Structural Steel Connection: The importance of steel cap plates at the top of a column

Why do Structural Engineers specify steel cap plates at the top of a steel column?  I get asked this all the time in the field.  Installers want to know why they can’t weld the top of the column directly to the bottom flange of the steel beam.

Steel Beam to Steel Column Connection

The web or vertical portion of a typical wide flange steel beam (I-Beam) is what transfers shear or load to the column below.  To adequately transfer large loads, the load must be applied uniformly over a large enough area, say several inches or more.  Another way to think of this is providing adequate bearing.  We can’t expect the steel beam to transfer its load to the column with a small bearing area, right?  If the steel beam web does not have adequate length along which to transfer the load it could buckle, which we call ‘web yielding’ or ‘w’eb crippling’.

web buckling

When we introduce a large point load from above, the given area we have to transfer the load may not be enough.  It is at these locations that Structural Engineers and steel detailers specify web stiffeners to prevent buckling.

Steel Connection

Please contact the Structural Engineer involved with your project, before you order your steel, to make sure you have the correct cap and base plates for your columns.  Most engineers will specify a bolted connection between the cap plate and steel beam, to comply with OSHA regulations and for ease of construction and inspection.  It is important that the steel beams be fabricated with holes in the correct location for installation prior to delivery.  Discuss using slotted holes with your engineer to give more installation flexibility in the field.  Our details also show a welded connection between the plate and steel beam, so that the bolts, which may potentially interfere with building finishes, can be removed once the welding is completed.

Learn more about Structural Steel by visiting other blog posts in our Structural Steel Category.

Framing Nails: What nail should you use when installing a hanger?

Framing nails used in wood framed construction are available in many sizes with different names that refer to a specific diameter and length.  In addition, many framing nails are designed to be used with nail guns for quick installation, while others are more commonly driven in place with a hammer. So which nail should you use when fastening a beam or joist hanger?  Does it matter?  The answer is yes!

Manufacturers of connection hardware, such as Simpson Strong-tie, always indicate what type of nails should be used with their products.  They typcially specify 16d, 10d or 8d common nails for the installation of hangers, straps and ties.  On occasion they will specify a 16d sinker nail or 10d x 1 1/2″ Teco nail.  So why is this important?  All of the hardware used in wood framed construction is tested and safe allowable loads are determined and published.  Structural engineers and designers will then specify hardware for connections based on these allowable loads and expect the hardware to perform as advertised.

When we conduct framing observations at residential construction sites, we typically find beam, josit and truss hangers installed with 8d or 10d nail gun nails.  These typical framing nails are usually .131″ Dia. x 3″ (8d framing nail) or .148″ Dia. x 1 1/2″ (10d Teco nail).  Using these typical framing nails in the installation of most hangers can result in a reduction of over 30% in hardware capacity, (see Simpson’s “Nail Design Information“).  In addition, many of the modern hangers use “Double Shear” or diagonal nails.  It is very important that these nails have the proper length to adequately penetrate both members in the connection.

This creates a bit of a dilema for a designer.  We are not always able to observe the installation of this hardware.  Should we assume that the connection hardware will be installed with incorrect fasteners and reduce the allowable capacity of the hangers we specify?  This results in larger, more expensive hangers with more fasteners.  On the other hand, larger common nails seem to be less common by industry standards these days.  They cost more and raise the price of construction.

Our approach has been a compromise.  There are many instances where hangers have extra capacity and smaller nails can be used.  We include “Lumber and Hardware Notes” or “Hanger Schedules” on our drawings, indicating the type of nails that should be used with the hangers we specify.  We try and specify 16d sinker nails (.148″ Dia. x 3 1/4″), 8d framing nails (.131″ Dia. x 3″) and 10d Teco nails (.148″ Dia. x 1 1/2″)whenever possible.

What has been your approach both in the field and during design?  Read other posts about residential construction in the Wood Framed Construction category of our blog.

Standard Common, Box and Sinker nail sizes

Standard Common, Box and Sinker nail sizes

Architectural Design and Structure

Three Forks Ranch

Thank you to ESA Architecture for our front and center image.  We feel this building is a perfect example of architectural design that encorporates and reveals structure.  The image on our home page is a picture of an infinity pool at Three Forks Ranch, in Wyoming.  Three Forks Ranch is a private hunting and fishing lodge we designed with ESA Architecture.  The structure is a combination of pre-cast concrete and large timber framing at a remote site in Wyoming.  Feel free to visit ESA’s website to see more images.

Vist the Architectural Design Catagory of our blog for more about Architectural Design.

Reinforced Concrete Beam Design: Concrete Beam Stirrups? What are they and why are they important?

We design reinforced concrete members and often we are asked to observe the steel reinforcement in field before the concrete is placed.  It is our job to make sure the concrete foundations, beams, columns, etc. are built the way they were designed.  During our observations we often find that steel beam stirrups, used in reinforced concrete design, are not installed correctly and it isn’t always clear to the installer why they are important.

Historically, beam stirrups had been used sparingly in residential construction.  However, in recent years concrete beam sizes have gotten shallower and spans have increased.  In our experience, this has been the result of architectural design and building occupant requirements. The increased cost of foundation elements, such as drilled piers, has also been a factor.  Increasing concrete beam spans, to reduce the need for additional piers, has resulted in the need for the use of steel stirrups.

Concrete beams vary in depth.  The deeper the beam, the more shear capacity.  When the depth is not adequate, steel stirrups must be added to increase the shear capacity of the beam.  These stirrups are usually one piece of steel that is bent into a rectangular shape.  Often small diameter steel is used, such as #3 and #4 rebar.  The stirrup typically wraps around the bottom and top bars of the beams.

A designer should specify the size, spacing and location along the length of the beam where the stirrups are required.  We like to specify the stirrup dimensions in our sections, so that the stirrup can be manufactured prior to installation.  Stirrups will be required at areas of high shear, such as bearing points and below large point loads.

The installer should be careful to fabricate the stirrup from one piece of steel and adequately overlap each end (contact the Structural Engineer or refer to the ACI code for variations).  Too often the stirrup is not pre-fabricated and the installer tries make the stirrup in the field, after the horizontal bars are already in place.  This is usually obvious, because the stirrup is constructed from two pieces with inadequate lap splice.  It is much easier and efficient to install a stirrup at the same time the horizontal reinforcement is being installed.  Always contact the Structural Engineer with any questions about size, shape, spacing and installation of stirrups prior to inspection.  This will help prevent last-minute changes, while the concrete truck is waiting.

Visit the Reinforced Concrete Design section of our blog for more on concrete design.

Section of a concrete beam with a closed stirrup

Section of a concrete beam with a closed stirrup