floor live loads both use a 1.6 factor in LRFD design?
The Ram Steel Gravity Beam and Column modules do not yet include user specified load combinations. The load combinations are hard coded into the program based on the selected code. Since the load factors on combinations of Dead + Roof + Live loads can vary depending on the live load type or magnitude we took the conservative approach of treating all the live loads with the same load factor. There is an enhancement logged (#120493) to change this so that the more exact code combinations can be used.
Why are my Roof Live loads ignored in the design?
RAM Structural System currently considers Snow OR Roof LL, but not both at the same time. In RAM Manager under Criteria - Members loads there is a toggle to select which the program should consider. Set the toggle to “Consider snow loads, Ignore roof live loads” when snow loads are modeled.
Note: Live Reducible, Unreducible and Storage type loads are always considered, it is only the Live - Roof type loads that are excluded when the option to consider snow loads is turned on.
Are my snow loads or storage live loads automatically added to the building weight for seismic load determination?
No, the program only uses the assigned Mass Dead Loads plus whatever self-mass options are turned on under RAM Manager - Criteria - Self weight when determining the total building mass or weight used in Seismic load determination (and in P-Delta calculations). The user should increase the Mass DL of applied surface loads to account for the weight of the snow load (or a percentage of the weight as required.
Note, the provided templates for load combinations do correctly consider snow load acting simultaneously with Dead, Live and Seismic loads, however.
This also applicable to Storage Live Loads, even if the magnitude entered for the Storage Live load is large (e.g. > 125 psf) no portion of the storage live load is automatically considered in the seismic mass. The user must increase the Mass DL (or manually alter the masses in Ram Frame) when part of a storage live load needs to be added to the seismic mass.
How can I apply a drift snow load?
Within the snow loads, only the top load counts. Since only the top snow load counts, the drift snow load should typically taper down from the max value to the flat-roof snow load as a minimum. The program gives a warning when any portion of the sloping plane of snow load is 0 or less magnitude.
In general, it’s best to define snow drift loads with M1 and M2 set to the highest value, and M3 set to the flat roof level. Then the loads can be applied in rectangular or trapezoidal areas as required. In the image below, the total snow load on the left is 50 psf tapering down to 30 psf on the right. This would be used in conjunction with a flat roof snow load of 30 psf applied first to the whole roof.
See Also
[[RAM Steel Beam Pattern Loading]]