Questions tagged [beam]

Questions about the theory and applications of structural elements designed to resist bending.

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12
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1answer
4k views

At what point does an I-beam becomes a H-beam?

According to BS5950, a beam section can be classified as plastic, semi-compact, compact or slender. For the same section area, a H-beam can take axial compression (without buckling) better than an I-...
12
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4answers
960 views

Easy nonlinear model for large deformation of a beam

I have a beam subject to twisting and/or bending forces as well as linear compression force along its main axis. It is modelled as an isotropic beam, but if anisotropic is not too far off then that's ...
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Buckling: Do buckling mode shapes of n > 1 occur in reality?

In the buckling of columns we know that: $$P = \dfrac{n^2\pi^2EI}{L^2}$$ The smallest value of P occurs when $n=1$ which gives a simple buckling shape (one wave): $$P_{cr} = \dfrac{\pi^2EI}{L^2}$$ ...
11
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4answers
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How do I calculate the forces on a desk and its legs?

I have a design for a desk, and I'd like to not just guess at how strong it'll be, but I can't find an explanation on how to figure out all the forces involved that doesn't assume I already know a lot ...
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3answers
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Strength of a welded steel gate with vertical bars vs. crossed diagonal bars

Looking for some hints on how to create a rough estimate for the following problem. Given two steel gates with the same dimensions, same material - e.g. everything is the same. The only difference ...
10
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3answers
5k views

Why is it unwise to choose cylindrical horizontal beam

I heard that it is unwise to choose cylindrical horizontal beam. My question is why?
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2answers
27k views

How to deal with a point force acting right on the hinge of a beam?

I have been attempting to solve a question where there is a point force acting on the hinge of a beam. Here is the problem: I am not sure how to deal with the 2 kN point force at $C$ ($C$ and $E$ are ...
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3answers
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How do you select the correct area to consiser for first moment of area calculations?

I have just begun studying mechanics of materials and I am struggling to understand intuitively how to select the area in first moment of area calculations. I was hoping someone has a relatively easy ...
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Unbraced Length for Lateral Torsional Buckling vs. Yield Strength

The AISC 360-10 Specification for Structural Steel Buildings gives provisions for calculating the maximum unbraced length of a compression flange that separates yielding moment from lateral torsional ...
7
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2answers
270 views

What does it mean when actual member reaction is 100% of allowed?

I received construction documents for an extensive residential remodel and looking at the engineering there's a glulam drop beam with a calculated member reaction that is 100% of allowed (3965 lbs @ 5 ...
7
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1answer
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Large deflection of a cantilever beam with distributed normal load

I have a strip of stainless steel encastree'd at one end to which is applied a constant pressure on one side, and I need to know what the deflection equation y = f(x) is at equilibrium. If the ...
7
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1answer
303 views

Energy of a 3D Timoshenko beam

I am interested in the kinetic energy T and the potential energy U of a three dimensional Timoshenko beam. For a 2D Timoshenko beam the energies read: $$ T = \dfrac{1}{2} \int\limits_o^l \left( \rho ...
7
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1answer
597 views

Tolerance for Beam With Cope End

I need to have some beams fabricated with copes on the ends. They will be set on top of existing steel trusses, and used to replace undersized purlins to shore up existing rafters (hence the need ...
6
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1answer
682 views

Design of transverse reinforcement in a wide beam

Besides satisfying nominal bar provision in the code, is there any special requirement in the provision of transverse reinforcement to ensure that the longitudinal reinforcement is fully effective in ...
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3answers
2k views

Does the length of a beam change upon deflection?

I have made a Python program in which I have computed the deflection of a beam. I am plotting the deflected beam and am now asked to show the length of the beam. If the beam is not deflected, it is ...
5
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4answers
7k views

How do you calculate Mcr (critical buckling moment)

When designing a steel beam, the resistance to buckling is related to Mcr; the elastic critical moment for lateral-torsional buckling. However the Eurocodes give no advice about how to calculate this ...
5
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1answer
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Strength of a Vertical Round Aluminum Tube with load cantilevered off it?

Can a vertical round tube safely support a cantilevered arm mounted near the top of the tube? ROUND VERTICAL TUBE: 6061 Aluminum 2 inch Outside diameter 0.25 inch wall thickness 3 feet height ...
5
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1answer
146 views

Effects of constructed tubular/annular beam upon moment of inertia

I would like to construct a hollow beam using either 2"x4" or 2"x6" structural lumber.1 My thought is to take 4 boards and butt them against each other in order to form a square beam2 with a hollow ...
5
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1answer
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What is the mathematical derivation for shear center of a beam?

For me, it's a bit hard to imagine where shear center can be, and what purpose it serves. The definition I found online is very vague: Shear center is defined as the point on the beam section where ...
5
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1answer
146 views

Why is this box girder made of lattice on one side only?

Here's a zoomed piece of this image from Wikipedia That's a close-up of a tower holding a railroad bridge. A and B are two of the four columns in the tower's corners. I assume that they are placed ...
5
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2answers
127 views

Longitudinal impact of steel rod on a steel plate to verify the contact duration

So I'm doing an experiment to verify the contact duration when a steel rod (Material: Mild Steel) is allowed to free fall longitudinally on a steel plate. The arrangement is real simple: The rod is ...
5
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1answer
74 views

Can the thickness of a guard post be derived purely from energy principles?

Consider an ordinary staircase with a banister rail supported by a wall at one end and a steel post of constant profile at the other end. The base of the post is embedded rigidly in the fabric of ...
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2answers
773 views

Given the tensile strength of a rod, how do you calculate the max weight that can be hung from it?

Given a 3.25" long, 0.125" diameter rubber rod with tensile strength 1200 psi supported on both ends, how much weight can you hung from the middle of it without breaking it? (This is not homework I am ...
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3answers
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Why does a continuous beam have less deflection than a pair of simply supported beams?

Inspired by this question, and also this answer to a separate question. I have two beam models, one with two independent spans (a full hinge), another is a continuous beam over both spans (only ...
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1answer
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Derivation of beam slope for a cantilever beam

I am trying to derive the maximum beam slope formula $\theta = \dfrac{wL^3}{24EI}$ for this cantilever beam: What method can I use for this derivation? So far I can only find simply supported beams ...
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3answers
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Are the points of inflection and contraflexure the same?

In theory of bending of beams, often these terms are used. There are no clear definition present. Most of them contradicting. Which of the property(bending moment and shear force) is zero in which ...
4
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1answer
145 views

Why are steel I-beams mostly symmetrical?

From what I understand most types of steel are strong both in tension and compression, yet they are still usually significantly stronger in tension. Suppose a common I-beam supported on both ends and ...
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2answers
2k views

Round beam deflection under its own weight

I'm trying to calculate how much simply-supported round beams of several materials and diameters deflect under their own weight plus an additional weight in the middle. I know the material densities ...
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2answers
3k views

Multiple Varying cross section beam deflection

How do we calculate the deflection of a cantilever with two or three cross-sections, and different forces acting at the end of each section?
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3answers
87 views

Natural frequencies of an orthotropic bending beam with circular cross-section

I am trying to find an expression for the natural frequencies of an orthotropic bending beam with a circular cross-section. I have tried searching for some literature on this, but have so far been ...
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2answers
129 views

Undesired vibration of a beam under foot traffic

The other day I was in the waiting room of a doctor's office in a new building as part of UCLA medical complex. A new construction as part of multi million dollar extension of existing medical complex ...
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1answer
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Deriving the deflection force equation for a beam that is fixed on both ends

I want to calculate the force-displacement equation for a beam that is fixed at both ends. I think I should use the superposition principle but I am not very sure how to get started. I know that for,...
4
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1answer
142 views

Is the order of operations in jacking a cable relevant?

If you have a prestressed concrete beam or slab and are jacking a cable from both ends, does the order of operations alter the result? For a symmetric curve, the tension diagram along the cable's ...
4
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1answer
99 views

Deformation of two perpendicular beams with parabolic properties

I want to calculate the deformation of the following structure: I have numbered the vertical beam 1 and the horzontal beam 2. The hight of beam 2 is given by: $h(x)=h\sqrt{1-x/L_2}$ So I thought I ...
3
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3answers
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Why do we assume the moment is zero when analyzing a simply supported beam?

When calculating the reactions of a simply supported beam under any type of loading we assume that the moment about the reaction points is zero. What if our assumption is wrong and in actual practice ...
3
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3answers
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Is it possible to convert uniform loads to point loads?

I want to perform a structural analysis on a portal frame using a finite element solver to get a displacements and reactions for a load. The software library only accepts loads applied to node, not to ...
3
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2answers
719 views

Why is there a moment at the cross section end when you cut a body?

I just started mechanics of materials, and it seems that my knowledge from statics has gotten rusty over the summer. But there was a new concept introduced in the first example problem today. There ...
3
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4answers
853 views

How does stiffness/rigidity affect the bending moment of a beam

My question may sound silly as it’s mainly good sense but I may be mixing things up. Let’s consider a flexible cantilever beam of a given length and material is fixed at one end and free at the other ...
3
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1answer
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Plotting deflection of a beam

I have a formula to compute the deflection of a beam. When I have computed the deflection I get a vector with non-negative elements representing the deflection in each position on the beam. When I ...
3
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3answers
85 views

Why does concentrated shear force on a beam influence the whole beam?

This might be a silly question but can someone explain how this shear force on the right side of the beam causes Qy throught the whole beam? I understand it mathematically but not conceptually so I ...
3
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2answers
9k views

Determining stiffness of a beam w/varying moment of inertia

I need to calculate $K$ or equivalent stiffness of a beam that has a varying $I$ across its length. $I$ has been given to me as a function of $x$, or length. I have been unable to find a proper ...
3
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1answer
2k views

Is the Qmax of an T-Beam equal if calculated both above and below the neutral plane?

I was doing a question involving a T-Beam question wrong since I was selecting the wrong area to use for $Q_{max}$ when trying to maximise the shear stress. $$\large{\tau}= \frac{VQ}{It}$$ A friend ...
3
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3answers
116 views

Why in the construction of power transmission towers , the rods are hinged oblique? [closed]

I know if a rod is hinged at its two ends and if there is no load act upon it along its length then the forces exist only along its length but i don't see how this characteristic comes into play ?
3
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3answers
956 views

How is shelf deflection determined?

Question: How is shelf deflection determined? More specifically, how is the deflection determined when one or more edges of the plate have reinforcing edging applied to them? I have been able to ...
3
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1answer
3k views

Effective length for lateral torsional buckling in a laced strut

When loaded vertically, a beam experiences compression in its top flange and tension in its bottom flange. The top (compression) flange would tend to buckle vertically much like a column, however, the ...
3
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1answer
85 views

Translating the name of a type of beam from Russian to English

I am trying to translate the name of a special kind of beam from a set of Russian-language drawings. The dictionary translates the name as simply supported beam, but it's not that simple. There is a ...
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1answer
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Is it acceptable, with reinforcement, to notch a floor joist more than 1/6 of the depth?

IBC 2308.8.2 states a rule of not notching more than 1/6 the depth of a wooden joist (emphasis added): Framing details. Joists shall be supported laterally at the ends and at each support by solid ...
3
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1answer
150 views

How to measure the unbraced length of lateral torsional buckling for negative moment in a W-shape steel beam?

I have a W-shape steel beam that is fixed-fixed and the top flange is fully braced for lateral torsional buckling. The negative moment at the end controls the design and there is no lateral bracing ...
3
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2answers
955 views

What exactly is the thin wall assumption in regards to beam cross sections?

What does it mean with respect to calculating stresses on a beam undergoing loading and when should it be applied?
3
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2answers
74 views

Is a force to the right positive?

I am trying to do this question and I have everything but one. Solving for $A_x$, $x$ force at $A$. $$\begin{gather} F_x = 6\cdot20 + A_x \\ A_x= -120\text{ kN} \end{gather}$$ Is this correct? ...