Architecture, Engineering & Planning EVstudio | Denver & Evergreen Architect | Colorado & Central Texas | Blog » Framing

Types and Design of Wood Trusses

Jim Houlette — Thu, 19 Jan 2012 21:00:42 +0000

Here are some great resources about the different types of wood trusses and how they are designed. If you have any questions about roof or floor trusses please let us know.

Alpine Engineered Products – Builder’s Guide

SBCA – Structural Building Components Association – Truss Configurations

Originally posted 2009-08-16 00:01:36.

Effect of Overdriven Fasteners on Shear Wall Capacity

Jim Houlette — Thu, 19 Jan 2012 21:00:34 +0000

APA Technical Topics - Form No. TT-012A

With the use of nail guns set too high and the speed of construction, many nails are over driven. Most building officials don't seem to notice or comment on this error. But, if they do you might have a lot of work ahead of you to add correctly placed nails or staples when the nailing is too close for additional nails. Before you start adding nails or staples consider that additional nails may cause more harm than good. If the specified nailing is at 4"o.c. or less, adding more nails can damage the sheathing and studs, reducing the shear wall capacity more than if the additional fasteners were not used. APA addressed this problem and issued a simple breakdown on when you need additional fasteners or not.

Please visit www.apawood.org for additional information and other useful technical bulletins.

Originally posted 2009-08-08 22:27:19.

Who Does Truss Drawings?

Sean O'Hara — Thu, 19 Jan 2012 21:00:09 +0000

Most residential projects and wood framed commercial projects utilize roof trusses. A roof truss is generally cheaper and less time consuming than a roof framed with rafters. It has these advantages because it is created from smaller pieces of lumber and it is fabricated in a shop and then only has to be set up in the field.

When we design a house we do a truss layout where we show where we anticipate each truss going and give information on what the shape of the top and bottom of the truss is, what the plate heights are and what the loading criteria is for the trusses. We also call out if the truss is a flat bottom truss, a scissor truss or an attic truss. Then the contractor gives our layout to their truss supplier. The truss supplier does the actual design of each individual truss based on our roof design. The reason for this is that each truss supplier may have a slightly different preference for how they construct the truss and they use their own proprietary software for the design. When the truss drawings are complete the contractor sends them to the Architect for review. Usually the drawings are right on when we review them.

The wrinkle that we are starting to see more of is municipalities requiring the submittal of truss drawings to their building department prior to the issue of the building permit. This can cost some time because typically the truss supplier doesn't do their design until after they are selected as the winning bid. In other words the truss supplier has to be selected prior to the completion of architectural drawings, otherwise expect another week or two added to the schedule. Oftentimes in a bid situation the contractor hasn't been selected at this point.

I've recently run into this new policy in Jefferson County and Commerce City, Colorado. I'd urge municipalities to look at this policy and consider going back to not requiring truss drawings to be submitted as part of the permitting process.

Originally posted 2008-10-05 16:57:45.

Reducing Holdowns Using OSB Exterior Sheathing

Jim Houlette — Sat, 31 Dec 2011 21:20:56 +0000

Tired of all the holdown straps?

You can eliminate many of the metal holdown straps from the upper floor wall to the main floor wall by modifying your OSB wall sheathing layout. When the OSB wall sheathing is continuous over the floor framing section or lapped with the rim board uplift loads can be transferred.

If you are utilizing prefabricated wall panels you can still have the OSB extend to the middle of the rim board. Just have the OSB extend below/above the plates as build in the shop. The only caution is, extra care is needed to prevent the overhanging OSB sheathing from being damaged during shipping.

This concept can also be utilized at the foundation sill plate to help reduce uplift loads that would otherwise require embedded straps such as STHD10RJs. In that situation, cross grain bending is not desired on the sill plate as the load transfers to the anchor bolts. The cross grain bending can be removed from the equation using mud sill anchors (MAS) attached to the exterior face of the OSB sheathing in lieu of anchor bolts. A great solution when utilizing prefabricated walls.

Typically, when building with wood you want many smaller fasteners that distribute the load out evenly rather than a few large fasteners concentrating the load.

EVstudio | Architecture and Engineering | Denver and Evergreen

Originally posted 2009-07-31 00:01:56.

It's Okay to be a Little Shifty

Dean Dalvit — Mon, 12 Dec 2011 23:46:20 +0000

This post is for all of those framers who diligently adhered to a structural framing plan to the letter and wound up with more backout framing than they knew what to do with once the plumber and mechanical contractor had their way with the joists.

According to Weyerhauser, the makers of the TJI joist products...
TJI® joists that do not occur under a joint in the sheathing may be shifted to avoid cutting them for plumbing or other mechanical. Joists other than the single joist to be shifted must remain at their specified locations. Joists meeting the clear spans of the Trus Joist L/480 live load span table may be shifted up to 6” (but not exceeding a maximum spacing of 24” o.c.).

Framers, have your guys lay out the plumbing and heating penetrations through the floor deck so you can adjust the layout as necessary and save yourselves considerable headaches during backout. While it is a bit unorthodox, we like to show the plumbing fixtures on our structural plans for this very reason. We don't always know how the mechanical contractor is going to run his equipment, so it's better to ask twice and cut nothing at all!

Originally posted 2008-08-11 15:05:28.

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Fastening to Concrete – Wedge Style Expansion Anchors

Adam Denton — Fri, 09 Dec 2011 23:50:00 +0000

A 5/8" diameter wedge style expansion anchor.

There are many situations were it may become necessary to attach a timber ledger to a concrete wall. It is desirable in many new homes to lower the elevation of the finished floor to make a home more wheel chair accessible. An example of the differences between the finished floor elevations that can be achieved with a framing ledger are shown below. Additionally, when an addition is made to a residential home a timber ledger is installed so that the finished floor of the new construction can match the finished floor elevation of the existing construction while utilizing the existing foundation wall.

Conventional Floor Framing vs. Ledgered Floor Framing

Wedge style expansion anchors work because of the friction forces developed between the studded collar (see photo at top) and the concrete. To begin a typical installation a hole must first be drilled into the concrete. All manufacturer’s instructions for hole size and depth should be followed carefully because the recommendations will vary between bolts sizes and manufacturers. Once a hole is drilled and cleaned the anchor may be driven into the hole with a hammer. The studs on the collar are slightly larger than the widest diameter of the bolt so initial contact with the concrete is only at these areas. Once the bolt is driven to its specified depth a pre-drilled timber ledger may be installed over the bolts. Finally, the nuts (with supplied washer) may be installed. As the nut is tightened, the studded collar with catch on the edges of the hole in the concrete and will be forced onto the wedge. The collar will expand as it is driven onto the wedge creating a greater frictional force against the concrete. With the nut installed to the proper torque the anchor may be safely loaded to the manufacturer’s design specifications.

Originally posted 2010-07-30 23:04:52.

Proper Bearing of I-Joist Rafters

Adam Denton — Mon, 21 Nov 2011 08:02:24 +0000

As today’s architectural design trends toward more open floor plans with vaulted ceilings there is a need for economical rafters that will span long distances while maintaining a shallow profile. In many situations pre-manufactured I-joist rafters fit that need.
As a structural engineer I have inspected many roofs framed with I-joist rafters, but very few that have not needed a correction due to the I-joist bearing. While typically not a problem when the roof is flat and the I-joists bear perpendicularly on the top plate, when the rafters are sloped proper bearing becomes an issue. The simplest way to properly bear the I-joists is to attach a full length beveled block to the top plate. As a structural designer I have not gotten a lot of positive feedback from framers when I specify this method. Beveling the end of the I-joist seems to be preferred over beveled blocking. The problem comes with the compromised stiffness of the I-joist’s web once the bottom flange is beveled.

As shown in the above detail, the web of the I-joist rafter should be reinforced with plywood to increase it’s stiffness at the bearing edge. As always, blocking should be provided between rafters as well as a mechanical clip (such as the Simpson H2.5) to counteract uplift forces.

Originally posted 2010-10-25 00:01:45.

Structural Modeling in Revit

Anthony Ries — Thu, 10 Nov 2011 07:39:06 +0000

With each new project we use more and more of the tools in our BIM toolbox. The latest tool that we have developed to the level of prime-time is our structural steel framing within Revit. We are currently working on a very unique custom home in the foothills of the Rocky Mountains which has some unique requirements. This home will be making extensive use of architectural concrete and structural steel.

Normally we can create a 3D walk through of a project as an additional architectural service. With this current project we have created the entire structural steel system in Revit and with a few mouse clicks are able to create a structural steel walk through. Below is a short video showing the preliminary structural steel layout only. No bearing walls or concrete is shown.

As a design and documentation tool we are able to pan, zoom and rotate in 3 dimensions to thoroughly vet our structural design and also more quickly identify problem areas where structure may be interfering with other trades or design features. Additionally it provides a solid model that indicate lengths of beams, joists and columns. This particular model will be shared with the steel fabricator in order to improve their accuracy, speed and reduce questions. We also use it internally to investigate how various pieces come together in a direct way.

With all of that said, 3D modeling is no substitute for expertise, education and practical experience. We use the software as a tool to further explore and document our designs, not as a crutch to develop the design.

Why It’s Critical To Have Correct Floating Sill Plates on Slab-On-Grade Floors in Colorado

Jim Houlette — Tue, 20 Sep 2011 07:42:49 +0000

When I’m requested to investigate a house with structural concerns one of the first things I check is proper floating sill plates on slab-on-grade floors. I’m always amazed how so many basements are finished without a floating sill plate or some way of allowing the slab to move without moving the structure. I believe the problem starts with engineers not specifically detailing a floating sill plate. If they do, many of them are detailing it wrong. Compounding the issue is most basements are finished by the homeowner with the help of contractors with no information on a proper floating sill plate detail.

The first thing everyone must understand is ALL slab-on-grades have a good chance at moving. The amount of movement, up or down, depends on the soil type, backfill compaction and existing or introduced water percentages. A soil report by a geotechnical engineer will typically provide you information about the risk your basement slab-on-grade may move. The risk can vary from low, moderate, high or very high. When a high or very high risk is encountered an elevated structural floor is typically the best choice, so the soil movement does not translate into the structure. With low to sometimes moderate slab risk a slab-on-grade can be used with precautions to isolate the slab-on-grade from the rest of the structure.

The most common error that contractors or even some engineers make is assuming the gypsum board wall sheathing will just crumble if the slab pushes on it. This is not a good assumption to make. Yes, gypsum board is brittle and falls apart fairly easily compared to other building materials, but it does have strength. When the gypsum board is placed full height from slab to ceiling any movement in the slab can be translated into the structure.

EVstudio | Engineers and Architectures | Denver and Evergreen

QCI Structural Solutions - Residential and Commercial Repairs

Originally posted 2009-08-02 17:38:00.

Understanding Trus Joist Pro Ratings and Minimum Deflections by Code

Jim Houlette — Wed, 07 Sep 2011 18:37:01 +0000

When designing floor joists engineers must check several design properties including moment, shear, bearing and deflection. In this article we are going to focus on deflection since it is a common complaint from homeowners.

For any structural member deflection limits must be satisfied based on the building code. For floor joists a live load deflection limit of L/360 and a total load deflection limit of L/240 must be met. For example, a floor joist spanning 20ft (240 inches) a deflection limit of 2/3inch for live loads and 1inch for total loads. This is the maximum deflection the designer should meet but it is only a minimum requirement. Some homeowners may notice this deflection and complain about bouncy floors.

To provide higher quality floors for homeowners a deflection limit of L/480 is commonly used. For the 20ft floor joist example the new deflection limit is now 1/2inch.

iLevel has taken this one step further by creating their TJ Pro Rating. The TJ Pro Rating is based on satisfaction percentages of homeowners. Below is some information provided by iLevel.

TJ-Pro™ Ratings factor in the human element in designing floors.
How a floor feels to a home buyer has always been subjective. Yet making floor performance predictable is exactly what we at iLevel have done. We invested over 10 years of research into “real world” floor situations and tested hundreds of floors that were built to precise deflection characteristics. Then we had thousands of people walk on them. We captured their preferences, put the information into a computer model and analyzed the data. TJ-Pro™ Ratings are the result. Based on the number of people who preferred one floor to another, we assigned a performance number. Our proven software programs use this information to analyze your design and assign it a performance number that is typically between 25 and 65. The higher the number, the more likely a home buyer will be happy.

Originally posted 2010-03-14 00:01:01.