Walls and Slabs

Background

ClearCalcs calculates in real time for instant results. When designing a wood beam to ASD or LRFD, four conditions must be checked for adequate design: bending strength, shear strength, deflection and bearing capacity. This article will outline how to interpret your ClearCalcs results and determine ways to optimize results. This will allow for more efficient and cost- effective designs.

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Summary

Moment Utilization

What is moment utilization?

Moment utilization is the ratio of adjusted moment capacity to governing moment demand. Moment capacity represents the maximum bending that the beam can resist without excessively deflecting and failing.

How is moment calculated?

Moment capacity is calculated by taking the bending design value of the wood member and multiplying it by various factors for wet conditions, temperature, or duration. The bending design value is determined by the material species and grade. For structural composite lumber or flitch beams, other factors may be included in this calculation.

If you need to decrease your moment utilization, here are 3 factors that you could modify:

1/. Increase the depth of the member. Consider using a 2x10 member instead of a 2x8 if your beam is not passing in design. This impacts the net area and section modulus, therefore increasing the bending capacity. 2/. Add additional support to the beam. Reducing the effective span length will decrease the applied loads and lower the moment demand per span. For example, if you have a joist 14 ft long, but you add an additional support at the midspan, there will be a lower governing load demand for the same supporting capacity. 3/. Use a stronger wood species or grade. Using a hardwood, rather than a softwood, or a No. 1 compared to a No. 2 grade will have higher strength properties and will increase the moment capacity per member.

Shear Utilization

What is shear utilization?

Similar to moment utilization, the shear utilization is the ratio of factored shear capacity to governing shear demand. Shear capacity represents the maximum shear force that can be supported before shear deformation or failure occurs.

How is shear calculated?

Shear capacity considers the specified strength in shear, which is determined by the material properties. This value is multiplied by two-thirds of the net cross sectional area and adjustment factors.

If you need to decrease your shear utilization, here are 2 factors that you could modify:

1/. Increase the member depth. Increasing the net area will result in a higher shear capacity. For example, if you have a 2x8 member close to failure, consider using a 2x10 member instead. 2/. Use a stronger wood species or grade. Strength values are based on species, grade, size and use and higher strength properties will increase the shear capacity per member 3/. Increase the number of plies. This will result in a higher cross sectional stiffness and will also increase the number of layers that can resist shear. For example, using a two-ply member instead of one ply with approximately double the capacity and halve the utilization

Bearing Utilization & Minimum Bearing Length (End Supports)

What is bearing utilization?

Bearing utilization is the ratio of bearing capacity at the most critical support divided by the governing bearing load. Bearing capacity represents the maximum allowable stress or pressure that the beam can support without deforming or failing. When the beam fails due to bearing stress, it can either crush itself or the support, potentially punching through the material. This is common in compression members.

How is bearing calculated?

In ClearCalcs, the bearing strength is calculated individually per support using the compression strength perpendicular to the grain. This value considers the bearing resistance per inch of bearing length, as well as wet service and temperature factors.

If you need to decrease your bearing utilization, here is one factor that you could modify:

1/. Increase contact area between the wood beam and the support in which the bearing surface exists. For example, if your bearing length is 3 inches, consider changing your supporting post to have a larger area where your bearing length can be 5.5 inches. This would significantly decrease bearing stress, as you nearly doubled your bearing length and the bearing stress it is applied over a greater area.

Governing Live / Short-Term Deflection

What is live / short-term deflection?

Live / short-term deflection is the deflection that only considers short term serviceability loads such as live, snow, wind, earthquake or roof live loads.

How is live / short-term deflection calculated?

Live / short-term deflection is calculated for each span individually as the maximum from each serviceability load combination. For cantilevers, the allowable deflection is taken as twice the simple span deflection. For I-joists, the shear deflection is added as required by the code. This value is determined considering the loading conditions and beam properties, typically using a deflection equation.

Governing Long-Term Deflection

What is long-term deflection?

Long-term deflection on the other hand considers long term loads including dead load compounded with the effects of creep.

How is long-term deflection calculated?

Deflection is calculated for each span individually as the maximum from each serviceability load combination. For cantilevers, the allowable deflection is taken as twice the simple span deflection.

If you need to decrease your long-term deflection, here are 2 factors that you could modify:

1/. Stronger beam stiffness, using a higher moment of inertia (I) or modulus of elasticity (E) will result in less deflection. This is because typical deflection equations are directly impacted by the material’s stiffness. For example, the deflection for a simply supported beam with a point load equals PL^2/48EI, where the deflection is impacted by load, length and stiffness. 2/. Shorter span lengths will increase stability and decrease the applied loads per span.

Diagrams

Graphed Load Combination

How to change the graphed load combination

The graphed load combination can be changed by selecting the preferred combination in the drop-down list at the top of the “Diagrams” section. By default, ClearCalcs graphs the “D+L”, or dead + live load combination.

How to determine the governing load combination for my beam design

The governing load combination for beam design will be the one associated with each of the maximum utilization ratio. You can view the governing loads in ClearCalcs under the “Governing Load Combination Determination” section. Further, all governing combinations will be stated in the summary section, once opened in “Detailed View” as shown below:

Shear Diagram

What is a shear force diagram?

A shear force diagram is a visual way of identifying the shear force distribution over the length of the member. To interpret your shear force diagram, any given point will show the location along the beam (x-axis) and the shear force (y-axis). This represents the internal stress the wood beam experiences due to the applied loads.

Moment Diagram

What is a moment diagram?

A moment diagram is a graphed distribution of moment capacity along the length of the member. To interpret your moment diagram, any given point will show the location along the beam (x-axis) and the moment capacity (y-axis). This represents the rotational forces that will occur due to exterior applied loads, any distance away from the point of interest. This bending moment value is calculated and plotted at each location along the beam.

Deflection Diagram

What is a deflection diagram?

A deflection diagram shows how much the beam will deflect at any given point along the length of the member. To interpret your deflection diagram, each point will show the location along the beam (x-axis) and the maximum deflection (y-axis). This represents the distance at which the beam will deform due to exterior applied loads.

Free Body Diagram

What is a free body diagram?

A free body diagram (FBD) is a visual diagram showing the magnitude and direction of all forces acting on an object. In ClearCalcs, the forces you may expect to see on the diagram include distributed, line, point and moment loads, axial loads, and support reactions. More detailed information about loads at a specific location can be found by hovering your cursor over the free body diagram.

To interpret your free body diagram, you can identify the member you are analyzing (in this case, the wood beam), as well as the magnitude, direction, and location of all forces. For example, in the following image, we have a 50 lb point load acting downwards at a location of 12.5 ft from the left edge of the beam.