Tension-only brace members are capable of resisting tensile forces but do not have resistance to compressive forces. They behave similar to regular frame members except they do not provide any stiffness in compression.

RAM Frame utilizes a nonlinear analysis algorithm to track tension-only members during solution of the structural model under applied lateral loads. The program applies the loads for a given load case to the structure and iteratively controls the stiffness of the tension-only members. In some cases, it is necessary to apply the loads incrementally in order for the analysis to converge to a solution. When this is necessary, the program incrementally increases the applied loads while it searches the correct equilibrium state. If the equilibrium state is not found, the loads are applied with smaller increments. The program automatically adjusts load increments as it processes lateral loads.

Further information on analysis with tension only members are provided in Additional Load Cases Generated for Analysis with Tension-Only Members .

It should be noted that only axial forces in the tension-only members are monitored during the iterative analysis. In other words, bending stiffness of tension-only members still exists if the member's ends are not released (or fixed) and it is not changed during the iterative process.

For models with tension-only braces, the superposition of analysis results is not appropriate for any two load cases for which the direction of the load in any given brace differs. This is due to the fact that for one load case some braces may be in tension while for another load case other braces may be in tension. Generally the load cases must first be combined and then the load combinations iteratively analyzed, rather than the load cases independently analyzed and then combined. Note, however, that RAM Frame does not analyze load combinations; it analyzes load cases and then combines the results in the combinations. Therefore, in order for these results to be valid, the following assumptions are necessary:

It is assumed that tension-only members do not carry Gravity loads. Hence, in RAM Frame the tension-only members are turned off when the Dead and Live Load cases are analyzed. This allows gravity load case results to be superimposed with lateral load cases without requiring that those combinations be explicitly iterated on.

In Eigen solution and dynamic load cases, and in the calculation of centers of rigidity, tension-only members are treated as regular brace members (i.e., they carry tensile and compressive forces) except that their axial stiffness (and bending stiffness if their ends are not pinned) is reduced by half. This is based on the assumption that tension-only braces generally occur in pairs, and that for any given load case one brace or the other will be in compression (inactive) while the other is in tension (fully active), which allows these load cases to be analyzed even when there are tension-only members.

For all other load cases (Wind, Seismic, User Defined lateral load cases, Virtual load cases, Nodal load cases, etc.), tension-only members are defined with their full stiffness in tension but no stiffness in compression. It is assumed that the user will not create user-specified load combinations that combine two lateral load cases for which different braces are in tension versus compression.

Direction of the lateral loads becomes important in an iterative analysis. For example, a positively directed load case will give different results than a negatively directed load case, and the differences will not be merely a difference in the sign of the resulting member forces. Therefore, RAM Frame provides an option whereby additional load cases can be created by selecting the Generate Additional Load Cases for Analysis with Tension-Only Members option when generating lateral load cases (implemented for wind and seismic load cases only).

For instance, there are 12 load cases generated for a regular IBC2000 (ASCE 7-98) wind load case. If this box is checked, the number of load cases becomes 22. Additional load cases are created to account for directional effects of applied loads so that the most severe loading case is captured. The following figure conceptually shows an example of this application:

When load cases are created in this way, with one load case with loads in the positive direction and another with loads in the negative direction, it is not necessary (nor correct) to use load combinations with both a positive and negative load factor. For instance, the following two load combinations are automatically created in the Load Combination module of RAM Frame for AISC LRFD 2^nd Edition in absence of tension-only members:

1.2 D + 0.5 Lp + 0.5 Rfp + 1.3 W1
1.2 D + 0.5 Lp + 0.5 Rfp - 1.3 W1

where W1 is a typical wind load case. However, if there are tension-only members, the program generates only the first load combination:

1.2 D + 0.5 Lp + 0.5 Rfp + 1.3 W1

Note that the second load combination (1.2 D + 0.5 Lp + 0.5 Rfp - 1.3 W1) is not valid. To consider a load case in a negative direction within a load combination, the user should first create additional load cases as explained above. The program then generates one more load cases (in which W2 = - W1) and iteratively solves both lateral load cases. Finally, the following load combinations are automatically generated by the program:

1.2 D + 0.5 Lp + 0.5 Rfp + 1.3 W1
1.2 D + 0.5 Lp + 0.5 Rfp + 1.3 W2

The above procedure is automated within RAM Frame if additional load cases are created.

If Load Combinations are generated automatically and there are tension-only members, the program allows combining results of one dead load case (or live load case) with only one lateral load case. Note that an iterative analysis is carried out for only lateral loads and dead or live loads are not considered during the iterative analysis. Therefore, superposing results of one dead load case with one lateral load case is based on the assumption that ignoring gravity loads in an iterative analysis does not significantly change the results. Even though this may not be correct if gravity loads cause significant P-Delta effects applied along with lateral loads, this can be compensated by turning on the P-Delta option. In this case, P-Delta effects are considered in the iterative analysis.

If there are no tension-only members in the model, the analysis is a typical (non-iterative) analysis. If the user has generated the additional (tension-only) load cases, the program gives a warning message stating that additional load cases have been created but there are no tension-only members. The additional load cases are not needed because their effect can be accounted for by the sign in the load combinations.

If there are tension-only members in the model but the user has not generated the additional (tension-only) load cases, the program gives a warning message indicating that an iterative analysis would be performed without the additional load cases.

The tension-only methodology has been implemented as described because of the ease and convenience given to the user. This implementation continues to allow load case results to be superimposed in load combinations, and does not require that the user generate and analyze in RAM Frame all of the combinations that will be used subsequently in all of the design modules.

When the model includes tension-only members a special set of analyses is performed. The tension-only analysis has been enhanced to eliminate the instability warnings that sometimes occurred during analysis previously, especially when the analysis was combined with a P-delta analysis.