Deck Design Do's and Don'ts: Part 3

Sawn lumber, unless protected, is highly susceptible to shrinkage, swelling, twisting, and bending when exposed to moisture.   For an outdoor deck, lumber is exposed to rain and changes in humidity, the latter of which can be a nightmare in Alabama.  The deck in the picture below shows signs of joist failure.  One saving grace for the deck is the fact that the two test stations that have toppled over are constructed of thin, hollow metal and are much lighter than they may appear.  

The second picture, shown below, shows how the joists are being held up by the deck.  The 1x ledger board used to support the joists is still in place.  Unfortuntaly, the beam has rotated away from the deck, allowing the ledger to slide out from under the joists.  The beam's rotation and deflection were exacerbated by the effects of weather exposure.  The few nails that connected the beam to the joists were nailed from the beam and into the end grain of the joist, leaving very little resistance to lateral separation.  The beam's lack of resistance to moving laterally allowed the effects of loading and weather to combine; ultimately causing failure of the deck without failure of the beam or joists.   

Small changes in the construction of the above deck, such as toe nails or joist hangers, could have made a big impact on the decks performance.  Thankfully, the employees working at the facility were unharmed. 

Statistically, deck collapses are responsible for more people being injured or killed than any other part of their residence.  Taking the time to consider low cost, high impact changes in all aspects of construction should be at the forefront of every builder.   

Click here for Deck Design Do's and Don'ts Part 2 and here for Part 1. 

Kerfing a Bowed Stud

Wood studs are not perfect and never will be.  Neither will any other material, plant, etc. that is made in nature.  Through experience, science, and understanding, we learn how to manipulate natural materials to be suitable for our needs.  Trees, which have branches in many aspects of our lives (pun intended!), are a top renewable resource.  After centuries of harvesting tress, and thus lumber, to build shelters, etc., you would think our understanding of the material would be very thorough.  But as some say, it can be hard to see the forest through the trees.  In other words, sometimes you need to see the big picture.  Below is an example of an instance where the big picture reveals much more than what is in front of you 

It is common for carpenters to kerf a wood stud to remove a bow in the stud to allow for the wall finish to be flat.  Depending on the function of the wall, such as load bearing, the kerf in the stud may be reinforced with a section of 2x material lapped and fastened to the full height stud.  Regardless of how the stud is kerfed, reinforced, etc. the most important piece of knowledge involved is the function of the stud.  Knowing what the stud supports can result in an appropriate repair solution. 

These 2x6 studs have a kerf that leaves just over an inch of solid material. 

Now, back to that forest and tree stuff.  The worker who cut these studs could only see the bowed stud (tree) in front of him.  Had he looked at the big picture (forest), he might have realized that kerfing a first floor stud in a four story building might not be a good idea.  And yes, this wall does carry floor trusses from each floor above. 



Deck Design Do's and Don't's: Part 2

For Part 2 (click here for Part 1) of Deck Design Do's and Don't's, I want to address guard post attachments.  Based on research at Virginia Tech, we now know that typical attachments used in deck construction do not meet code required design loads.  One such instance can be seen in the picture below. 

The 4x4 post has been notched and nailed to the exterior rimboard of the deck.  As you can see, the nails in the thin portion of the member have created a split in the wood along the grain.  Similar notched and un-notched connections with bolts also fail to meet the code design loads.  

Use of hold-down anchor brackets, as shown in the picture below, are required to be installed at guard posts to meet the design loads.  The primary method of failure for typical bolted connections stems from the rotation created in the rimboard.  The rimboard attachment to the joists must support that rotation.  Hold-down anchors provide a means of transferring the load out of the rimboard and into the floor system.  

Photo Courtesy of Simpson Strong-tie.

For more information on deck design, including links to deck design guides, you can look at my previous post here.  For more information about guard post connections you can look here for details and here for design.   

Deck Design Do's and Don't's: Part 1

I recently visited a wood framed observation platform that I am replacing with new designs.  The owner wants to keep a small section of the platform that is newer while replacing the older portion that is failing in some areas.  I will post some of the failure items over the next few weeks so as not to show everything at once.

For Part 1, we can discuss the importance of post caps when bearing a girder on a beam.  See the photograph above.  Reasons to use a post cap for this condition include:

1. Increases lateral strength of the connection.
2. Eliminates use of toe-nails, which when improperly installed can split the top of the post.
3. Contains movement of the wood girders relative to the top of post that results from the shrinking, swelling and twisting of wood as it reaches its equilibrium. 

Ultimately, you do not want the condition in the photograph to result in the photograph below. 

For more information about deck design, please see my previous post here.

Lessons from a Tornado

The April 27 tornadoes left many opportunities to study structural performance of both commercial and residential buildings.  One area that I have observed was a residential development located just out of the path of an EF3 tornado that made its trail not far from my home.  With the exception of one building's collapse, damage to each home generally consisted of a portion of roof missing, as seen in the photo below. 

A close up inspection of the rafter heel condition at the top plate highlighted the failure of the roof system.  A birds-mouthed rafter with (3) toe-nails into the wall top plate is a common framing detail used in residential structures.  This detail was used throughout this development.  There appear to be two realistic modes of failure for this detail as highlighted in the pictures below.  The first mode is failure of the 2x rafter at the birds-mouth.  This failure mode occurred in a small portion of rafters witnessed. 

The second mode of failure is at the toe-nail connection.  Although this connection works reasonably well for lateral shear, the typical limit state for uplift loading is nail withdrawal from the wall top plate.  The nails in the picture below appear to have seen very little stress during failure.     

Athough this is a common detail, I avoid detailing a rafter/truss to top plate connection with toe-nails. A Simpson H2.5A anchor, available at local hardware stores, is inexpensive and provides a significantly improved connection capacity. This syle of clip results in a more consistent installation and brings a different connection failure mode that does not include nail withdrawal. For a small home with a simple framed roof, such as homes in this development, a small investment in H2.5A clips can have a major impact on structural performance.

Tuscaloosa, Alabama Tornado Meets Wood Truss

I pulled the picture below from The Birmingham Business Journal's coverage of the April 27, 2011 tornado damage.  I have been slow to look at online pictures and videos of the event for a myriad of reasons.  I lived in Tuscaloosa for several years while studying for my BS and MS.  Even though many people have seen the devestation via TV, Web, or first hand, I could not resist sharing this picture here on the blog. 

A close look at the stick of wood embedded in the bumper of this car, and the wood lying on the ground in front of the car, reveals that this "stick" is actually from a metal plate connected wood truss.   I do not know the context of this picture to understand how this may have happened or the location of adjacent buildings from where this truss may have lived.  Whatever the process, the end result is amazing.  

Follow Through!

I spent April Fool's Day travelling to two different sites with wood frame structures.  While I observed several items that will eventually make this blog, one that stood out the most is shown below.  The anchor bolt installation was consistently inconsistent, with about 1 in 3 installed correctly.  From cut-off anchor bolts cast outside the edge of the sill plate to anchor bolts embedded into wood studs to the 1/2" diameter in lieu of 5/8" diameter, it was a free-for-all for picture taking.  But please, when everything comes together and all you have to do is place the nut on the bolt, just do it.  That nut has threads just waiting to be twisted onto the end of the bolt. 

The Mullions are just not that bad.

In our last installement, we looked at some rather unfortunate mullions framed in a stairwell.  Let's take a close look at the framing around the wall.  The upper portion of the wall is gable framed above trusses that bear at the top plate.  This is good because it allows the trusses to brace the top plate for out-of-plane wind resistance.  To lessen the axial load on the wall, a three-ply beam spanning the width of the stairwell was placed approximately 3-4ft inside the exterior wall.  Somehow, someone missed a few inches of framing, which resulted in the trusses NOT bearing on the exterior wall.  If the trusses do not bear on the wall, how can they brace the wall to resist out-of-plane wind pressure? 

Mulligan for Mullions

It has been a while since I looked at this picture, but I remember this condition quite well.  As some context, this condition occurs at the fourth floor level of a stairwell.  This wall condition spans from the roof framing down to the intermediate landing between the third and fourth floor.  If we look a bit closer, as in the photo below, we can see that only two studs run the full wall height at each side of the window.  This project was located in a region with a wind speed of 100mph.  As is, this condition required remediation to the mullions and the header-mullion connections to satisfy design requirements.  Had the mullions between the windows been constructed full height, instead of spanning between the headers, the load path would be easier to follow and require less remediation.  

 

Unbearable!

I do feel bad about not getting a second post for January.  I had a slight delay due to the birth of my first child.  My little girl weighed in at 6lb 11oz and made it into this world 13 days before her due date, which coincidentally was today.  I will try to make up for the lapse in posting with two pictures in one post.  (Yes I know, it is cheating when it shows the same thing.)

As the title hints, the corner of this wall is not actually bearing on the slab.  Since this corner is at a shearwall (hopefully the hold down anchor was installed after this visit), at some point during its life this stud pack will see a compression load.  Will this cause the building to fail?  Most likely not.  It will cause the interior gypsum board to buckle.  That would sure be a mess to clean up.  

A second picture below better indicates the true gap between the sill plate and the slab.  You can also see separation between some studs and the sill plate.  If someone were to tighten the anchor bolts to cinch the sill plate to the slab, more separation could occur, which does not correct the problem.  You could fill the gap with an epoxy grout.  This building uses prefabricated walls which should be fabricated to the correct height.  If you fill the gap, what happens at the top of the wall?  



  

OverSTRESSED

What an interesting week.  I have spent the last three days out of the office in full long-john gear, two pairs of socks and gloves and a boggin.  For the last two days I climbed into, through, and around 20 attics.  Most of the projects we see are under construction.  I learned a lot of things the last two days by observing roof trusses and framing that have been in place over a decade.  Particularly, I learned the importance of proper truss member bracing.  As shown in the picture, weak axis bending is a limiting factor in truss design.  The truss manufacturer relies on the installation of bracing to prevent this mode of failure.  When the bracing is not installed, the web members are overstressed and, as shown, can fail. 

Relocating Hold Down Anchors

Here is a common issue that can arise in the placement of hold down anchors.  In this picture, two cast in place anchors interfered with the installation of the hold down anchors.  The easiest fix from the Framer's perspective is to move the hold down to the nearest stud for a clean installation.  This presents several items that the Engineer needs to consider prior to approving this change. 

• As you can see in the picture, the fasteners provided with the anchor are too long for one stud.  Hold down anchor manufacturers generally require an attachment to a minimum of two studs.  Additional studs may be required to meet the manufacturer's specifications.
• If the hold down anchor is relocated away from the edge of the panel, does the shear wall diaphragm boundary element change? The tension load needs to be transferred into the stud pack with the hold down anchor.  I recommend requiring the specified edge nailing be used to fasten the sheathing to the new stud pack at the relocated hold down anchor.  
• By relocating the hold down anchor, the length of the shear wall changes.  This may be critical in a high wind or seismic area.  The design of the shear wall should be verified based on the field changes.

Merry Christmas and Happy New Year. 

At First Glance

One of the things I quickly learned in the field is that not everything is what it appears to be.  Whether it is an epoxied anchor bolt that you can pull out by hand or a hurricane clip wedged between two pieces of wood with no nails, the closer you get to see and touch the structure the more confident you will be with your field observation.  The girder truss in the above picture looks to be properly supported.  A stud pack below the truss and hurricane clips on each side of the truss are standard fare in a region with a low wind design speed.   If I stood across the room and looked up at this condition, I would feel satisfied.  Walking further into the structure, however, I saw that the stud pack may not be adequate.   Have a look at the picture below.  

When observing the framing in apartment complexes or housing units, the framing quickly becomes redundant.  Continuing to walk through each room in each unit is still important to not miss the problem conditions that exist isolated to specific units.   



Beat to Fit - Part Deux

On this episode of "Beat to Fit, Paint to Match", we have this dandy plate washer that was bent to allow the sheetrock to be attached to the wall.  The stud wall in question consists of 2x4's at 16.  The plate washer, if I remember correctly, is 2"x2".  You can see that the problem is the location of the anchor bolt, which barely fits behind the edge of the sill plate.  For a 3 1/2" wide sill plate, there is a tight tolerance for locating the anchor and placing a 2" plate washer.  Had this been a 3" plate washer, there would be a 1/4" tolerance each way from the center of the plate.  

I have to ask, at what point would it be better to use post-installed anchors instead of cast-in-place?  From conversations with two different Contractors, a Titen HD is about twice the material cost of an L-bolt.  For a job with 50+ housing units, that would be a substantial impact on the overall cost.  In my opinion, labor savings may cover the cost difference, especially when bolts and/or studs have to be replaced when they align with each other (see previous posts).  If the framing crew is working on contract, and not by the hour, then the labor savings may not be seen by the owner.     

Beat to Fit

I woke up last Monday morning in a hotel in Mississippi.  It was one of those almost cool, humid Southern mornings with an overcast sky and the promise of rain.  I was excited to get to the job site and get to work.  My excitement was more about getting finished so I could make it home to my first Preparing for Parenthood Class than it was for being on-site.  I do enjoy being on-site, but for that day it was trumped by thoughts of the arrival of my first child.  Anyway, I should at least say that I was pleasantly suprised by the framing conditions I found.  I am not that pessimistic, but problems often arise in wood framing construction. 

I was on my 20th, and last, house when I discovered the beauty below.  I can understand the need to cut a stud because the anchor bolt is in a bad position.  I have seen that happen many times now.  This may be the first case where someone decided to beat the stud into submission rather than use a power saw.  Maybe it was early in the morning and this guy was full of extra energy.  Maybe he was just wielding his authority with his all-mighty hammer.  Maybe the wiley ole-timer did this and had the panel set in place before the young thinker could get the saw out of the tool box.  I just could not help being reminded of the old saying "Beat to fit, paint to match."  

Plumber versus Window Jamb - Take #2

Here for Take #2 of the Sawzall Massacre*, someone did a fine job carving up another window jamb.  Reminds me of the Dim Mak scene from Jean Claude Van Damme's Bloodsport:  

 

Official: "Cut stud with saw"

Sawzall Frank: "Okay, which stud?"

Official: "The two in the middle!"

I have to wonder if the window manufacturer counted on the frame to transfer the load from the header into the single remaining stud below.  Maybe this will not affect the opening and closing of the window in the future. 

*The Sawzall Massacre has been filmed live at a jobsite near you. 

Plumber versus Window Jamb

It is amazing what someone can do with a reciprocating saw.  Here you can see where someone cut the studs of a prefabricated wall panel to fit around the pipe cast in the slab.  Three support studs were cut to allow the pipe to stay in its location.  This is not good.  Thankfully, for a small opening and a one story structure, the single stud would probably never fail.  That was before the plumber decided to core the 3 1/2" studs with a 2 1/2" hole saw.  Let's just hope this opening was reinforced before it was covered with sheet rock.  It is usually best to review the framing AFTER the rough plumbing installation.