Introduction to Lateral Force Resisting Systems

1. CE 636 - Design of Multi-Story Structures T. B. Quimby UAA School of Engineering Introduction to Lateral Force Resisting Systems

2. Basic Concepts • The LFRS is used to resist forces resulting from wind or seismic activity. • Buildings are basically big cantilever beams. They are supported on one end only and the loads are perpendicular to the beam. • As in a beam, buildings are designed for strength (shear and flexure) and serviceability (deflection).

3. Constraints • Strength • Shear • Flexure • Serviceability • Deflection • Spatial Requirements

4. Structural Systems for Cantilever Beams • Braced Frame (Vertical Truss) • Moment Frame • Infilled-Frame • Shear Wall (~ solid beam) • Tube systems • Combinations of the above

5. Braced Frames • Braced Frames are basically vertical truss systems. • Almost exclusively steel or timber. • Highly efficient use of material since forces are primarily axial. Creates a laterally stiff building with relatively little additional material. • Has little or no effect on the design of the horizontal floor system. • Good for buildings of any height. • Bracing may intrude on the spatial constraints. • May be internal or external.

6. Types of Bracing • Different types of bracing • Single Diagonal • Double Diagonal • Chevron Bracing • Story height knee bracing (eccentricity braced frames • May be single story and/or bay or may span over multiple stories and/or bays

7. Bracing

8. Large Scale Bracing • Multiple Floors • Multiple Bays

9. Moment Frames(AKA Rigid Frames) • Columns and Girders joined by moment resisting connections • Lateral stiffness of the frame depends on the the flexural stiffness of the beams, columns, and connections. • Economical for buildings up to about 25 stories. • Well suited for reinforced concrete construction due to the inherent continuity in the joints. • Design of floor system cannot be repetitive since the beams forces are a function of the shear at the level in addition to the normal gravity loads. • Gravity loads also resisted by frame action.

10. Moment Frame Behavior • Note the bending in the typical beam, column and joint.

11. Infilled Frames • Common in many countries. • Used for buildings up to 30 stories. • Steel or concrete frame infilled with concrete or masonry. • Infill behaves as a strut in compression. • Tension contribution is ignored. • Due to random nature of masonry infill, it is difficult to predict the stiffness and strength of this system. • No method of analyzing infilled frames has gained general acceptance.

12. Infilled Frames

13. Shear Walls • Generally constructed with concrete, masonry, or plywood. Sometimes steel. • Shear walls have high in-plane stiffness and strength. • Well suited for tall buildings up to about 35 stories. • Shear walls may intrude on the spatial constraints. Best suited to residential and hotel construction. • Can be used around elevator and/or stair cores.

14. Shear Wall Building

15. Coupled Shear Walls • Special case of shear walls. • Two or more shear walls in-plane, coupled with a stiff beam or slab at each level. • Tends to behave like a moment frame system with very stiff columns. • The coupling reduces lateral deflections. • Forces in the coupling elements can be quite large.

16. Coupled Shear Wall Building • Free body of left shear wall has additional reactions from the coupling members.

17. Wall-Frame Structures • Combination of shear walls and rigid frames or combination of braced and rigid frames. • Shear walls and braced frames tend to deflect in a flexural mode while the rigid frames tend to deflect in a shear mode. • In a wall-frame structure, both the shear walls and rigid frames are constrained to act together, resulting in a stiffer and stronger structure. • Good for structures in the 40-60 story range.

18. Wall-Frame Building

19. Tube Systems • The basic idea is to make a rectangular tube out the the perimeter of the building. • The tube is made up of closely spaced columns connected by stiff spandrel beams creating very stiff moment frames. • Frames parallel to direction of force act like webs to carry the shear. • Frames perpendicular to the direction of force act as flanges. Flange forces are not uniform. • Best applied to rectangular or circular plans. • Suitable for both steel and concrete. • Use for buildings of 40 stories or more. • Frames are repetitive and easily constructed.

20. Tube System • Gravity Loads taken by frames and interior columns. • Aesthetically, the system gets mixed reviews because of the small windows and the repetition.

21. Tube Variations • Tube-in-Tube or Hull-Core • Inner tube is usually around an elevator or service core and can be made very stiff with shear walls or braced frames. • Bundled Tubes • Introduces additional “web frames” which reduces shear lag which makes flanges more efficient. • Allows for more architectural variation. • Sears Tower, Chicago • Braced-Tube • Utilizes a large scaled braced frame in place of rigid frames • Allows for wider columns spacing and smaller spandrels.

22. Outrigger-Braced Structures • Structural “depth” is increased (i.e. the moment of inertia of the structure is increased) • Shear strength is unchanged. • Utilizes a braced core with stiff outriggers to mobilized outer columns in tension and compression. • 4 to 5 outriggers appear to be the economical limit.

23. Outrigger-Braced Structures • Under Lateral Loads: • Columns on one side are in tension • Columns on other side are in compression.

24. Suspended Structures • Used primarily to achieve some architectural purpose. • Floor are hung from a truss on an upper level • Tension members can be smaller than columns would be in same place. • Accumulated lengthening of tension members may cause extreme deflection problems at lowest hung floor. This can be controlled by hanging 10 or less floors from a single truss. • Limited to “shorter” structures since structural depth is small at base, making lateral deflections large. • There are several variations on the theme.

25. Suspended Structures • Suspension does little to help the LFRS.

26. Core Structures • Core carries all gravity and lateral forces. • Core may be braced frame or shear wall. • Floors are cantilevered off of the core. • Creates a column free interior. • Building width is limited by capabilities of the cantilever. • Building height limited by stiffness of core. • Structurally inefficient.

27. Space Structures • Three dimensional triangulated frame. • Highly efficient and relatively light weight. • Bank of China building in Hong Kong is a classic example. • Ingenuity required to get the gravity and lateral loads from the floors into the space frame.

28. Hybrid Structures • Combinations of the various types of systems. • There are almost limitless combinations. • May be necessary to achieve architectural goals. (“Postmodern” architecture intentionally tries to get away from simple prismatic building shapes.) • The development of large scale computer based analysis has made design of odd shapes possible.