1. Frames are pinned at their base
2. Rigid haunches are provided at the knees and the ridge
3. Knee-haunches extend up to the second purlins from the eaves
4. Ridge-haunches extend up to the second purlins from the ridge
5. Purlins are uniformly spaced

For the combination of factored dead, live and wind loads three possible mechanisms are invesigated:

Mechanism 1 This mechanism for dead and live load combination assumes  plastic yielding at the intersection of column and knee-haunch and at second purlins from the ridge. This mechanism normally occurs when the same sections are used for columns and rafters.
Mechanism 2 For this second mechanism under dead and live load combination, hinges form in rafters at the second purlins from the ridge and eaves at the beginning of haunches. This mechanism normally occurs when column sections are stronger than rafters.
Mechanism 3 For the combinations of factored dead, live and wind loads, only one mechanism is investigated wherein plastic yielding takes place at the second leeward purlin from the ridge and at the intersection of haunch and leeward column.

For each of the mechanisms, output gives the over-capacity factor, the reactions at base and the moments at ridge and eaves.

Base reactions are used for designing foundations. Note that the base reactions are factored.

Moments at ridge and eaves are used for designing connections and may turn out to be somewhat higher than the plastic moment capacities of members.

Over-capacity factor is ratio of loads that can be sustained and the loads applied on the structure. An over-capacity factor greater than or equal to 1 indicates that the structure can safely carry the applied load combinations for the assumed mechanism.

Plastic analysis gives no indication of the deflection and stability aspects for which separate checks must be made.

© 1999-2009 Rudra Nevatia