Load paths
UDL on a beam from tributary area
Turn a floor pressure into a uniform line load on a beam: multiply the tributary width by the load. Here is the method, beam self-weight, and the two-way equivalent UDL.
Sam Rivera··5 min
Writer
Sam Rivera
Structural engineer · Educator · Austin, TX
Sam teaches statics and reinforced-concrete design, and has graded more tributary-area sketches than anyone should. He writes the explainers he wishes his students had.
Fundamentals
Concrete slab self-weight calculation
Slab self-weight is thickness times density. Reinforced concrete is about 25 kN per cubic metre, so a 150 mm slab weighs about 3.75 kN per square metre. Here is how to get it right.
Sam Rivera··4 min
Codes & standards
Typical floor live load values by occupancy
Floor live load depends on use, and codes set minimum values by occupancy. Here is a reference table for offices, homes, retail, storage and more, in kPa and psf.
Sam Rivera··4 min
Codes & standards
Roof live load vs snow load
A roof carries both a minimum roof live load and a snow load, and the larger governs. Here is the difference, how each is found, and which one to design for.
Sam Rivera··4 min
Load paths
Lintel load calculation, explained
A lintel carries the wall and floor above an opening. Here is how to find its load, the triangular masonry-load idea, and a worked example for a door or window head.
Sam Rivera··5 min
Fundamentals
One-way vs two-way slabs: which tributary model to use
Whether a slab spans one way or two changes how load reaches the beams, and what shape the tributary areas take. Here is how to tell them apart and when the distinction actually changes your numbers.
Sam Rivera··6 min
Fundamentals
Triangular and trapezoidal loads on beams
A two-way slab loads its beams with triangles and trapezoids set out by the 45-degree rule. Here is where the shapes come from, the equivalent UDL, and a worked example.
Sam Rivera··4 min
Fundamentals
The 45-degree rule for tributary area
The 45-degree rule sets tributary boundaries by bisecting the angles between supports. Here is what it is, how it relates to the midline method, and when it applies.
Sam Rivera··4 min
Fundamentals
Tributary width for a beam, explained
A beam's tributary width is half the spacing to the parallel member on each side. Multiply it by the floor load for the uniform load on the beam. Here is the method with numbers.
Sam Rivera··4 min
Codes & standards
Snow load calculation (ASCE 7), explained
ASCE 7 turns ground snow into a roof snow load with a handful of factors. Here is the flat-roof and sloped-roof formula, what each factor means, and a worked example.
Sam Rivera··5 min
Codes & standards
Eurocode load combinations explained (EN 1990)
EN 1990 sets the load combinations for European design. Here are the ULS expressions 6.10, 6.10a and 6.10b, the partial and psi factors, and a worked example.
Sam Rivera··4 min
Fundamentals
How to calculate tributary area for a wall
A wall collects a tributary strip, not an area like a column. Here is how to find its tributary width, turn it into a line load, and where the strip method applies.
Sam Rivera··5 min
Fundamentals
Tributary area on an L-shaped floor
The re-entrant corner of an L-shaped floor is where hand tributary sketches drift. Here is why, how clipping to the outline fixes it, and the area-balance check that proves it.
Sam Rivera··5 min
Codes & standards
Load combination worked example
One column, every gravity combination, worked end to end in both ASCE 7 and Eurocode. See exactly how service loads become a governing factored reaction.
Sam Rivera··4 min
Codes & standards
Eurocode vs ASCE 7 load combinations
ASCE 7 and Eurocode reach similar safety by different bookkeeping. Here is a side-by-side of the gravity combinations, the factors, and a worked comparison.
Sam Rivera··4 min
Codes & standards
ASCE 7 load combinations explained
ASCE 7 gives the factored load combinations that size members. Here are the gravity LRFD and ASD combinations, what the factors mean, and a worked example.
Sam Rivera··5 min
Fundamentals
What is superimposed dead load (SDL)?
Superimposed dead load is the permanent weight added on top of the structure's self weight: finishes, services, ceilings and partitions. Here is what counts and typical values.
Sam Rivera··4 min
Codes & standards
What is the governing load combination?
The governing load combination is the one that produces the largest design effect on a member. Here is how to find it, why it varies, and a worked example.
Sam Rivera··4 min
Fundamentals
How to calculate a wall line load
A wall's line load is its weight per metre of length: height times thickness times unit weight. Here is the method, typical material weights, and a worked example.
Sam Rivera··4 min
Fundamentals
Tributary area vs influence area
Tributary area is the floor that loads a member; influence area is the larger area whose load affects it. Here is the difference, the K_LL factor, and why it drives live load reduction.
Sam Rivera··4 min
Fundamentals
How loads transfer from a slab to beams
A slab hands its load to the beams that support it, as a uniform strip in one-way action or as triangles and trapezoids in two-way action. Here is how, with the load shapes.
Sam Rivera··5 min
Fundamentals
Area load vs line load vs point load
Loads come in three forms: area loads on slabs, line loads on walls and beams, and point loads at columns. Here is what each means, its units, and how they convert.
Sam Rivera··4 min
Load paths
Column load takedown by hand, step by step
How to take one column down by hand: tributary area per floor, load per floor, accumulate down the building, then factor. A full worked example with numbers.
Sam Rivera··5 min
Fundamentals
Tributary area: edge column vs corner column
An edge column carries about half a bay; a corner column about a quarter. Here is why, with the geometry, the load ratios, and a worked comparison.
Sam Rivera··5 min
Fundamentals
How to calculate tributary area for a column
A column's tributary area is the floor region nearest to it. Here is the half-bay method, the interior, edge and corner cases, and a worked example.
Sam Rivera··5 min
Codes & standards
LRFD vs ASD load combinations, explained
LRFD factors the loads up and the strength down; ASD keeps loads near service and limits the stress. Here is the difference, the ASCE 7 gravity sets, and a worked example.
Sam Rivera··4 min
Codes & standards
Factored loads vs service loads, explained
Service loads are the real, unfactored loads; factored loads are them multiplied by load factors for strength design. Here is the difference and when each is used.
Sam Rivera··4 min
Codes & standards
ASCE 7 live load reduction, explained simply
A column carrying ten floors is not realistically loaded to the full design live load on every one at once. ASCE 7 lets you take credit for that. Here is the formula, the K_LL factors, and a worked reduction.
Sam Rivera··6 min
Codes & standards
Partition load allowance explained
Movable partitions are covered by a load allowance rather than counted one by one. Here is what the allowance is, typical values, and when a partition is counted as dead load instead.
Sam Rivera··4 min