Category: Lateral Systems

Little did we know when we introduced our first holdown in 1966 that our product innovations would lead us to solutions that can help hold together five-story buildings during an earthquake or allow builders to more easily retrofit structures and install larger window and door openings in homes. Our offering of lateral-force resisting systems, including Wood and Steel Strong-Wall® shearwalls, Strong Frame® moment frames and new Strong-Rod™ Systems, gives designers and engineers added design flexibility in wood-frame construction and the confidence that almost anything is possible.

Soft-Story Retrofits

In February 2007 I had the opportunity to volunteer for a Soft-Story Sidewalk Survey for the San Francisco Department of Building Inspection. The purpose of the survey was to inventory buildings in San Francisco that appeared superficially to have soft or weak first stories. The volunteers were given a list of addresses to review and we recorded if the building was more than three stories tall, had five or more dwellings, and estimated what percentage of the ground level had openings in the walls. No structural analysis going on, just counting stories, mailboxes, doors and windows.

San Francisco soft-story structure. Photo credit: USGS. — San Francisco soft-story structure failure. Photo credit: USGS.

A collapsed house in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images. — A collapsed soft-story in San Francisco from the 1989 Loma Prieta earthquake. Photo credit: Adam Teitelbaum, AFP, Getty Images.

Building Drift – Do You Check It?

Guest Blogger Sam Hensen, Simpson Strong-Tie Southeast Engineering Manager — Sam Hensen

[Simpson Strong-Tie note: Sam Hensen is the Simpson Strong-Tie Engineering Manager for the Southeast U.S. and the latest blogger for the Structural Engineering Blog. For more on Sam, see his bio here.]

Just as bending and shear checks performed on gravity loaded beams do not ensure that the beam will comply with required deflection limitations, adherence to allowable shears and aspect ratio limits on shearwalls does not mean the structure will comply with required drift limitations. Shearwalls that are too flexible may prevent the structure from meeting drift limitations even if the shearwall design has adequate strength.

Seismic

For seismic load applications, section 12.12.1 of ASCE 7-22 states that the design story drift of the structure shall not exceed the allowable drift listed in table 12.12-1. For light-frame buildings, the maximum permitted drift is 2.5% of the story height. This limitation is put in place not merely for serviceability reasons, but is an inherent effect of current seismic design provisions that is required to be checked to ensure life safety.

Taking Wood-Framed Construction to New Heights

When I had more hair and less of it was gray, I worked on a project as an assistant engineer doing the calculations for a mixed-use building in San Jose, California. Final design consisted of four stories of wood framing over a concrete podium slab and another level of below-grade parking. At that time, my firm had designed many two- and three-story residential buildings, but common thinking was you switched to steel or concrete for taller structures because of perceived limitations in wood-framed construction.Continue Reading

Preventing Roof Tiles from Becoming Wind-Borne Debris in High Wind Regions

I tend to think of designers as dealing with either wind or seismic design, yet the Southeast region contains everything that Mother Nature can throw at a building. This includes high seismic areas along the New Madrid and Charleston faults, hurricanes along the Gulf and Eastern coast, and tornado prone areas throughout the South and Midwest. Sam participated in the investigation and was a co-author of the Damage Study and Future Direction for Structural Design Following the Tuscaloosa Tornado of 2011, which gives him some very recent experience with tornado damage. This week, Sam will be discussing a topic not often thought about by structural engineers – the importance of proper roof tile attachments. Here is Sam’s post:

According to recent studies by the Insurance Institute for Business Home and Safety (IBHS), roof coverings are a major problem area in wind-related events and account for 95% plus of home claims after the event.

Preventing roof tiles from becoming wind-borne debris in high wind regions is essential for several reasons, and may also have an effect on insurance premiums. In this post, I’d like to discuss two reasons that roof tiles can pose a significant threat to life safety:

Why a Structural Boundary Member Between a Truss/Rafter is Not Optional

Blocking or boundary member?

In my experience traveling across the country observing wood-framed construction, it was apparent that east of the Rocky Mountains, structural wood members in-line with supporting walls between roof framing cease to be installed. Some may call these wood members blocking and deem them as optional. And often in a humid environment, installation of these members may be ardently resisted in order to provide ample attic ventilation and prevent mold growth. It is important, however, to understand that this blocking creates the structural boundary members for the roof diaphragm and it is not optional.

Top 3 Roof Deck Design Considerations for High Wind Events

Was it JFK who said, “The time to repair the roof is when the sun is shining?” He was likely using the roof as an analogy for the economy, but I take things literally and wanted to talk about roofs. The time to think about the design of your roof and its function in a high wind event like a hurricane or tornado is right now.

During a high wind event, a roof deck is expected to perform many functions. It should prevent water intrusion from rain, withstand impacts and protect those inside from hail. It also needs to act as a diaphragm – transferring lateral loads to shear walls and resisting the vacuum effects of wind uplift forces.Continue Reading

Flexible or Rigid? Multi-Story Light-Frame Structure Design Considerations

I like to think I’m flexible, but I’ve been accused of being rigid at times. I guess that’s what therapy is for. If you were to ask a light-frame structure diaphragm that same question, you would likely get multiple conflicting answers. The 1988 UBC first introduced parameters to evaluate diaphragm rigidity. Earthquake Regulations Section 2312(e)6 stated:

Figure 1. Flexible Diaphragm Definition from ASCE/SEI 7-22

Provision shall be made for the increased shears resulting from horizontal torsion where diaphragms are not flexible. Diaphragms shall be considered flexible for the purposes of this paragraph when the maximum lateral deformation of the diaphragm is more than two times the average story drift of the associated story. This may be determined by comparing the computed midpoint in-plane deflection of the diaphragm under lateral load with the story drift of adjoining vertical resisting elements under equivalent tributary lateral load.