__Questions & Answers__

By John F Mann, PE

Brief answers to questions posed by readers in search-engine queries are provided below. Please contact us for further information.

July 10,2012

(Q) How much point load weight can an 8 x 7.023 aluminum Ibeam support?

For any beam, load capacity is a function of several factors. See "Beam Design - Basic" and other related pages on this site.

Key factors are; (1) Number of supports, (2) Locations of supports, which determine span length (or lengths for continuous beam), (3) Distance between points where compression edge of beam has lateral bracing, (4) Allowable bending and shear stresses for the material used, (5) Allowable deflection, (6) Load distrubution.

Load capacity tables are published by numerous technical organizations and material suppliers. However, most load-capacity tables are for uniform load only. Perhaps most well-known are load capacity tables for wide-flange steel beams in the AISC Manual and tables for open-web steel joists by Steel Joist Institute. Wood joist manufacturers publish allowable uniform loads in product publications.

Aluminum beams should be designed per Aluminum Design Manual 2010 published by the Aluminum Association.

Point load applied at midspan produces bending moment that is two times the moment caused by uniform load (along entire length of beam) having the same total load as the point load. Therefore, for a beam having allowable bending capacity that supports uniform load with total value of W pounds (unit load "w" in pounds per linear foot times length of beam "L" in feet), allowable bending capacity for point load at midspan is only W/2 pounds.

Midspan deflection due to point load of W at midspan is 60% greater than midspan deflecton due to the same total uniform load (wL = W).

Of course, shear capacity, bearing capacity and other basic design requirements must also be verified.

April 26, 2012

(Q) What design load is required for a deck?

As for all buildings, we must design for weight of permanent materials ("dead load") and weight of potential variable elements ("live load"), including persons and furniture.

Design loads specified by the building code are; (1) Minimum values and, (2) Uniform over entire surface area of the space.

Total design load is dead load plus live load.

For wood-framed residential floors, uniform dead load for the entire floor assembly is most often taken as 10 psf with wood flooring, including drywall underneath and floor joists. For tile flooring, dead load of 20 psf is typically used. This dead load is applicable for all elements that support floor joists.

For a typical wood-framed deck, dead load is much less. For deck joists, dead load is only the weight of decking, which is generally not more than 2 psf. For elements supporting deck joists, dead load should be at least 5 psf.

For residential construction, uniform live load for interior space is either 40 psf (first floor and all other areas except bedrooms) or 30 psf (bedrooms).

The building code specifies that a deck must be designed for the same live load "as the floor area served".

Therefore, for a first floor level deck, uniform live load is 40 psf. Note that, for elements supporting deck joists, this live load is 8 times greater than the conservative design dead load (5 psf) and at least 10 times the actual dead load of decking (2 psf) plus joists (2 psf)! So, a beam must have much more capacity than the small capacity required to support weight of decking and joists.

For a second floor level deck adjacent to bedroom space, a valid case might be made to use 30 psf as the live load, especially if all rooms on second floor are bedrooms. However, my opinion is that 40 psf live load should always be used for any residential deck.

A "deck" is defined (per code) as being supported on two opposite edges. This condition is different than a "balcony" which is supported only on one edge (typically at the building wall). Minimum uniform live load for a balcony is 60 psf.

September 4, 2011

(Q) Can tree roots crack a basement wall?

Most definitely, tree roots can and do crack foundation walls around basements. Roots cause lateral pressure that adds to lateral pressure from soil against the wall.

Roots grow towards water, and water tends to collect along a foundation wall which is essentially an underground dam.

Roots that cause damage generally are from large trees that are within 20 feet or so of the wall. However, I have seen extensive damage to a foundation wall (in southern New Jersey) caused by a root from a tree that was actually 40 feet from the wall! We did not believe that this could be true until we excavated to replace the wall and found the root growing along a 30-foot length of foundation wall.

Most often, damage from a tree root will result in localized cracking and inward movement, in contrast to the uniform inward movement caused by soil pressure. Sometimes you might see a few block pushed in near base of wall, sliding relative to block below - this is the type of unusual occurrence that indicates a tree root.

March 26, 2011

(Q) How much load is on a ridge beam?

As for any beam element, a structural ridge beam (along ridge of roof) must be designed to support weight of permanent materials ("dead load"). In general, a beam must also be designed to support weight of temporary / variable elements ("live load").

For the ridge beam, primary live load is snow. However, governing codes also specify "minimum live load" to cover the potential for a wide variety of potential loads (persons, construction materials).

As discussed briefly below, a ridge beam must also resist wind uplift force.

For downward ("gravity") loads, calculating design loads for a ridge beam is relatively easy for simple conditions; uniform dead load, uniform live load and two supports.

Snow load is defined (by code) along the horizontal span of roof rafters or joists, not the sloped length. Dead load is most often also applied along the horizontal span, although, to be completely accurate, weight of dead load should first be calculated along the sloped length and then calculated for the horizontal span. For relatively low slope (up to about 30 degrees from horizontal), there is almost no practical difference.

For a ridge beam with two supports, the following information is required;

(1) Length of ridge beam, between supports; ( Lrb )

(2) Horizontal span (length) of rafters (or roof joists) on each side of the ridge beam, from ridge beam to support wall (or other beam) at far end of rafter. Spans are designated S1 and S2.

(3) Uniform dead load pressure in psf (wDL)

(4) Uniform live load pressure in psf (wLL)

Unit Load is calculated as pounds per linear foot (PLF) on the ridge beam. Total load is then Unit Load times length of beam.

Unit Dead Load (PLF) = [wDL x (S1 + S2)/2 ]

Total DL (lbs) = Lrb x Unit DL

Unit Live Load (PLF) = [wLL x (S1 + S2)/2]

Total LL (lbs) = Lrb x Unit LL

Total Load (lbs) = Total DL + Total LL

Design "reaction" force applied (by the beam) to each support is half of the total load. This force must be supported by elements below, all the way down to the foundation.

For an example, assume a ridge beam with length of 20 feet, supporting rafters that are 16 feet long on each side of the beam.

For wood-framed construction, with asphalt shingles, dead load is most often taken as 10 pounds per square foot (psf), However, dead load must always be determined for actual conditions.

Snow load can be highly variable, depending on location. Also, snow load on the roof must be calculated according to standard code provisions. However, for many locations, snow load of 30 psf is required.

Total Unit DL = 10 psf (16 feet + 16 feet)/2 = 160 PLF

Total DL = 160 PLF x 20 feet = 3,200 lbs

Total Unit LL = 30 psf (16 feet + 16 feet)/2 = 480 PLF

Total LL = 480 PLF x 20 feet = 9,600 lbs

Total Load = 3,200 lbs + 9,600 lbs = 12,800 lbs

Each support must have capacity to resist 6,400 lbs, down to foundation.

The ridge beam must have adequate capacity (strength & stiffness) to support 640 PLF total load (160 PLF DL + 480 PLF LL). For this relatively long ridge beam, manufactured wood (such as LVL) will typically be required.

Also, the beam must resist wind uplift force, due to wind uplift pressure on roof surfaces. Calculation of design wind uplift load (to be resisted by the ridge beam) is similar, though more complicated. Standard code provisions (ASCE 7) specify greater wind uplift pressures applicable to "edge strips" compared to interior roof areas. Gross wind uplift is offset by 60% of "actual" dead load, which should be only the actual weight of permanent materials.

Tiedown connectors are often required at ends of long ridge beams to resist net wind uplift force. Additional connectors are then also necessary at base of support posts.

October 14, 2010

(Q) What is maximum length of cantilever for I-beam type floor joist?

The building code typically sets a limit of 2 feet for the extension (cantilever) of a floor joist over a foundation wall or other support, unless engineered design is provided.

As for any beam element, the joist must be sized to have adequate strength and stiffness for applicable design loads. For the part of a joist extended (cantilevered) past a support, stiffness (to resist deflection at free end) is relatively more important compared to simple-span beam design.

For strength (bending and shear) of the cantilevered part, only those loads on the cantilever (with length C) must be considered. Shear (V; at support) is equal to total load on the cantilever. For uniform loading (w), moment (M; at support) is equal to wC^2/2. For point load (P) at free end, moment is PC. Moments causes tension stress at top of beam and compression at bottom.

Shear must not exceed allowable shear force specified by the I-beam joist manufacturer. Section modulus must be large enough so that M / S does not exceed allowable bending stress.

Total reaction force (at support) must also not exceed limits published by the manufacturer, for full design load on the entire beam.

Blocking between joists must be installed at the support to prevent tilting or twisting. For joists, a rim board is also installed across ends of joists.

Deflection at the free end should be calculated for the combined loading of; (1) Dead load on the entire beam, and (2) Live load on the cantilever part only.

Load on the interior span (L) between supports causes upward movement (deflection) at the free end of the cantilever. Load on the cantilever causes downward deflection at the free end.

In general, deflection of the free end increases very quickly as the length (C) of the cantilever increases. For uniform load, deflection is a function of the fourth power of length (C^4).

Of course, the remainder of the beam (between supports) must also have adequate strength and stiffness. Live load should be considered only on the simple-span part.

For further information, see the article on this site [to be added].

May 17, 2010

(Q) Why were floor joists installed perpendicular to rafters in older structures?

Presumably this question is referring to attic floor joists, which are most often installed parallel to rafters to resist outward thrust force from rafters.

Attic floor joists may have been installed perpendicular to rafters to minimize span length (of attic floor joists) by taking advantage of interior cross walls, especially when there was no center bearing wall perpendicular to rafters. Another potential reason would be to allow for installation of HVAC ducts within the joist spaces, eliminating the need for a separate duct chase box-out below the ceiling.

Of course, some other method of providing resistance to outward thrust force from rafters must then be provided, or a structural ridge beam must be installed.

In well-constructed older buildings, when attic floor joists are perpendicular to rafters, you will find rafter tiebacks installed from the support wall across several attic floor joists. This was a typical way of at least attempting to provide resistance to outward thrust force from rafters. Use of a structural ridge beam in buildings before about 1970 was relatively unusual.

However, another reason (which is also the case today) is that the builder did not understand that some element must resist outward thrust force from low ends of rafters.

If a structural ridge beam is installed, attic floor joists may be installed perpendicular to rafters without any other element providing resistance to outward thrust. See "Structural Ridge Beam" for more detailed explanation.

(Q) How far can manufactured roof trusses span without interior support?

Manufactured wood roof trusses can be designed to span more than 80 feet without interior support, depending on conditions of design. However, fabrication, transportation and construction can require special and costly effort. For example, trusses may have to be fabricated as two-part "piggyback" assemblies, which require much more permanent lateral bracing. More important is the need for careful design of supports, as discussed below.

For spans over 60 feet, temporary bracing should be designed by a professional engineer. Numerous collapses have occurred during construction due to lack of adequate temporary bracing.

Key conditions of design are truss configuration, design snow load, wind uplift pressure and support requirements.

Consult local wood truss manufacturers for more information. Structural Building Components Association (SBCA) is another source of general information.

Considerations for light-gage steel trusses are similar. TruSteel Division of Alpine Engineered Products lists maximum spans of 80 feet plus.

http://www.cascade-mfg-co.com/docs/TrussSpanFlyer.pdf

Support at each end of the truss should be carefully designed. Gravity-load design reaction forces at each end of the truss, applied downward to supports, can be relatively large.

Wind uplift reaction forces (often "overlooked" by architects) may be especially problematic. The truss manufacturer is NOT responsible for design of tiedown connectors (or any other building elements) to resist wind uplift forces, which must be transferred all the way down to foundations.

Use of interior walls for support may be warranted to minimize design problems with exterior wall supports.

For evaluation of existing roof trusses over interior walls, see "Bearing Wall Removal" on this site.