Question

Identify three structural concepts to provide a stiff load path for lateral load resistance

Answer 1

lateral load resisting systems, are broadly classified into Frame Systems, Structural Wall Systems, Hybrid Systems and Tubular Systems.

In the present study, three specialized systems, each one from the foremost category, namely Special Moment Resisting Frame (SMRF), Shear Wall System (SW), and Dual System (DS) are explored in detail towards their structural behaviour and performance.

• SPECIAL MOMENT RESISTING FRAME (SMRF)

Moment resisting frames can dissipate a large amount of energy, but they often suffer from large lateral displacements. Moment resisting frames are structural systems consisting of beams, columns, and joints with rigid connections (moment resisting connections).Moment resisting frame specially detailed to provide ductile behaviour is called as Special Moment Resisting Frame.

Generally, the strength and stiffness are proportional to the storey height and column spacing. These systems are frequently used as structural skeletons in RC, steel and composite buildings and bridges. Moment resisting frames are classified into Special Moment Resisting Frame (SMRF),Intermediate Moment Resisting Frame (IMRF) and Ordinary Moment Resisting Frame (OMRF) based on ductility. The deformed shape of columnsis generally characterized by double curvature, thus inelastic demand can beconcentrated at both ends. Frame shows higher stiffness, strength, and ductility than free standing columns because of their deflected shape. Framebehaviour significantly depends on the relative rigidity of structural members (beams and columns) and connections (beam to columns and base to columns).

• SHEAR WALL SYSTEMS (SW)

Shear Walls are vertical systems which are frequently combined with RC, steel and composite framed structures to control lateral deflections. Structural walls subjected to in plane lateral loads undergo deflection in due of the wall behaving in flexural and shear mode. Primary mode is dependent on height to length ratios and their overall behaviour is normally a combination of the two modes. These systems are generally classified according to their height to width (H/L) ratio (also known as vertical aspect ratio) in squat and slender (or cantilever) walls. Squat walls have low slenderness and their H/L ratios vary between 1 and 3. Slender or cantilever walls are those with H/L > 6. Under horizontal loads, the ratio of bending to shear deflections of structural walls increases with the system aspect ratio H/L. Consequently, squat and slender walls are governed by shear and flexural modes, respectively. Both squat and cantilever walls have high in plane stiffness and strength.

• DUAL SYSTEMS (DS)

Rigid moment resisting frames are ductile systems with high resistance, but their lateral stiffness is often inadequate to prevent large drifts under earthquake forces. To reduce storey and roof drifts, moment resisting frames are often connected to Bracing Systems or Structural Walls, also known as "Hybrid Frames or Dual Systems‟.

In the Dual System, both frames and shear walls contribute in resisting horizontal forces. Frame bends in accordance with shear mode, whereas the deflection of the Shear Walls is by a bending mode like the cantilever walls. As a result of the difference in deflection properties betweenframes and walls, the frames will try to pull the Shear Walls in the top of the building and try to push the walls at bottom. So the frames will resist the lateral loads in the upper part of the building, which means an increase in the dimensions of the cross section area of the columns in the upper part of frame more than what it needs to resist the gravity loads, while the Shear Walls will resist most of the vertical loads in the lower part of the building.