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Posts Tagged ‘Structural Engineering’

Just give me a price…

September 13, 2012 Leave a comment

How many times do you get a call and the caller just wants a price?  It happens to us quite often and since we are a custom timber frame shop, each project needs to be estimated and that takes time.

Step 1:  Sandy (me) or Mike gets the info, sometimes architectural drawings, sometimes a sketch drawn by the contractor, sometimes a described project by the homeowners.

Step 2: Estimating Dept.  reviews the provided information, comes up with a material list, allowance for steel (if needed), sometimes a sketch for the potential client to review.

Step 3: Sometimes our structural engineer needs to take a look at the estimated project, like she did for this one.  42FT scissor truss….What’s the roof pitch and where is this located?

Step 4:  Back to me (Sandy) material list entered and priced out.  Wood species, in this case, Douglas Fir, surfaced 4 sides and free of heart center…check.

Step 5:  On the bench.  Owners will review, set design complexity, adjust as needed and back to me. Quite a lot of thought goes into this process.

Step 6:  Presentation created and presented.

Wow, that’s a lot of work.

So how about you just give me a price?

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Timber Connections – Steel

July 11, 2012 Leave a comment

So there are really two types of joinery for heavy timber frames, traditional and steel.  There is a hybrid of the two so maybe that could make a third type but typically as soon as you start adding steel plates it’s considered a steel joint, even if it is a concealed plate.

The use of steel plates for timber joints is a fairly straight forward concept.  The force transferred from the timber (in compression or tension) to the steel plate by steel pins.  These pins can be bolts, lags, nails, etc.  The steel plates then carry the load along the plate to the next timber member.  The steel pins in this member then transfer the force back into the timber frame.

The plate configuration and the number of steel pin connectors all depend on the forces that need to be transferred from one member to the next.  The simplest configuration is a plate on one side of the member.  This is also the weakest version because the bolt is in single shear.  In single shear there are two members trying to slide past each other in opposite directions.  So the two members create a shear plane.  Sometime a simple connection is all that is needed and it will be cheaper.

Then you can also place a piece of steel in the middle of the timber member.  This is called a knife plate because it cuts into the beam.  This is a stronger connection because the bolts are in double shear.  In single shear there were two members sliding past each other.  In double shear there are three members that create two shear planes.  Knife plate connections are a concealed connection.  The bolts can be countersunk and plugged to hide the connection.  This is also a more expensive connection because more time is required in the fabrication of the timber to cut out the space for the steel.

The most common configuration for steel plate connection in trusses is having a steel plate on each side of the timber.  This is also a double shear connection but it is stronger than the knife plate because you have two surfaces of steel and one of wood where the knife plate is vice versa.

There are other ways to strengthen a steel connection.  One way is to make the steel plate thicker.  Larger diameter bolts can be used or shear plate can be added to the connection.  The exact configuration of the connection needs to be designed for each individual case.  Depending on the configuration and the loads at the connection a number of possibilities are available.

Ode to Gravity

May 9, 2012 1 comment

You are a constant reminder

And you remind us constantly

Of your presence and your power

And your rule which we must obey

It can be rain falling from the sky

Or my glass falling as I brush by

As I hurry out the door,

My keys dropping to the floor

You are always there,

Ever consistent,

Ever constant,

Ever present

I was hired to oppose you

To confront you, to face you

To take on your challenges

Whatever you might throw my way

You work against me day after day

You push and I push back

Locked in this life long battle

I will fight to resist your unchangeable way

I will work around your ever present force

But I will always respect you, my dear gravity,

Respect the fact that you are always there

Respect the fact that you will always act

Why does it matter where the windows are?

April 27, 2012 Leave a comment

Window and door opening or fenestrations, where they are located and their sizes do affect the design of a structure.  How much it affects your structure depends on where your project is located.

How exactly do these affect the structure?  Structural engineers need to design for either wind or seismic forces.  Depending on your location one of these will control and that is the one that needs to be designed for.

We are going to kick off with wind loadings.  When wind blows on a structure it exerts a force.  This force affects the building in several different ways.  First up is uplift which occurs at the roof.  When the wind blows it can create an upward force on the roof, even on enclosed structures.  This force can lift the roof off of a house if the roof is not properly attached to rest of the building.  On open structures, like picnic pavilions, the roof acts like a kite, a large kite but still a kite that catches the wind and this is also an uplift example.

The next is racking which is caused when the wind blows on the side wall of a building and the building leans to one side.  This leaning is what we call racking.  If there is too much racking in a building the window tolerance can be overcome and they can crack.  The last two are sliding and overturning of the building.  These are more self explanatory.  Both of these forces are countered by proper attachment of the building to the foundation.

In typical wood frame construction, diaphragms of plywood or decking, studs or joists, and nailing are used to transfer these forces around and through the building.  The window and door locations affect these systems as well as openings in the floor for stairs.  If too much of the plywood is removed other systems may need to be added to the building.  Some options that an engineer might use are drag struts and moment frames.  In some instances metal straps and fasteners can be used to help resolve the force transfers between diaphragms or where loads have concentrated down to the foundation.

It is important that the window and door location are known.  There isn’t as much concern with smaller windows but if a wall is going to be basically glass then that will be an important consideration to take into the design process.

Hay in the Hay Loft?

April 19, 2012 Leave a comment

When a structural engineer wants to know what the space will be used for, they are typically trying to figure out what live loads should be applied to structure.  Depending on the usage of an area the loading will vary.

For example, if the space is going to be used as a storage area the live load is going to be higher than if the space was used as a residential living space.  Typically, the higher the volume of people the higher the loading is going to be, like corridors, classrooms, and other assembly areas.  Another area that higher loading is used is in storage areas.  Structural engineers have no idea what could be placed in this space, it could be anything from your collection of teddy bears and pillows to antique cast-iron tools.  Because of this range the engineers tends to lean towards a more cautious number.

There are general rule of thumbs that are used to come up with the live load but when this question is asked the engineer is looking for anything specific that might add weight to the structure or need design consideration.  Two good examples of this is first a hot tub.  The floor under a hot tub needs to be designed so there isn’t excessive deflection under the weight of the water.  The second example is a spiral stair case.  The spiral stair case is great for a compact space but structurally the stairs put a large point load on the floor under the center pole.  This point load needs to be transferred through the framing system.

So this question is again asked for accuracy in the design of the structure.  Also, don’t give the engineer a hard time if they ask if hay is going to be stored in the hay loft.  You would be amazed how many people call the space a hay loft but don’t plan on ever putting any hay there.

Location, Location, Location

April 12, 2012 4 comments

Okay so maybe those are the three rules for real estate but a structural engineer is very interested in the location of the project.  So, why does the structural engineer want to know where the project is located?  Are they going to make a house call?  No they are not going to show up at your house; they actually use this information when they design the building.

The structural engineer is looking for the physical address of the project.  They are going to use this information.  Please provide it.  Other information that works is the map and parcel number and even the longitude and latitude of the job site works.  Seems fairly simple right?  Oh, the following is a list of answers that will get you another call from the engineer:

  • Just give the state that the project is located (i.e. – New York, Iowa, etc.)
  • General directions (i.e. – 55 west of Albany, 1.5 hours south of Manchester, etc.)
  • The architects or contractor office address

The location will determine the magnitude of the natural forces acting on the building.

Depending on where you live in the United States (or in the world) the loading requirements will vary.  Here in the mountains snow loading is a large factor in the design.  With timber frames, unbalanced snow likes to control the design.  There are maps and charts that layout the snow loads for towns across the U.S.  Even the general chart for the U.S. has some areas labeled as case study areas.  These are areas that the snow loading really varies.  Case study areas are typically located in areas of mountain ranges.  The elevation of the mountain affects the amount of snow load on the building.  To address this some states have put together their snow loading maps and if they haven’t the building department for the town needs to be called.

If you are closer to the coast, in the eastern U.S., high winds from hurricanes need to be accounted for.  If you are out west, wind isn’t an issue but earthquakes are.  The proximity of a building to a fault line can really affect the lateral loading.  The engineer doesn’t want your house to walk off the foundation.  But more on lateral loading when we talk about the “Where are the windows and doors going to be located?”

Another thing that structural engineers have to think about is flood zones, especially in coastal areas.  Depending on the flood zone that you’re building is in will affect the design of the house.  The building codes have specific requirements on building systems like mechanical, electrical and even the material that are used if you are listed in an A or V zone.

The location of the building is important information to a structural engineer.  With this information they can accurately put together the loading that is required for that specific building location.  This can lead to a more economical design and less changes in the long run.

Structural Engineer! What’s that?

April 3, 2012 2 comments

If you have ever built a commercial building or ordered trusses from a manufacturer then you might have come across structural engineers.  Actually, if you have built anything that was structural in nature then you might have wanted a structural engineer or should have had a structural engineer.  The structural engineer typically comes into a project with annoying questions like “What is this space going to be used for?” and “Where are the windows and doors going to be located?” and “Where is the project going to be located?”  All you are thinking when these are being asked is that these are just preliminary drawings and that everything might change in a week.  So who is this person and why do you need them on your job?  Well…

A structural engineer is responsible for the analysis and design of the support or framing system of a building.  I like to refer to it as the bones of the building.  While designing the building, the engineer will take safety and performance into consideration.  For performance or serviceability this is the design for vibrations from machinery, floor vibration or deflection that could cause discomfort, or even building deflection or sway.  The occupants of a building might feel uncomfortable if the building sways or moves too much, especially on the upper stories.

How can you check the qualification of your engineer?  The easiest thing to do is to check if the engineer has either a Professional Engineer (PE) license in civil engineering or their Structural Engineer (SE) license in the state which your project is being built.  Some states don’t offer a SE license so that is why there is a difference in the title.  There is no general licensing for engineers, it is state specific and each state has their own requirement to obtain and maintain their license.

The other thing that you can do is check references and talk with past client about their experience with this particular engineer.  It’s just like selecting an architect or contractor, do your homework and you should be able to put a design team together that meets your needs.

Okay, so that is a general definition of the responsibilities of a structural engineer, how does it pertain to timber framing?  Timber framing for engineers involves the same responsibilities and analysis but it all pertains to one specific material.  Not every structural engineer is familiar with heavy timber framing and traditional joinery.  So, if you have a heavy timber frame make sure that the engineer on the project has experience in this material and traditional joinery.

So what is the deal with those annoying questions anyway?  Now, that is just going to have to be another post on another day.