Requirements and Solutions for Sunroom Lateral Design

Jay H. Crandell, P.E.ARES Consulting

October 6, 2010Technical Committee MeetingNational Sunroom Association

Outline• Background on Sunroom Lateral Design• Codes, Standards, and Performance Requirements– 2009 IBC– 2009 IRC– ASCE 7‐05– AAMA/NPEA/NSA 2100‐02– AC213, AC340

• Lateral Force Distribution Methods• Various Methods to Resist Lateral Loads• Discussion

Background for Lateral Design of Sunrooms and Screen Enclosures

Hurricane Charley

HUD, 2005

International Building Code (2009)

Is this a Sunroom?• La Plata F4 Tornado Study

Room projection, all windows

IBC 2009 (cont’d)

1. Parts and portions (i.e., sunrooms) must be designed to support structural loads defined in the code.

2. For occupancies or uses not covered, appropriate design loads are at discretion of building official.

IBC 2009 (cont’d)

What are the drift limits for wind lateral loads? For seismic design, the code defers to ASCE 7‐05.

IBC 2009 (cont’d)

None of these values are intended to apply to lateral (in‐plane) shear deflection or drift.

IBC 2009 (cont’d)

Footnote h to table gives deflection limits for only out‐of‐planebending of aluminum structural members in sunrooms.

IBC (cont’d)

Does this reference standard have more stringent deflection limitations than Table 1604.3?

Does it address in‐plane shear deflection limits?

IBC (cont’d)

…of course (nothing new here)

IBC (cont’d)

If glass is not designed as part of the LFRS, then its potential effect on the system must still be considered and provided for in the design. (How?)

IBC (cont’d)

Has anyone here been asked to do load tests on site?

IBC (cont’d)

How many sunrooms are placed over existing or new decks? Who is responsible to ensure adequacy to support vertical and lateral loads? How are sunroom manufacturers dealing with this issue? (deck collapses are one of the more common structural failures resulting in injury)

How much additional lateral or vertical load might the sunroom cause? How are the piers to be adequately braced for the additional lateral load?

IBC (cont’d)

Everything must be designed for wind loads.

Decreases due to shielding “by other structures” is not permitted.

But, sunroom additions are generally shielded by the main structurein one of the two orthogonal loading directions (SKETCH CONCEPT).

Calculation of wind loads will not be considered in detail. The code givestwo options: (1) ASCE 7‐05 or (2) tabulated “all‐heights” method in code.

IBC (cont’d)

Glazing on buildings is not included as a contribution to lateral resistance, but it must resist wind pressure as well as debris impact in the wind‐borne debris region.

How much more important is wind pressure and wind debris resistance if glazing is also used to resist lateral wind loads for a sunroom that is part of the habitable space of a building?

Lateral force resisting systems are generally designed with a minimum safety factor of 2.0 for wind. Glazing is typically designed for a safety factor of 1.5 as is the case with many building envelope components.

IBC (cont’d)

Changes in ASCE 7‐2010 are significant for 2012 IBC

IBC (cont’d)

Proper wind exposure characterization has big impact on wind load. ASCE 7‐10 commentary provides new insights. Topographic wind speed up also important.

IBC (cont’d)

Nothing escapes seismic design!IBC generally defers to ASCE 7‐05 for seismic design requirements.

IBC (cont’d)

Exceptions are important!What about sunrooms attached to these structures?Can or should they also be exempted in some cases?

IBC (cont’d)

IBC (cont’d)

Applicable to sunrooms?

IBC (cont’d)

Applicable to sunrooms?

IBC (cont’d)

IBC (cont’d)

Minimum loads are 10 psf for wind, irrespective of the actual wind load for a given wind speed region? Does this really mean the design loads? Not clear.

IBC (cont’d)

ICC‐ES AC340 Acceptance Criteria for Patio Covers is an example of a means of providing evidence to gain approval as an alternative means of design.Other “approved agencies” can provide the same testing and evaluation service.

IBC Conclusions

• Indicates in general that sunrooms (as a building portion) must be designed to resist full design wind and seismic loads that are required for the structures to which they are attached.

• Safety factors are not indicated (only design load levels)

• Drift limits are not clearly indicated.• Seismic requirements defer to ASCE 7.• Applicability of exemptions for sunrooms not clear.

International Residential Code (2009)

Same as IBC

IRC (cont’d)

All portions of dwellings must safely support loads, even portions such as sunrooms that are not prescriptively addressed in the code or its reference standards

IRC (cont’d)

Engineering is required for parts and portions not addressed in code.

IRC (cont’d)

Can also use alternative means and methods of design same as IBC.Can be used as basis for equivalence to prescriptive requirements in IRC that may differ in basis from the engineering requirements of the IBC.

IRC (cont’d)

• IRC address only prescriptive construction (light‐frame wood and steel, ICF, etc.)

• Wind speed limited to 100 mph in hurricane prone regions, and less than 110 mph elsewhere (R301.2.1.1).

IRC (cont’d)

IRC (cont’d)

Topographic winds speed up effects are considered in the IRC by adjusting basic wind speed.

Exposure B is the “default” exposure for IRC, but other exposures apply when applicable and prescriptive requirements are adjusted accordingly.

IRC (cont’d)

But, what is the basis of the IRC wall bracing provisions such that an equivalent bracing design for a sunroom addition can be achieved?

For that matter, what is the performance basis of wood frame wall bracingas designed per the IBC? ….. These will be address later.

IRC (cont’d)

Provides tabulated wind loads especially for design of screened enclosures.Are these considered Category I sunrooms per AAMA/NPEA/NSA 2100‐02?These requirements are not included in IBC Appendix I.

ASCE 7‐05• Wind loads are fairly straight forward.

• Simplified wind load tables often used:

ASCE 7‐05 (cont’d)

• Seismic forces and design requirements are a bit more complicated and challenging, particularly when dealing with mixed structural systems (e.g., a sunroom with glazing and aluminum frame tied to a wood frame building with wood structural panel bracing).– How do you prove seismic/deflection compatibility for different systems?

– Would the building be required to be designed using the seismic response parameters for the sunroom because they “share” load when tied together?

– At what point should the attached structure be considered “incidental” in its impact to the overall building?

ASCE 7‐05 (cont’d)• If feasible/approved, seismic loads can also be greatly simplified (by professional judgment):

Is anything more complicated really necessary for a sunroom addition?What is an appropriate R‐factor to use? (also Ω and Cd)

Use 1.0 instead of 1.2 for 1‐story structure (based on ASCE 7‐05 Eq. 12.14‐11)

ASCE 7‐05 (cont’d)• The other big challenge is how to best distribute lateral forces within the structure and two its various parts.

• Two extremes are recognized with somewhat arbitrary limitations on use of either (right answer is somewhere in between):– Rigid Diaphragm (forces distributed based on relative stiffness of shear walls)

– Flexible Diaphragm (forces distributed based on tributary area)

• For light‐frame wood construction, rigid diaphragm is probably the most accurate approach, but flexible diaphragm approach is permitted and is much simpler and is most commonly used.

Voluntary Specs for Sunrooms(AAMA/NPEA/NSA 2100‐02)

• Could not find referenced in 2009 I‐Codes ???– It is mentioned in Ch.20 of FBC with modification of Ch.5 to defer to ASCE 7 design loads.

• Category I sunrooms are exempt from minimum structural requirements (or only the fenestration products in these sunrooms?):

VSS (cont’d)

• Structural requirements:

VSS (cont’d)• Structural requirements for out‐of‐plane bending of roof assemblies are addressed:

VSS (cont’d)

• Out‐of‐plane wind pressure resistance of glazing is also addressed:

Minimum safety factor for glazing is 1.5. For roof assembly, it is 2.5. What is the implied failure sequence? Consequences?

Performance Requirements for Wall Bracing (Precedents)

• IRC 2009 Wall Bracing (wood framing):– Safety Factor = 2.0 (wind and seismic)– Drift limit (none applied; although for systems addressed about 0.5”

+/‐ at ASD design load for 8’ wall or h/180)– Seismic Parameters are different from IBC and based on historically

accepted performance.– Design shear strength based on various sources of test data for

systems addressed (some monotonic and some cyclic tests applied)– Whole building system factor and partial restraint factor is applied to

bracing calculation (1.2 wind; 1.33 seismic)– Partial restraint factors de‐rate tested fully‐restrained shear values to

account for lack of full‐restraint in end use. The whole building factor accounts for the “rest of the building” not considered in evaluating lateral resistance.

– Nominal (unfactored) shear values based on testing and SDPWS (e.g., 700 plf for WSP, SFB, etc.; 400 plf for GWB and LIB – all with interior gyp included) ‐‐ this is the EXCHANGE RATE when bracing is removed from or added to a dwelling to accommodate a sunroom addition.

Performance Requirements for Wall Bracing (Precedents)

• IBC (wood framing – Special Design Provisions for Wind and Seismic):– Safety factor (~2.0 min. for wind; ~2.8 min. for seismic)– Drift limit defined by ASCE 7 for seismic (apparent shear stiffness

parameter provided to support drift calculation)– Drift limit for wind is whatever is considered “permissible” (design

values at 2.0 safety factor give about 0.5” deflection at design load for 8’ wall height or h/180).

– Nominal shear design values mostly based on monotonic tests for both wind and seismic.

– Seismic parameters are R=6.5 for WSP and R=2.0 for all other bracing systems for wood frame walls (big difference and only two categories to cover a wide range of different bracing methods and properties).

– Competitively and accurately assigning appropriate relative seismic parameters is the big controversy with gaining acceptance of alternative materials and methods for seismic design – stifles innovation or otherwise makes it difficult and costly.

Performance Requirements (cont’d)• AC213 Interim Criteria for Glass Shear Walls and

Diaphragms for Patio Covers:– Requirements for testing wall systems (1 or more bays)– Requirements for testing roof systems (1 or more bays)– ASTM E72 test method without hold‐down applied (essentially

equivalent to E564 test approach and permits partial restraint) ‐‐ see figure next slide

– Aspect ratio of segments and overall length of assembly will be important as the test is not necessarily a “fully‐restrained” condition (i.e., no hold‐down)

– Allowable load determined by safety factor of 3 or load at 1” drift

– Three tests required (minimum)– Requires use of R=1 for seismic design, unless unspecified

interpretation of cyclic test results is used to determine an alternate value.

AC213 – Glass Shear Wall Segment Test (Single Bay) w/o Hold‐down

Performance Requirements (cont’d)

• Safety factor in AC213 is greater than that used for wood frame building lateral design

• Drift limit is greater – This is good from a seismic standpoint because it will better ensure deflection compatibility, i.e., drift of the building will not so readily cause forced displacement of the sunroom resulting in damage due to deflection incompatibility

– Oddly an exception requiring ½” deflection limit is required for “patio covers that may have detrimental impacts on an adjoining structure due to excessive deflection”.

Performance Requirements (cont’d)

• AC340 Acceptance Criteria for Patio Covers– Specifies specific wind load methods to use in ASCE 7 for

freestanding and attached patio covers with and without enclosure walls.

– Defaults to conservative use of wind load provisions for attached condition (e.g., loads always based on building size and corner zones without account of wind shielding, stagnation zone, etc.)

– Requires wind and earthquake lateral analysis per code loads and load combinations

– Roof diaphragm resistance can be quantified by ASTM E455 testing in lieu of engineering analysis

– Minimum safety factor of 2.5 required for lateral design (different than previous performance criteria/precedents)

NOTE: Sunroom industry should consider developing its own consensus structural design standard for sake of consistency and design efficiency.

Lateral Force Distribution Options

• OPTION #1 (transfer lateral load to building)– Rigid roof diaphragm approach– How rigid is rigid enough to qualify? (diaphragm must not deflection more than twice the wall that supports it’s lateral reaction) – How to figure this when roof diaphragm is attached to a conventional home? (need rules of thumb)

– Depends on aspect ratio of sunroom addition– Even so, outward wall is still going to experience significant portion of load (it’s really a system)

– Anchorage to main building must be detailed– Verify adequate bracing in building for additional load (what about added torsional moment? Is this incidental?)

SKETCH CONCEPTS ON FLIP CHARTFairfax Co. VA Permitting: Must analyze existing structure if using this approach!

Lateral Force Distribution Options

• OPTION #2 (resist loads in sunroom and building)– Flexible roof diaphragm approach

– ½ load in sunroom and ½ load on building (still introduces load to building, but not with torsionalmoment)

SKETCH CONCEPTS ON FLIP CHART

Lateral Load Distribution Options

• OPTION #3 (Independent structure with “incidental” attachment to main building)– Disadvantage is sunroom system needs to provide separate bracing system at building line (e.g., portal frame, etc.)

– Advantage is that sunroom can be designed and built without concern for condition of existing building or its code compliance (sunroom is an independent portion).

SKETCH CONCEPT ON FLIP CHART

Methods of Resisting Lateral Load

• Cantilevered roof diaphragm (rigid diaphragm)– Building must be verified as adequate to resist added load (add supplemental bracing as needed?)

– Sunroom walls are assumed to be “just along for the ride” laterally (but must still support gravity and uplift loads and out‐of‐plane wind loads)

– Diaphragm must have tested capacity (AC213)

– Diaphragm anchorage to building must be designed detailed (or included as part of standard detail in diaphragm testing)

Methods of Resisting Lateral Load

• Cantilevered Columns (e.g., fixed base)

• Glass/Frame Assembly (fixed panels)

• Moment Frame (Steel, Alum., Wood Portal)

• Narrow Shear Wall Segments (various proprietary products)

• K‐Brace

• Cable Set

Cantilevered Columns

ATI‐ES Code Compliance Research ReportCCRR‐0140 (8/30/2010)

Portal Frames (wood moment frame)

Various versions now recognized in the 2009 IRC as prescriptive construction compatible with other IRC wall bracing methods.

K‐Brace (Screen Enclosure)

Cable Braces (Screen Enclosure)

Parting Thoughts: Foundations

• Anchorage to Foundation (overturning, shear, wind uplift) – recent testing showing ACI AppDvery conservative.

• Foundation Types:– Slab– Thickened Edge Slab– Stem Wall (Masonry or Concrete/ICF)– Permanent Wood Foundation– Deck/Pier (bracing required)– FPSF (used with Slab, Stem Wall, or PWF)

THANK YOU!

• Questions?