SE APPRENTICE. Today s Speakers. Lateral Load Systems. Lateral Loads

Transcription

1 Today s Speakers Carrie Bremer, PE Schaefer Stephen Metz, PE SMBH, Inc. Today s Moderators SE APPRENTICE Lecture Two- Systems & Paths Lisa Willard, PE SE Solutions, LLC SE Apprentice Brian Quinn, PE SE Solutions, LLC Content Created by Carrie Bremer & Stephen Metz; Event produced by SE Solutions, LLC 2 Lateral Loads Lateral Loads are usually caused by wind and seismic loads on buildings. They can also be a result of hydrostatic or soil loads. The part of the structure that resists lateral loads is called the Lateral Load Resisting System. Lateral Load Systems Common Lateral Load Resisting Systems Braced Frames Moment Frames Shear Walls- Concrete, Masonry or Wood 3 4 Lateral Load Systems Braced Frames Demonstration Video 5 6 1

2 Braced Frames Braced Frames Braced Frame Configurations 7 8 Moment Frames Moment Frames 9 10 Shear Walls Shear Walls

3 Shear Walls Stability Shear Moment Sliding Overturning Serviceability Design Examples 15 3-story, 60,000 sq. ft. steel-framed office building 200 ft. x 100 ft. with 25 ft. column bays 16 Design Example Variables Design Example Cases Floor Construction Cladding Type Lateral System Thin slab on bar joists (light) Composite slab on steel beams (heavy) E.I.F.S. with stud backup (light) Brick with stud backup (heavy) Moment frames (flexible) Braced frames or shear walls (stiff) Case Floor Construction Cladding Type Lateral System 1 Light Light Flexible 2 Light Heavy Stiff 3 Heavy Heavy Stiff 4 Heavy Heavy Flexible

4 Design Example Results Case 1: Light floor with moment frames (Case 4 is similar) 19 Case Cs coefficient Building Weight, W (kips) Base Shear, V (Cs*W, kips) Seismic Horizontal Deflection, d** Effective Steel Tonnage (Tons) 1: L-L-F , : L-H-S , : H-H-S , : H-H-F , **Wind deflection limit of 1.26 controlled in the short direction with a wind base shear of 147 kips 20 Design Example Costs Hierarchy Keys to Economical Structures Most Economical 1. Light floor Light cladding Flexible lateral system 2. Light floor Heavy cladding Stiff lateral system 3. Heavy floor Heavy cladding Stiff lateral system Least Economical 4. Heavy floor Heavy cladding Flexible lateral system Lighter floor and cladding systems reduce seismic loads and steel tonnage Compared to moment frames, braced frames reduce the overall foundation, anchor bolt installation, steel tonnage, and erection costs Shear walls can double as bearing walls and vise-versa, further reducing steel frame tonnage and saving money Choosing a System Aesthetic Considerations-Exterior Façade, Floor plans, building layout Structural System-Lateral load resisting system has to work with the rest of the structural system. For example, we probably won t use a steel moment frame in wood construction Cost Constructability and Construction Schedule Braced Frames

5 Load Path Braced Frames Loads applied to the building cladding Load Path Diaphragms Diaphragms Diaphragms Flexible or Rigid? Diaphragms Flexible Metal deck Plywood or OSB sheathing Rigid Concrete slabs (formed and on metal deck)

6 Flexible Diaphragms Rigid Diaphragms Rigid Diaphragms Rigid Diaphragms P 1 =V 1 = w l 2 P 2 =V 2 =w l 2 w I= b d3 12 d 1= d 2 b 1= b 2 I 1= I Flexible Diaphragm Rigid Diaphragm

7 Load Combinations Load Combinations The building code acknowledges that it is unlikely that the building structure will experience the full magnitude of multiple loads at the same time. Load combinations are the way the code acknowledges this. There are two different series of Load Combinations: Strength Design (LRFD) and Allowable Stress Design (ASD) Load Combinations Load Combinations Different load combinations control for different pieces and parts of the building. Each building and system component must be designed for the worst case load combination Column Load Braced Frames P DL = 75psf x 37 x 27.5 = 19k 2 2 P LL = 25psf x 37 x 27.5 = 6.4k

8 Column Load Column Load W= 21psf x 26 x = 12k 2 2 P WL = 12k x 26 / 18.5 = 17k Column Load 1.0 DL+1.0LL 1.0 DL+.75LL+.75WL P DL = 19k P DL = 19k P LL = 6.4k P LL = 6.4k x.75 = 4.8k 25.4k P WL = 17k x.75 = 13k 36.8k 45 Questions? Carrie Bremer carrie.bremer@schaefer-inc.com (614) Stephen Metz smetz@smbhinc.com (614) Which University is the answer to this session s challenge question? Cal Poly, San Luis Obispo Oregon State Purdue The Ohio State University 8