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WHILE YOU’RE UNDER MY ROOF

You should be dry and safe in the face of a storm.  While waterproofing design requirements are typically the responsibility of the Architect, issues with water infiltration often result in structural concerns.  So which roofing type do you think is best for high wind and coastal construction?  Is it 3 tab or Architectural asphalt shingles?  The ubiquitous clay tile roofing of south Florida?  Or 5V crimp or standing seam metal roofing?

I have personally witnessed asphalt shingles transform into lethal frisbees as far inland as Gainesville, FL during Hurricane Frances back in 2004.  Outside of my anecdotal evidence, FEMA has documented after the more recent Hurricane Michael “widespread poor wind performance of asphalt shingles.”  Before closing the book on this roofing material, it should be noted that many failures were due to poor installation which would have a likewise effect on all roofing types.  There are multiple options depending on wind speeds for asphalt roof shingles as tested in ASTM D3161 where Class A passes tests at 60 MPH and Class F passes at 110 MPH (and yes, I did not get those backwards).  Another standardized test method, ASTM D7158, rates shingle performance against up to wind speeds of 150 MPH.

And clay tiles? Those things are bullet proof, right? Wrong!  Clay tiles set with mortar have been shown to be particularly vulnerable at hips and ridges.  Often, loose tiles  become projectiles themselves, further injuring nearby structures.  These tiles also do not fair well under impact loads and are liable to shatter or crack upon impact by wind borne debris, later leading to leaks and eventually structural damage to sheathing or framing.  FEMA 499 Technical Fact Sheet No 7.4, Tile Roofing for High Wind Regions notes that “foam and mechanical-set attachment methods have historically performed better than applied using the mortar-set attachment method.”

Last but certainly not least, metal roofing is a solid candidate for high wind installations.  According to FEMA “metal roof coverings appeared to perform better than other roof covering on buildings built to the Florida Building Code, [however] metal roof damage to some buildings was observed after Hurricane Michael.”  References like FEMA 499 Technical Fact Sheet No 7.6 can provide guidance on minimum roof slope requirements for hydrostatic vs hydrokinetic panels or additional fastening requirements near eaves.  Consult with your preferred design professional for proper specification of roof covering, attachment methods and adequate underlayment so that you can confidently state “you’re safe while under my roof.”

Author: Enrique Fernández, PE

THE BATTLE OF THE SOFTWOODS

Let’s not talk about lumber prices, it’s too early for that.  Instead, let us settle this argument once and for all.  Which is the best softwood lumber to construct with?  Is it Southern Yellow Pine (SYP) or Spruce Pine Fir (SPF)? This is a matter with structural, constructability and economic implications.

It’s best to begin at the beginning.  So let’s review an essential wood property.  Specific gravity is defined as the ratio of a material’s density when compared to water.  The higher the figure, the greater the density.  Anything above 1 will sink and everything below 1 will float.  As can be seen in the table below, the specific gravity of SYP is about 24% greater than SPF and 18% greater than DF.  The denser structure of SYP contributes to it outperforming other species in bending, tension and compression.

It appears that where strength is needed, SYP is the champ among these softwoods.  However, this increased density means greater weight per board.  The denser, harder wood may also prove to be more difficult to drive nails or start screws into.  If constructing an interior partition on an elevated floor, one could benefit from the increased workability and extreme light weight nature of SPF lumber.

To be clear, when SYP is needed for strength and therefore specified, it SHALL not be substituted with SPF.  A beam or compression member that has been sized using SYP will only have a fraction of its intended strength if built with SPF.  This can lead to life safety liabilities that no one wants.  Consult your local professional Engineer or Architect for any substitution requests.

Author: Enrique Fernández, PE

Expansion Anchors at the Edge

Like the old refrain goes, “there are only two kinds of concrete: cracked concrete and concrete that will crack.”  Engineers and builders do their utmost to limit and control cracking to preserve structural integrity and aesthetics by employing various methods.  We create control joints, construction joints, expansion joints, provide temperature and shrinkage reinforcing using welded wire fabric, fibers and rebar.  In synopsis, we go thru a lot of effort to mitigate cracking concrete.  So then, why would a builder ever expressly create a condition where concrete is likely to crack and compromise structural integrity?  This cementitious sin is more common than you might think, and the guilty party is often an expansion anchor installed in a close edge condition.

Expansion anchors inherently work by, you guessed it, expanding!  When installed, the expansion anchor is hammered into a predrilled hole in the concrete.  Then a wedge at the base of the anchor is drawn up thru a sleeve which expands against the drilled hole within concrete.  Naturally, this expansive force creates stresses within the concrete that must be resisted.  While concrete is a natural champ in compression, it is an abysmal wimp in tension.  So when these stresses need to be resolved near a slab edge, resultant forces can create splitting within the concrete that will likely propagate cracks extending out from the anchor.  These cracks can compromise the integrity of the connection and aesthetically ruin your slab edges.

However, there is a time and place for expansion anchors though.  They are widely available and an economic solution for anchoring within interior slab conditions or when set within minimum edge distances on foundations.  Specific anchors can vary based on the manufacturer, but for a typical 5/8” diameter anchor to achieve full capacity, the minimum edge spacing is usually 10”.  Some anchor manufacturers allow lesser minimum edge distances, down to 4-1/2”, but with significant reductions in capacity.  In the absence of manufacturer recommendations tested per ACI 355, the minimum edge distance of expansion anchors (displacement or torque controlled) must be checked for side-face blowout failure and cannot be less than 8 anchor diameters per ACI 318-11 appendix D8.3 and ACI 318-14 section 17.7.3.

Architects, engineers, contractors, and do-it-yourselfers may be asking themselves right now, “Ay bendito! How can I attach a sill plate at an exterior wall to my slab without wedge anchors!?!”  The limitations of expansion anchors (wedge and sleeve anchors) noted above preclude their use in exterior wall applications near slab edges.  Well, despair not, you can always specify a cast-in-place anchor like the TIEMAX Edge Bolt which has higher holding power and is specifically designed for close edge conditions as little as 2”.  Also, if you need is a post-installed solution, then there is always the drill and epoxy TIEMAX Stud or the Large Diameter Tapcon that comes in many sizes and flavors.  These are all available from Fastening Specialists Inc:

Author: Enrique Fernández, PE

Glazing Protection

Do you sit outside during a hurricane?  Not likely, even if you are invited to a hurricane party.  So why would you allow a hurricane into your home or business by not protecting windows, doors and openings adequately?  Luckily, there is guidance for glazing protection recommended, or in some cases mandated, by the Code for hurricane prone areas.  These approved products and methods can keep the storm outside and even incidentally protect your home or business from an errant brick launched by a not so peaceful protestor.

Impact resistant windows can be costly but offer permanently in place protection that does not require last minute installation and allows a view to what is happening outside.  Their construction can be a laminated glass/film assembly or fenestration made of polycarbonate that is 200 times stronger than glass.  Impact testing for such assemblies against large and small missiles is covered in ASTM E1996-20 Standard Specification for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Windborne Debris in Hurricanes.   “Missiles” can range from a small steel ball weighing as little as 0.1 ounce to an 8 foot long 2×4 lumber board launched at 80 feet per second.

Impacted windows are then tested against hurricane wind pressures per ASTM E1886 Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials.  This ensures that impacted windows continue to perform throughout the storm event.  For homes and businesses located within 3000 feet of the shoreline, window frames are typically made of vinyl or fiberglass to mitigate corrosion and provide a durable assembly.  It is obviously important to attach these windows per the manufacturer’s approved recommendations which typically include provisions for pressure treated wood bucks and stainless steel anchors, like those pictured below and available from Fastening Specialists Inc.

Alternatively, shutter systems are also available for opening protection.  These function independently from the windows and doors they protect from missile impact, but do not typically protect against cyclic wind pressures which will need to be resisted by the door or window itself.  Shutter systems come in many forms including roll-up, accordion types, corrugated panels, etc.  Some require installation prior to the storm event and others are in place permanently, but must be operated to be set in the closed position.  In the event that a system is electrically operated, these units should be able to be manually overridden in the likely event that there is a power failure.  Some systems offer translucent or clear panels that allow a view to the outside and light to pass into the structure during a power outage.  Lastly, the minimum glazing protection permitted by the Code is structural wood panels no less than 7/16” in thickness and spanning no greater than 8 feet.  The minimum fastening schedule for this type of protection is also specified in the International Residential Code section R301.2.1.2.

Garage doors are a common point of failure in the structure’s exterior envelope.  These are very large openings that can increase internal wind pressures substantially if failed.  Roof sheathing can be peeled off from the combined forces from within and without the structure.  Use the appropriate roof sheathing fasteners to mitigate this risk as covered in a previous blog, The Rise of Roof Sheathing Ring Shank Nail.  Additionally, impact and wind pressure rated garage doors are increasingly available from a myriad of manufacturers and should be specified for designs in hurricane prone areas.  As evidenced, elevated internal pressures can often lead to significant structural failures locally or throughout the building.  Interestingly, the Code used to allow for the design of these elevated internal pressures in lieu of glazing protection, but since has been repealed due to significant material losses as a storm enters a building.  Fortunately, with regards to this, the times are a changing, as described in more detail by a previous blog addressing the updated wind code adopted in the Florida Building Code 2020.  Consult your local professional structural engineer for the enduring design of your next project.

Author: Enrique Fernández, PE

Wind passed by ASCE 7-16

A very venerable and very senior engineer (senior, like briefly came out of retirement senior) once berated a team of younger engineers by accusing them of “confounding the wind Code with their blasted academia and just making it all too complicated!”.  While his grumbling may contain more than just a hint of truth, he was justly rebutted by one junior engineer who said “yeah, things were simpler when the whole Code was a pamphlet that fit in your back pocket.”

I’ll spare you the details of this battle of the ages, however the wind Code continues to grow with the noble aim to increase building safety while safely reducing costly conservatism.  Our most recent update to the Florida Building Code 2020 edition is set to be adopted as of January 01, 2021 and brings into the fold the ASCE 7-16 Code in lieu of ASCE 7-10.  New, and probably overdue for the soon to be adopted wind Code provisions is a standalone map for Risk Category IV buildings (essential facilities and structures whose failure would pose a substantial hazard to the community).

No real surprise here, as Risk Categories III and IV used to share a map in the previous Code edition which seemed to defy the purpose of having two separate Risk Categories in the first place for wind calculations.  Now, in fairness, for those who can remember back to prior ASCE code editions when only one wind map existed and Wind Importance Factor was used to differentiate Occupancy Categories, even then both Categories III and IV shared an Iw = 1.15.  Apart from this addition, the new ASCE 7-16 is largely unchanged in the hurricane prone parts of the USA.  The middle and western parts of the country have seen some reductions in wind velocities however, and it’s nice to see they’re getting some attention.  These areas used to be covered by a uniform swath of 115mph for Risk Category II buildings and now they have winding wind patterns of their own.

Don’t get winded just yet, there are more changes to the wind provisions as we wind our way through the new ASCE 7-16.  There is also a new Factor for Ground Elevation, Ke, which adjusts velocity pressures using ratios that correlate to air density.  As one gains elevation above sea level, air is more rarified and this air mass density decrease is now taken into account for its reduced effect on wind pressures.  Additional changes include new roof zoning with enhanced corner zones (zone 3), new field zone (1’), eave zones (2e and 3e), ridge zones (2r and 3r), and rake zone (2n).  A new enclosure classification titled “Partially Open” has been added.  Also important to note, external pressure coefficients, GCp, have been notably increased for all roof zones for an overall increase of 28%, and this will presumably elevate wind design pressures in many cases.  Alas, there are more details and numerous changes to be found in the new ASCE 7-16 and I would recommend you pick up a copy (it makes excellent bedside reading) or hire and consult with your local professional structural engineer who is obligated to sail the winds of change.

Author:  Enrique Fernández, PE

The Rise of the Roof Sheathing Ring Shank Nail

The year 2020 will live in infamy for many reasons, among them is this record hurricane season.  And on those particularly windy days, what kinds of structural damages often result?  Most Florida residents have seen shingles fly by like high speed frisbees or palm fronds strewn across their streets and yards.  But there are almost always an unlucky number of homeowners who have to contend with major roof sheathing failures that have debilitated their structures and let the storm inside.  Roof corners, edges and sometimes whole swathes of roof sheathing are peeled away from the structure like a delicate clementine tangerine.

“Why”, you might ask.  “Wasn’t this built to code”, you may contend.  Well, times are a changing.  The Florida Building Code 7th edition 2020 has officially been released and is set to take effect beginning December 31, 2020.  Many jurisdictions will be adopting this new version of the code in 2021 and subsequently enforcing it.

Updates to the Florida Building Code tend to have far ranging effects since Florida is considered to be a leader in high wind design and mitigation techniques due to its exposure and vulnerability to hurricane events.

Among the updates in the FBC 2020 code revision, is an important set of fastening requirements that help prevent the damaging uplift forces that strip structures of their roof sheathing.  In accordance with the new Code, ring shank fasteners with standardized dimensions and material properties designated in ASTM F1667 will be specified as the fasteners of choice. 

Three common variants of these Roof Sheathing Ring Shank (RSRS) nails are typically specified.  RSRS-01 (2-3/8” x 0.113”) nails, RSRS-03 (2-1/2” x .131”) nails and RSRS-04 (3” x 0.120”) nails.  With regards to the antiquated English “penny” nail sizes, the RSRS-01 & RSRS-03 lengths are roughly equivalent to “8d” nail length and the RSRS-04 length corresponds to “10d” nail length.

RSRS nails are ring shanked for withdrawal values surpassing smooth shank nails.  They also feature full round heads that increase pull thru capacities.  These features help seriously mitigate two common failure modes of roof sheathing subjected to uplift forces.  RSRS nails are available in the hot-dipped galvanized finish and 316 stainless steel for superior corrosion protection.  The new Code requirements also mandate more stringent roof sheathing thicknesses based on project wind speeds as noted in the 2020 Florida Building Code Residential 7th edition Table 803.2.2.

While the Code provides some prescriptive tables for sheathing and minimum nailing requirements, it is always advisable to consult a professional licensed Structural Engineer to ensure the design and construction of your new structure can stand up to the elements of mother nature.  Take these steps to make sure that the “Rise of the Roof Sheathing Ring Shank Nail” only happens in sales figures and never out the framing once it’s driven in.

Author: Enrique Fernández, PE