Sustainable Wood High-Rise in Australia

http://www.theage.com.au/victoria/wooden-highrise-to-bring-cheers-to-old-brewery-site-20110227-1ba1p.html


The use of wood as a sustainable structural material is increasing as the industry moves toward the use of life-cycle assessment to determine a materials 'green' properties. For this structure, wood will ultimately be responsible for insulating the building and providing a power source, in addition to being the material of choice for the structural framing.

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.