tmurray
SAWHORSE
We had something similar. In most homes we were inspecting they installed 4ply beams, but only needed 3 ply by code. By doing this, they had a lot easier time ensuring the splices were located properly in the minimum required plys.
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Garage Portal Header Splices (plural)
- Thread starter jar546
- Start date
jfmann77
REGISTERED
Joint in member of built-up wood beam that occurs between supports should never be allowed unless written acceptance is provided by person responsible for structural design of that beam. While some joints between supports may end up being acceptable, there is always high risk that such joint greatly reduces design capacity of beam.
Of greatest risk is joint near midspan of "simple span" beam which is basic beam with two supports.......like header beam over garage door opening.
Imagine having 2-member beam, with each member continuous..........then making vertical sawcut in one member at midspan.......which produces same condition as installing beam with joint in one member. Essentially, only one continuous member is then effective at midspan.
Comment in previous post about outer member maybe not being required for structural capacity could be valid..........yet such notion is not most likely result.........and should not be considered valid unless person responsible for structural design of beam were to verify such, in writing.
Of greatest risk is joint near midspan of "simple span" beam which is basic beam with two supports.......like header beam over garage door opening.
Imagine having 2-member beam, with each member continuous..........then making vertical sawcut in one member at midspan.......which produces same condition as installing beam with joint in one member. Essentially, only one continuous member is then effective at midspan.
Comment in previous post about outer member maybe not being required for structural capacity could be valid..........yet such notion is not most likely result.........and should not be considered valid unless person responsible for structural design of beam were to verify such, in writing.
jfmann77
REGISTERED
Of course beam requirements specified on plans should govern. If plans specify multi-member beam, then joint in any member between supports should not be considered acceptable without written approval of person responsible for structural design (PRSD).
For long beam (girder) with 3 or more supports..........joint near intermediate support might not be much problem for beam supporting only floor joists, since that location is near where calculated bending moment in beam is zero for constant ("uniform") loading condition. However, other loading conditions can surely occur.........most important of which would be large concentrated ("point") load near joint location.........such as from column supporting upper floor.
Also........joint in member eliminates shear capacity of that member at joint location......which must be carefully considered.
Perhaps most deceptive is joint over intermediate support of multi-span beam, with 3 or more supports. Although such condition is most often acceptable, if multi-span beam is designed as "continuous" beam.......then joint over support would be major structural defect........similar to joint at midspan for simple-span beam..........since highest bending moment in beam occurs over support. However, for such design, person responsible for structural design (PRSD) should provide clear notations on plans to highlight & emphasize need for continous beam over support, without joint.
Overall ..........joints in members of built-up beams, especially between supports, should be considered defects unless proven otherwise.
For long beam (girder) with 3 or more supports..........joint near intermediate support might not be much problem for beam supporting only floor joists, since that location is near where calculated bending moment in beam is zero for constant ("uniform") loading condition. However, other loading conditions can surely occur.........most important of which would be large concentrated ("point") load near joint location.........such as from column supporting upper floor.
Also........joint in member eliminates shear capacity of that member at joint location......which must be carefully considered.
Perhaps most deceptive is joint over intermediate support of multi-span beam, with 3 or more supports. Although such condition is most often acceptable, if multi-span beam is designed as "continuous" beam.......then joint over support would be major structural defect........similar to joint at midspan for simple-span beam..........since highest bending moment in beam occurs over support. However, for such design, person responsible for structural design (PRSD) should provide clear notations on plans to highlight & emphasize need for continous beam over support, without joint.
Overall ..........joints in members of built-up beams, especially between supports, should be considered defects unless proven otherwise.
bill1952
SAWHORSE
Well I'll play. Built a small barn. Beam supporting attic is four 2x12s. I calculated four. Building department said three were enough. 32' with an 8x8 post in the middle, do two 16' spans. No joint more than 4' from a bearing point or another joint. I planned it for 490 plf but really 350 plf is probably the right number, 30 vs 20 psf storage. Building inspector termed it overbuilt.
jfmann77
REGISTERED
Would need further information to calculate required beam size...........though for now, using the following conditions;
(1) Roof span (width) = 24 feet ...........so roof joists / rafters 12 feet horizontal span on each side of ridge beam
(2) 10 psf uniform dead load
(3) 20 psf uniform snow load
(4) Joint in ridge beam over center column; two 16-feet "simple" spans
(5) Douglas-Fir, Grade 2 lumber.........design properties per NDS
(6) Load Duration Factor = 1.15 (snow)
(7) Ridge beam span = 16-feet (two spans; 32-feet total)
For stated conditions;
>> Unit design load = 371 PLF
>> Design bending capacity of three (3) 2x12s is 67-percent of required capacity............not acceptable.
>> Calculated Live Load deflection (for snow) is 0.51 inches which is just less than allowable of 0.53 inches for Span / 360 limit.......or much less than allowable of 0.80 inches for more liberal limit of Span / 240.
>> Design reaction force at each end of each beam is 3,608 lbs...........so 7,216 lbs at center column.
For 30 psf uniform snow load;
>> Unit design load = 491 PLF
>> Design bending capacity of four (4) 2x12s is 51-percent of required capacity............not acceptable.
>> Design bending capacity of four (4) 2x12s is 67-percent of required capacity............not acceptable.
>> Design bending capacity of LVL 11-1/4 x 3-1/2 is 118-percent of required capacity.........acceptable. LL deflection = 0.67 inches
If beam is designed (and installed!) as one continuous 32-feet long beam...........bending moment over center column is same as bending moment at midspan for 16-feet simple beam.............so requirement for bending-strength remains same as for simple-span beam. However, deflection is greatly reduced if that were important.........which is not likely for barn. Also, design force at center column is increased.
(1) Roof span (width) = 24 feet ...........so roof joists / rafters 12 feet horizontal span on each side of ridge beam
(2) 10 psf uniform dead load
(3) 20 psf uniform snow load
(4) Joint in ridge beam over center column; two 16-feet "simple" spans
(5) Douglas-Fir, Grade 2 lumber.........design properties per NDS
(6) Load Duration Factor = 1.15 (snow)
(7) Ridge beam span = 16-feet (two spans; 32-feet total)
For stated conditions;
>> Unit design load = 371 PLF
>> Design bending capacity of three (3) 2x12s is 67-percent of required capacity............not acceptable.
>> Calculated Live Load deflection (for snow) is 0.51 inches which is just less than allowable of 0.53 inches for Span / 360 limit.......or much less than allowable of 0.80 inches for more liberal limit of Span / 240.
>> Design reaction force at each end of each beam is 3,608 lbs...........so 7,216 lbs at center column.
For 30 psf uniform snow load;
>> Unit design load = 491 PLF
>> Design bending capacity of four (4) 2x12s is 51-percent of required capacity............not acceptable.
>> Design bending capacity of four (4) 2x12s is 67-percent of required capacity............not acceptable.
>> Design bending capacity of LVL 11-1/4 x 3-1/2 is 118-percent of required capacity.........acceptable. LL deflection = 0.67 inches
If beam is designed (and installed!) as one continuous 32-feet long beam...........bending moment over center column is same as bending moment at midspan for 16-feet simple beam.............so requirement for bending-strength remains same as for simple-span beam. However, deflection is greatly reduced if that were important.........which is not likely for barn. Also, design force at center column is increased.
jfmann77
REGISTERED
whoops..........correction for first line under "30 psf snow".............should be for three (3) 2x12s..............51-percent of required capacity
bill1952
SAWHORSE
Yeh, you need more data. Only center 14' of attic floor in 28' wide building. No roof loads on this beam (which is 60 psf here anyways). Thank you though.
jfmann77
REGISTERED
Appears I glossed over "supporting attic" in initial description.
Based on new information...........14-feet wide attic floor area extending 7-feet on each side of beam.
1) Width of building = 28 feet ...........so attic floor joists spanning 14 feet each side of floor beam (girder).
(2) 10 psf uniform Dead Load........entire attic floor........might be less for part of floor area not supporting live load, though does not alter results significantly.
(3) 20 psf uniform Live Load.........."light storage"............only on 7-feet of attic floor on each side of beam
(4) Joint in floorbeam (girder) over center column; two 16-feet "simple" spans
(5) Douglas-Fir, Grade 2 lumber.........design properties per NDS
(6) Load Duration Factor = 1.00
(7) No joints in members of built-up beam except at center column.
For stated conditions;
>> Unit design load = 350 PLF..............using Live Load Reduction Factor = 0.96
>> Design reaction force at each end of each beam = 2,848 lbs...........so 5,696 lbs at center column.
>> Three (3) 2x12s........design bending capacity is 61-percent of required capacity............not acceptable.
>> Calculated Live Load deflection = 0.43 inches......... OK; less than allowable of 0.53 inches for Span / 360 limit.
>> Four (4) 2x12s.......design bending capacity is 81-percent of required capacity............not acceptable.
>> Calculated Live Load deflection = 0.32 inches.........OK; less than allowable of 0.53 inches for Span / 360 limit.
>> LVL 11-1/4 x 3-1/2.........design bending capacity is 143-percent of required capacity.........acceptable.
>> Calculated LL deflection = 0.38 inches..........OK
If low ("knee") wall is installed under roof joists to form useable attic floor space...........at least some load from roof may be applied to attic floor joists.
Such condition usually will not govern design...........due, in part, to code provisions..........via Load Combination factors.......which allow for reduction of loads when roof snow load and floor live load act at same time. Also, "load sharing" occurs between roof joists and attic floor joists. However, for conservative design, weight of low wall should likely be considered as being applied entirely to attic floor joists.
Based on new information...........14-feet wide attic floor area extending 7-feet on each side of beam.
1) Width of building = 28 feet ...........so attic floor joists spanning 14 feet each side of floor beam (girder).
(2) 10 psf uniform Dead Load........entire attic floor........might be less for part of floor area not supporting live load, though does not alter results significantly.
(3) 20 psf uniform Live Load.........."light storage"............only on 7-feet of attic floor on each side of beam
(4) Joint in floorbeam (girder) over center column; two 16-feet "simple" spans
(5) Douglas-Fir, Grade 2 lumber.........design properties per NDS
(6) Load Duration Factor = 1.00
(7) No joints in members of built-up beam except at center column.
For stated conditions;
>> Unit design load = 350 PLF..............using Live Load Reduction Factor = 0.96
>> Design reaction force at each end of each beam = 2,848 lbs...........so 5,696 lbs at center column.
>> Three (3) 2x12s........design bending capacity is 61-percent of required capacity............not acceptable.
>> Calculated Live Load deflection = 0.43 inches......... OK; less than allowable of 0.53 inches for Span / 360 limit.
>> Four (4) 2x12s.......design bending capacity is 81-percent of required capacity............not acceptable.
>> Calculated Live Load deflection = 0.32 inches.........OK; less than allowable of 0.53 inches for Span / 360 limit.
>> LVL 11-1/4 x 3-1/2.........design bending capacity is 143-percent of required capacity.........acceptable.
>> Calculated LL deflection = 0.38 inches..........OK
If low ("knee") wall is installed under roof joists to form useable attic floor space...........at least some load from roof may be applied to attic floor joists.
Such condition usually will not govern design...........due, in part, to code provisions..........via Load Combination factors.......which allow for reduction of loads when roof snow load and floor live load act at same time. Also, "load sharing" occurs between roof joists and attic floor joists. However, for conservative design, weight of low wall should likely be considered as being applied entirely to attic floor joists.
bill1952
SAWHORSE
Interesting. I did use 5 psf for dead load. Nothing but joists - exit 16 OC - and decking - 1x10s. All rough sawn white pine. (before you say "graded", NY amends IRC to allow rough sawn) I also was fine with 1/180 deflection.
Thank you.
Thank you.
jfmann77
REGISTERED
Grade (not "graded") refers to grade of wood..........which, for each species, defines design properties such as "Reference" values for bending stress and compression stress...........as well as modulus ("E") value that is used to calculate deflection.
Dead load (weight) of typical wood-framed attic floor;
Wood floor sheathing (plywood) = 2 psf 3/4-inch thick wood boards have essentially same or greater weight
Floor joists = 2 psf
Drywall ceiling = 2 psf
Insulation = 1 psf
Total = 7 psf...............although typically "round up" to 10 psf.............actual weight may be used as long as reasonable.
f wood flooring is ....or might be......installed, then 10 psf must be used.
Though any owner may consider greater amount of deflection acceptable..............building code specifies limits that should be maintained for new construction...... via standard calculations.
Table R301.7 in IRC 2021........which is essentially same as table in prior codes going back to "beginning of time"..........specifies limits for deflection.........see R301.7......... due to Live Load only.......not including dead load except for Lintels supporting masonry veneer (see footnote "e" to Table);
Floors: Span / 360
Drywall ceilings: Span / 240
Deflection due to dead load has not been specified by building code............although it should always be considered for wood-framed construction which has well-known inherent property of long-term deflection due to sustained load.........that is, dead load.......which can continue for very long time.
For attic floor with drywall ceiling underneath..........conservative approach would use Span / 360.........while liberal approach would use Span / 240.
However, when attic floor is definitely to be used as floor for storage...........Span / 360 is most appropriate.
Although for separate thread..........deflection is always relative...........such that, relative to column, deflection of floor should consider deflection of not only floor joists but also members..........such as main girder...........providing support for floor joists. Failure to consider such "total deflection" often is cause........or key contributing factor........ for problems such as cracking of tile floors and other deflection-related defects with finish materials.
Dead load (weight) of typical wood-framed attic floor;
Wood floor sheathing (plywood) = 2 psf 3/4-inch thick wood boards have essentially same or greater weight
Floor joists = 2 psf
Drywall ceiling = 2 psf
Insulation = 1 psf
Total = 7 psf...............although typically "round up" to 10 psf.............actual weight may be used as long as reasonable.
f wood flooring is ....or might be......installed, then 10 psf must be used.
Though any owner may consider greater amount of deflection acceptable..............building code specifies limits that should be maintained for new construction...... via standard calculations.
Table R301.7 in IRC 2021........which is essentially same as table in prior codes going back to "beginning of time"..........specifies limits for deflection.........see R301.7......... due to Live Load only.......not including dead load except for Lintels supporting masonry veneer (see footnote "e" to Table);
Floors: Span / 360
Drywall ceilings: Span / 240
Deflection due to dead load has not been specified by building code............although it should always be considered for wood-framed construction which has well-known inherent property of long-term deflection due to sustained load.........that is, dead load.......which can continue for very long time.
For attic floor with drywall ceiling underneath..........conservative approach would use Span / 360.........while liberal approach would use Span / 240.
However, when attic floor is definitely to be used as floor for storage...........Span / 360 is most appropriate.
Although for separate thread..........deflection is always relative...........such that, relative to column, deflection of floor should consider deflection of not only floor joists but also members..........such as main girder...........providing support for floor joists. Failure to consider such "total deflection" often is cause........or key contributing factor........ for problems such as cracking of tile floors and other deflection-related defects with finish materials.
bill1952
SAWHORSE
no drywall ceilings or insulation in barn