Lateral Design Considerations for Mid-Rise Wood Structures

Transcription

1 Lateral Design Considerations for Mid-Rise Wood Structures 120 Union, San Diego, CA Togawa Smith Martin By: R. Terry Malone, PE, SE Senior Technical Director Architectural & Engineering Solutions Marselle Condominiums Trochalakis, PLLC Photographer: Matt Todd Photographer 5 stories of wood over 6 stories concrete (podium) 2 above grade This presentation is copyrighted by Woodworks

2 The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-aia members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

3 Course Description Research, full-scale testing and code requirements continue to evolve for mid-rise construction. This presentation will focus on important engineering considerations related to the lateral design of mid-rise* wood buildings. Implementation of a well-considered design requires the understanding of diaphragm and shear wall flexibility and their effects on the horizontal distribution of forces through the structure. In mid-rise, multi-family buildings, corridor only shear walls are becoming very popular as a way to eliminate exterior shear walls. The special design issues this situation creates will be addressed. *Definition: A mid-rise building can be described as something between a high-rise and low-rise structure that is, between four and ten stories with heights ranging between 35 to 75 feet tall.

4 Learning Objectives Design Considerations Discuss important lateral engineering issues that need to be considered to successfully design a mid-rise structure. Diaphragm and Shear Wall Flexibility Understand the relationship between diaphragm and shear Wall stiffness on the horizontal distribution of shears within a mid-rise structure. Tall Shear Walls With Multi-story Effects Discuss the behavior of shear walls under multi-story effects and the impact on the distribution of shear forces within the structure. Overview-Open Front and Non-Open Front Structures Review key points in the analysis of open front diaphragms and non-open front designs.

5 Mid-rise Codes and Standards Crescent Terminus building- credit: Richard Lubrant. The analysis techniques provided in this presentation are intended to demonstrate one method of analysis, but not the only means of analysis. The techniques and examples shown here are provided as information for designers to consider to refine their own techniques.

6 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front

7 Design Considerations Methods of analysis for open-front or tall shear wall mid-rise structures are currently evolving as more complex building geometries are becoming more prevalent. Building shapes and footprints are driving the design procedures for all lateral resisting systems and materials. Design requirements: IBC Analysis: o Method of analysis shall take into account equilibrium, general stability, geometric compatibility, and both short-and long term material properties. o Shall be based on rational analysis in accordance with well established principals of mechanics. Analysis shall provide complete load paths. Considerations: Make sure that you have complete continuous lateral load paths. Address vertical / horizontal irregularities. Investigate diaphragm and shear wall flexibility/stiffness. Consider the multi-story effects on shear wall stiffness. Check the effects of offset diaphragms and shear walls. Verify the horizontal distribution of forces within the diaphragm and to the shear walls.

8 A 35 Typical Unit B Open Front 76 6 C D 35 Typical Unit Non-Open Front E SW Typical SW Typical F Open Front & Non-open Front Floor Plan w/ and w/o offsets

9 Non-open front ASCE 7-10 Section (c), Light framed construction, meeting all conditions: Longitudinal-Typically permitted to be idealized as flexible, provided exterior shear walls of adequate stiffness exist. However, diaphragm can be semi-rigid, rigid or open front, even if exterior walls exist. Transverse-Traditionally assumed to be flexible diaphragm. If token or questionable wall stiffness occurs at the exterior wall line, consider using semi-rigid analysis using the envelope method-(conservative), or semirigid, or idealize as rigid. Open Front Structures: SDPWS : Includes flexible and rigid analysis Loading parallel to open side - Model as semi-rigid, which shall include shear and bending deformation of the diaphragm, or it can be idealized as rigid. Loading perpendicular to open side (Transverse)-traditionally assumed to be flexible. Drift at edges of the structure the ASCE 7 allowable story drift when subject to seismic design forces including torsion, and accidental torsion (strength, multiplied by Cd). No set drift requirements required for wind (Drift can be tolerated).

10 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Open Front Overview-Mid-rise: Exterior shear Walls

11 Complete Load Paths 1 Discontinuous diaphragm chords 2 3 A SW8 Strut/chord 10 B SW1 C Strut Strut/chord Strut /chord Collector SW5 Collector (typ.) Collector (typ.) Open Opening in diaph. Collector (typ.) Vertical offset in diaphragm Collector (typ.) Strut chord MRF1 Support Multiple offsets E Strut (typ.) SW2 Strut chord Strut/chord F 4 Collector SW3 Offset shear walls and struts Strut/chord Offset struts SW4 D Support Analysis: ASCE7-10 Sections: Design shall be based on a rational analysis At diaphragm discontinuities such as openings and re-entrant corners, the design shall assure that the dissipation or transfer of edge (chord) forces combined with other forces in the diaphragm is within shear and tension capacity of the diaphragm. ASCE7-10 Section 1.4-Complete load paths are required including members and their splice connections

12 Complete Load Paths A SW8 Strut/chord B Strut SW1 Strut/chord C Strut /chord Collector SW5 Collector (typ.) Collector (typ.) Open SW6 Transfer area Collector (typ.) Collector (typ.) Strut chord MRF1 Support E Strut (typ.) SW2 Strut chord Strut/chord F SW7 Collector SW3 Strut/chord SW4 D Support SDPWS Collectors for shear transfer to individual full-height wall segments shall be provided. 4 Where offset walls occur in the wall line, the shear walls on each side of the offset should be considered as separate shear walls and should be tied together by force transfer around the offset (in the plane of the diaphragm). ASCE7-10 Section 1.4-Complete load paths are required including members and their splice connections

13 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front

14 ASCE7-10 Section 12.3 Diaphragm Flexibility Seismic Section The structural analysis shall consider the relative stiffnesses of diaphragms and the vertical elements of the lateral force resisting system Is any of the following true? Start Yes Is diaphragm untopped steel decking or Wood Structural Panels 1 & 2 family Vertical elements one Light framed construction Dwelling of the following : where all of the following are met: 1. Steel braced frames 1. Topping of concrete or 2. Composite steel and similar material is not concrete braced frames placed over wood structural 3. Concrete, masonry, steel SW panel diaphragms except or composite concrete and steel shear walls. for non-structural topping not greater than 1 ½ thick. 2. Each line of vertical elements of the seismic force-resisting system complies with the allowable story drift of Table Yes Idealize as flexible No No Idealize as flexible Structural analysis must explicitly include consideration of the stiffness of the diaphragm (i.e. semi-rigid modeling), or calculated as rigid in accordance with 2015 IBC Section or ASCE 7-10 Section Is diaphragm concrete slab or concrete filled steel deck? Yes No Average drift of walls Envelope Method Allowed for semi-rigid modelling Yes No Is span to depth ratio 3 and having no horizontal irregularities? Yes No (Could apply to CLT or Heavy timber diaphragms) Maximum diaphragm deflection Is maximum diaphragm deflection (MDD) >2x average story drift of vertical elements, using the Equivalent Force Procedure of Section 12.8? Idealize as rigid

15 ASCE7-10, Section 26.2 Diaphragm Flexibility Wind Start Is diaphragm untopped steel decking, concrete filled steel decks or concrete slabs, each having a span-to-depth ratio of two or less? Yes Diaphragm can be idealized as rigid No Is diaphragm of Wood Structural Panels? Yes Diaphragm can be idealized as flexible No Is diaphragm untopped steel decking, concrete filled steel decks or concrete slabs, each having a span-to-depth ratio greater than two? Yes Justify by Calculation as flexible, semi-rigid or rigid per 2015 IBC Section or ASCE 7-10 Section Wind ASCE 7-10 Section Diaphragm flexibility The structural analysis shall consider the stiffness of diaphragms and vertical elements of the MWFRS

16 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front

17 Current Examples of Shear Wall Multi-story Effects and Mid-rise Analysis Current Examples of Mid-rise Analysis-Traditional Method Thompson Method-Woodworks Website Webinar Paper Design-Example.pdf SEAOC/IBC Structural Seismic Design Manual, Volume Structural Engineers Association of California. Sacramento, CA Current Examples of Mid-rise Analysis-Mechanics Based Approach Not currently addressed or required by code Shiotani/Hohbach Method-Woodworks Slide archive Diaphragm-Design-WSF pdf FPInnovations-Website NEW Seismic Analysis of Wood-Frame Buildings on Concrete Podium, Newfield 2016 WCTE: A Comparative Analysis of Three Methods Used For Calculating Deflections For Multi-storey Wood Shear Walls: Grant Newfield, Jasmine B. Wang FPInnovations-Website A Mechanics-Based Approach for Determining Deflections of Stacked Multi-Storey Wood-Based Shear Walls, Newfield Design Example: Design of Stacked Multi-Storey Wood-Based Shear Walls Using a Mechanics-Based Approach, Canadian Wood Council FPI Traditional Traditional MBA + moment APEGBC Technical & Practice Bulletin Revised April 8, and 6 Storey Wood Frame Residential Building Projects (Mid-Rise) -Based on FPInnovations Mechanics Based Approach

18 Traditional Shear Walls: Max. A/R=3.5:1 Walls assumed to be a cantilever wall fixed at the bottom at each floor, rigid wall sections and, out-of-plane diaphragm stiffnessrigid Doesn t account for multi-story shear wall effects T = sw1 + sw2 + sw3 VR V3 + V2 + Stacked Shear Wall Elevation Traditional Method-Thompson Example SW = 8vh3 EAb + vh + h a 1000G a b Source: WoodWorks Five-Story Wood-Frame Structure over Podium Slab Design Example

19 Shiotani / Hohbach Method Hohbach-Lewin, Inc.-Structural and Civil Engineers VR V3 Equivalent beam/column sw = Σ V i h i 2 6EI T 3L T h i V2 + + V i sw = Σ V i h i 3 3EI T The equation is judged to be adequate as long as the aspect ratios of the walls are reasonable Shear wall deflections are approximated based on this equation and extrapolated for multistory behavior. Shear walls for this building type have continuous rod tie down systems that are integral to their multistory behavior. column elements simplify the model, over shell or plate elements. Provides a reasonable approximation for shear wall deflection (conservative). It also captures the flexural deformation of the full height wall. Procedure Break the wall up into multi story components Calculate the deflection of the wall s story(s) components A column section is used, then back calculated for the required moment of inertia to produce the expected deflection at the top of the multistory shear wall. The sum of those deformations are greater than the traditional multi-story shear wall deflections

20 New Research and Analytical methods-tall Shear Walls Currently not addressed or required by code: Engineering preference and/or judgement Testing shows that the traditional deflection equation is less accurate for walls with aspect ratios higher than 2:1. (Dolan) Current research suggests that The traditional method of shear wall analysis might be more appropriate for low-rise structures. Multi-story walls greater than 3 stories should: Consider flexure and wall rotation. Rotation and moment from walls above and wall rotation effects from walls below. Moment from walls above M i H 2 i + V i H 3 2 EI i 3 EI i Rotation from walls above and below. Total displ. Traditional walls Tall Wall Method Traditional Method Allowable story drift for traditional and tall shear walls is checked floor to floor. Allow story Total displ. of drift Tall Wall. More flexible. SW A/R=2:1 flr.-flr. A/R h/d 2:1 A/R=3.5:1 flr.-flr. Tall Shear Wall MBA Tall SW Stiffness based approach Floor to floor A/R s and Stiffness of Shear Walls Acting as a continuous wall

21 V Flexibility does not allow dead load to aid in resisting wall rotation M Semi-balloon framed (Very flexible) Stiffness might allow dead load to aid in resisting wall rotation, provided joints are strategically placed. (justify by calculation) T C If diaphragm out-of-plane stiffness=flexible Analyze entire wall as a tall wall Should consider as flexible because it is unknown where rim joist splices will occur Rim joist Compression blocking Diaphragm out-of-plane Flexibility Platform framed If diaphragm out-of-plane stiffness=rigid (steel beam, conc. beam) Analyze entire wall as traditional floor to floor

22 Tall Wall Deflection θ 1 H 2 + H 3 + H 4 i = M ih i 2 2 EI i + V i H 3 3 EI i θ 1 H 2 + H 3 + H 4 + H 5 + V ih i H G v,i t i e n,i + H i d v,i L a,i + H i i +α 1 (H 1 + H 2 +H 3 + H 4 + H 5 ) L i +α 1 (H 1 + H 2 +H 3 + H 4 ) L i 4 θ4 + i_1 j`1 5 M j H j + V j H2 j EI j + H 2 EI i i_1 d a,j j`1 j L j θ 1 H 2 + H 3 +α 1 (H 1 + H 2 +H 3 ) L i 3 θ3 + θ 1 H 2 +α 1 (H 1 + H 2 ) L i θ2 α3 α4 α 1 (H 1 + H 2 ) L i θ1 1 α2 α1 Deflection-Bending +Rotation translates to top (Wall rotation) α1 α 1 H 1 L i Deflection-Wall rotation) translates to top Included in 1 Note: Increased wall flexibility can increase the period of the building, lowering the seismic force demands.

23 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front

24 2015 SDPWS Horizontal Distribution of Shear (Revised) Distribution of shear to vertical resisting elements shall be based on: Average drift of walls Maximum diaphragm deflection Analysis where the diaphragm is modeled as : o Idealized as flexible-based on tributary area. Can under-estimate forces distributed to the corridor walls (long walls) and over-estimate forces distributed to the exterior walls (short walls) Can inaccurately estimate diaphragm shear forces o Idealized as rigid-distribution based on relative lateral stiffnesses of vertical-resisting elements of the story below. More conservatively distributes lateral forces to corridor, exterior and party walls Allows easier determination of building drift Can over-estimate torsional drift Can also inaccurately estimate diaphragm shear forces Maximum diaphragm deflection (MDD) >2x average story drift of vertical elements, using the ELF Procedure of Section 12.8? Calculated as Flexible Note: Offsets in diaphragms can also affect the distribution of shear in the diaphragm due to changes in the diaphragm stiffness. o Modelled as semi-rigid. Not idealized as rigid or flexible Distributed to the vertical resisting elements based on the relative stiffnesses of the diaphragm and the vertical resisting elements accounting for both shear and flexural deformations. In lieu of a semi-rigid diaphragm analysis, it shall be permitted to use an enveloped analysis.

25 Exterior shear walls Rigid support or Partial support TD1 TD2 Flexible Semi-rigid Rigid Semi-rigid Rigid TD2 TD1 Seismic Loads Support Support Wind Loads as applicable I1 I2 I3 Rigid or spring Support?? Unit with Exterior Wall Support I3 I2 I1 Support Unit without Exterior Wall If flexible diaphragm Condition A Condition C Full support (SW rigid) Condition B No SW support Corridor only SW Partial support (Decreasing SW stiffness) Varying Degrees of Stiffness Effects of Exterior Walls

26 Multi-story Effects of Tall Shear Walls open front? Effects of openings? Semi-rigid? Flexible? Note that possible changes in diaphragm flexibility can occur from floor to floor: non-open front (flexible) vs. non-open front/open front (rigid or semi-rigid).

27 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Overview-Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front The following information is from an on-going example calculation and is subject to further revisions and validation. The information provided is project specific, and is for informational purposes only. It is not intended to serve as recommendations or as the only method of analysis available.

28 Case Study-Non-Open Front Floor Plan With Offsets Flexible analysis Rigid analysis 1 Semi-rigid (Envelope) A.5 B A 35 Open Front E SW4 SW SW2 SW Non-Open Front Max. A/R=4:1 Typical Unit SW1 Varies 26 typ. ASCE 7-10 Section (c), Light framed construction, meeting all conditions: All Light framed construction Non-structural concrete topping 1 ½ Each elements of the seismic line of vertical force-resisting system complies with the allowable story drift of Table Longitudinal-Allowed to be idealized as flexible, provided exterior shear walls exist and is in compliance with ASCE 7-10 Section (c). If calculations show that the story drift at a line of lateral force resistance exceeds the allowable limit, flexible diaphragm behavior cannot be used. C D E F

29 Automatic Tensioning Systems/Devises Coupler ATS isolator nut take-up device and bearing plate Semi-balloon framing Restraints are required at roof and each floor level to get best results Software programs are available for design Must be installed per manufacturers recommendations Steel rod Platform framing Overturning Number and size of studs as required by design Anchorage into foundation as required by calculation and per manufacturers recommendations Resistance

30 156 Determination of Non-Open Front Diaphragm Flexibility 26 sw corr. Corridor sw corr. Sym. C.L. Corridor SWAver. Diaph sw ext From Tall wall calc. Tall or narrow SW Check Total deflection-diaphragm flexibility 2x Aver.sw Aver.sw Offset Offset Ext. SW sw ext V M Full depth Torsion + acc. torsion Sym. C.L. Seismic Corridor I3 Shear Deflection Nail slip deflection. Kdiaph. I2 Computer Model Bending I1 Bending Stiffness = A d2 2 Displ. sw ext From Tall wall calc. Shear Deflection per USDA Research Note FPL-0210 Positive moment Moderately stiff. support Check exterior SW stiffness effects

31 Roof Plot- Longitudinal Loads- A/R*=1.5:1 (8 wall) Base Line= k 46.4 k k Base Line=0 Base Line=0 A/R 1.5:1 wall A/R 2:1 wall A/R 3.5:1 wall Bending increases at start of offset Offset Offset Partial support Full cantilever End Not full support k k k Full cantilever *Aspect ratios Floor to Floor

32 Sym. C.L. Determine Diaphragm and TD Nailing, and Chord Forces 35 TD2 I2 TD1 I1 3 Chord Slip at TD TD2 TD TD1 Collector TD2 6 I3 3 Collector 2 4 TD1 Shear TD2 Shear Basic Diaphragm Shear Longitudinal Loading

33 Check Story Drift-3D model or 2D plane Grid = + Spring support If Shear walls Collectors Same as Drift by semi-rigid or rigid analysis only (Open front) Collectors ASCE Story drift check (Non-open front) C.M. Same as + =

34 Story Drift Check Longitudinal + Torsion (8 ext. walls) Notice curvature (bending) of offset sections C.M. ASCE Story drift check (Non-open front) Significant shear distributed to corridor walls (cantilever action) Maximum drift (open front)

35 Presentation Topics Design Considerations Complete Load Paths Diaphragm Flexibility Tall Shear Walls-Flexibility Horizontal Distribution of Shears 120 Union, San Diego, CA Togawa Smith Martin Mid-rise: Exterior shear Walls Overview-Mid-rise: Open Front The following information is from an on-going example calculation and is subject to further revisions and validation. The information provided is project specific, and is for informational purposes only. It is not intended to serve as recommendations or as the only method of analysis available.

36 Relevant 2015 SDPWS Sections (a) SW (b) SW (c) SW SW Force Open front L SW Force SW SW Force L W Open front L Open front Cantilever Diaphragm Plan W Plan Cantilever Diaphragm W Cantilever Diaphragm Figure 4A Examples of Open Front Structures New definitions added: Open front structures Notation for L and W for cantilever Diaphragms Relevant Revised sections: Horizontal Distribution of Shears Torsional Irregularity Open Front Structures (d) SW SW SW SW Open front Force Cantilever Diaphragm L L Open front W Cantilever Diaphragm

37 Open Front Structures: (Figure 4A) For resistance to seismic loads, wood-frame diaphragms in open front structures shall comply with all of the following requirements: 1. The diaphragm conforms to: a. WSP-L /W ratio 1.5: b. Single layer-diag. sht. Lumber- L /W ratio 1: c. Double layer-diag. sht. Lumber- L /W ratio 1: The drift at edges shall not exceed the ASCE 7 allowable story drift when subject to seismic design forces including torsion, and accidental torsion (Deflection-strength level amplified by Cd. ). 3. For open-front-structures that are also torsionally irregular as defined in , the L /W ratio shall not exceed 0.67:1 for structures over one story in height, and 1:1 for structures one story in height. 4. For loading parallel to open side: a. Model as semi-rigid (min.), shall include shear and bending deformation of the diaphragm, or idealized as rigid. 5. The diaphragm length, L, (normal to the open side) does not exceed 35 feet.

38 Case Study-Open Front Floor Plan With Offsets Rigid analysis Semi-rigid (Envelope) No exterior shear walls typical Transverse shear walls typical A.5 B A SW4 SW SW2 SW Open Front Corridor walls typical Non-Open Front Typical Unit C D E.5 SW1 Shear panels are required over corridor walls 26 typ. E F

39 156 Determination of Open Front Diaphragm Flexibility 26 Sym. C.L. sw Corridor L Limit= Torsion + acc. torsion Sym. C.L. Diaph Corridor V Full depth K1 Seismic I3 Shear Deflection Nail slip deflection Kdiaph. I2 I1 Shear Deflection per USDA Research Note FPL Corridor only shear walls M Check drift at edges (extreme corners) including torsion plus accidental torsion (Deflection-strength level x Cd. ) Seismic-meet ASCE7 drift Wind-deflections can be tolerated. Bending Computer Model

40 Final Diaphragm (Total) Deflections sw corr. 35 sw corr. 1 2 Check for diaphragm flexibility 3 4 Corridor Diaph 8 4 Corridor sw corr Roof Corridor Seismic 5 th Floor I3 I2 I1 sw corr Verify Diaphragm Flexibility: If flexible, add blocking If still flexible, decrease nail spacing to stiffen up Corridor th Floor

41 Reference Materials The Analysis of Irregular Shaped Structures: Diaphragms and Shear Walls-Malone, Rice-Book published by McGraw-Hill, ICC Woodworks Presentation Slide Archives-Workshop-Advanced Diaphragm Analysis NEHRP (NIST) Seismic Design Technical Brief No. 10-Seismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers SEAOC Seismic Design Manual, Volume 2 Woodworks-The Analysis of Irregular Shaped Diaphragms (paper). Complete Example with narrative and calculations. Diaphragms_Paper1.pdf

42 Method of Analysis and Webinar References Offset Diaphragms Offset Shear Walls Diaphragms Openings Mid-rise Design Considerations Information on Website Presentation Slide Archives, Workshops, White papers, research reports

43 Questions? This concludes Woodworks Presentation on: Lateral Design Considerations for Mid-rise Structures R. Terry Malone, P.E., S.E. Senior Technical Director WoodWorks.org Contact Information: THANK YOU